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Title: Ecology of the Opossum on a Natural Area in Northeastern Kansas
Author: Henry S. Fitch
Lewis L. Sandidge
Release Date: August 24, 2011 [EBook #37199]
Language: English
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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Volume 7, No. 2, pp. 307-338, 5 figures in text
August 24, 1953
Ecology of the Opossum on a Natural Area
in Northeastern Kansas
BY
HENRY S. FITCH
AND
LEWIS L. SANDIDGE
UNIVERSITY OF KANSAS
LAWRENCE
1953
UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
Editors: E. Raymond Hall, Chairman, A. Byron Leonard, Robert W. Wilson
Volume 7, No. 2, pp. 307-338, 5 figures in text
Published August 24, 1953
UNIVERSITY OF KANSAS
Lawrence, Kansas
PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1953
24-7812
Ecology of the Opossum on a Natural Area in Northeastern Kansas
BY
HENRY S. FITCH and LEWIS L. SANDIDGE
On the 590-acre University of Kansas Natural History Reservation where
our study was made, the opossum, _Didelphis marsupialis virginiana_
Kerr, is the largest predatory animal having a permanently resident
population. The coyote, raccoon and red fox also occur on the area but
each ranges widely, beyond the Reservation boundaries. With the
passing nearly a century ago of the larger animals of the original
fauna, the buffalo, elk, deer, antelope, wild turkey, gray wolf and
others, lesser herbivores and carnivores including the opossum and
animals of similar size fell heir to their key positions of
predominance at the peak of the food pyramid. These smaller animals,
however, exert less powerful effects in controlling the general aspect
of the biotic community, and affect it in different directions. The
over-all ecology is greatly altered. The flora and fauna both are
undergoing successional changes which will continue for a long time
and probably will culminate in a biotic community much different from
the original climax.
The opossum plays an important part in this process of change; being
relatively large, numerous, and of omnivorous habits, it variously
influences, directly and indirectly, the populations of its plant and
animal associates, through a complex web of interrelationships.
Several excellent field- and laboratory-studies of the opossum have
been published (Hartman, 1928, 1952; Lay, 1942; Reynolds, 1945;
Wiseman and Hendrickson, 1950) and the life history of this remarkable
marsupial is already well known. The purpose of our study, therefore,
was to gain a better understanding of the ecological relationships of
the opossum in the particular region represented by the study area. To
accomplish this, we gathered data concerning the animal's responses to
climate and varying weather conditions; its annual cycle of breeding,
growth and activity, movements, principal food sources, numbers,
population turnover, and natural enemies. Although we did gain a
somewhat better understanding of the opossum's ecology, results are
remarkably meager in proportion to the large amount of time expended.
The hours of work daily in setting and tending a line of live-traps
ordinarily were rewarded with only a few records, sometimes none.
Comparable time and effort directed to the study of smaller and more
abundant kinds of animals has been far more productive of data. Field
work was carried on in parts of 1949, 1950, 1951 and 1952.
[Illustration: FIGURE 1. Map of the University of Kansas
Natural History Reservation showing locations where opossums
were live-trapped.]
Because opossums are nocturnal and rarely seen in the course of their
regular activities, the present study is based mainly on information
gained by live-trapping them. Several different sizes of traps of the
type described by Fitch (1951) were used. The most successful were
2' × 8" × 8" in dimensions although many of the larger ones were also
used. They were constructed of hardware cloth having a half-inch mesh.
Live-trapping was begun in October 1949 by Fitch with a line of about
a dozen traps. In the following month Sandidge joined in the field
work. The trapping was continued throughout the winter and spring of
1949-1950 and was resumed the following fall and more traps were
added from time to time until a maximum line of approximately 60 was
attained. Sandidge's participation ended in December, 1950. The
live-trapping was continued on a reduced scale by Fitch through the
winter and spring of 1951 and some was done sporadically in the fall,
winter and spring of 1951 to 1952.
Traps were baited with a variety of foods such as carcasses of small
vertebrates, meat scraps, canned dog food, ground horse meat and bacon
grease. At each capture, sex, weight, and individual formula of the
opossum, based on toe-clipping and ear-clipping (Fitch, 1952), were
recorded. Also recorded was the exact site of capture as located in
one of 84 divisions of the Reservation and estimated in feet from some
named landmark. Notes on breeding condition, pelage, injuries,
parasites and general appearance were also taken at the time of
capture. For opossums caught in 1951 and 1952, the hind foot
measurement was recorded.
Often, attempt was made to follow the released opossum to determine
the direction and distance of its homeward travel but this was
difficult because of brushy terrain and secretive habits of the
animal. An opossum being followed would almost invariably take refuge
in a tree if it caught sight of the observer. Other information
regarding the animal's habits was obtained from tracks in snow or soft
soil and from the distribution and contents of scats. Carcasses of
opossums which had fallen victim to predators were found on a few
occasions and in some instances clues as to the identity of the
predator were obtained. One hundred and seventeen opossums were
live-trapped and handled a total of 276 times. Six of these were dead
when first found in the traps. The remaining 111 were marked and
released. In addition, 207 pouch-young carried by adult females were
recorded and 115 of these were individually marked by toe-clipping.
Some of the opossums that were marked while in the mother's pouch were
subsequently recaptured when they were well-grown, independent young,
or adults, affording information on growth and dispersal.
HABITAT
The habitats of the Reservation have been described briefly by Fitch
(1952) and by Leonard and Goble (1952). More than half the area
consists of steep wooded slopes with mixed second growth forest,
consisting of elm, hickory, oak, walnut, ash, honey locust, hackberry
and osage orange, in about that order of abundance, with thickets of
blackberry, crabapple, wild plum and grape. Fallow fields and
pastures of the upland and valley floors alternate with the woodland.
The varied habitat provides numerous different food sources. Along the
edges of the hilltops there is a nearly continuous limestone outcrop
with a lower outcrop paralleling it. These rock ledges, well
distributed throughout the area, provide an abundance of den sites and
most of the opossums definitely trailed to a home base were found to
be utilizing dens in the rock ledges. Two small creeks on the area
have some water for most of the year. As compared with wooded
bottomland of larger stream courses in Douglas County and those
counties adjoining it, the Reservation area probably supports a
relatively low population density of opossums. "Sign" has been found
in much greater abundance in near-by areas supporting a heavier
woodland.
Every part of the Reservation is used by opossums, but their activity
is concentrated in the woodland, and all dens found were in woodland.
Most parts of the fields are within 100 yards of the edge of the
woodland and no point is more than 700 feet from the edge. Most of the
opossums' foraging in fields was concentrated along the edge;
otherwise they tended to follow creeks and gullies and they follow
well worn trails more often than they do in the woods. Within the
woodland, activity tended to be concentrated along the small streams,
and along the rock ledges where den sites were plentiful. Throughout
the annual cycle, and from year to year, there were minor shifts in
areas of concentrated activity depending on seasonal changes in food
sources such as thickets of wild plum, crabapple, blackberry and
grape, with fruits ripening at slightly different times of year. The
areas adjoining the Reservation offer somewhat similar habitat
conditions, part woodland, part pasture land and some cultivated
fields with corn or other crops which provide food sources for the
opossum.
Under original conditions the area that is now the Reservation
probably was marginal habitat for opossums, consisting mainly of open
grassland with trees in small and scattered clumps, if indeed they
were present at all. There has been steady encroachment of shrubs and
trees, originally chiefly confined to near-by bottomlands such as
those of the Kaw and Wakarusa valleys. Concurrently, the original
hardwood forest of the bottomlands has mostly disappeared, and the
land has been taken over for intensive agricultural use. The new
upland forest provides a habitat different in many respects from the
original bottomland forest. The species composition, in trees and
other plants, is somewhat different, with more xeric types,
especially on steep south slopes. Logs and large old hollow trees are
scarce. The lack of such potential den sites is compensated for by the
abundance of holes and crevices along hilltop rock ledges.
BEHAVIOR
Undisturbed opossums were seen in the course of their normal
activities on only a few occasions, and behavior is known to us mainly
from the sign and from observations made on those that were
live-trapped. Ordinarily those taken in live-traps were found curled
up in deep sleep from which they did not arouse until touched or until
the trap was moved or jarred. Reactions to humans varied greatly in
individuals and was not necessarily correlated with age or sex. Adult
males were uniformly hostile to the trapper and reacted with harsh,
low growls, with back arched and hair bristling. Although many adult
females and young of both sexes were similarly hostile in behavior,
others were not. Some cowered silently in the trap. Others showed
hardly any uneasiness. A small proportion of them feigned death when
handled or even before they were touched. Feigning was especially
frequent in response to clipping of toes and ears when the animal was
marked. In some that were handled, the feigning reaction was weak or
incomplete, the animal arising almost immediately after collapsing or
beginning to collapse in the feint.
Those that feigned death usually maintained the deception for not more
than two or three minutes after a person had moved away out of sight.
The opossum first raised its head and sniffed, listened, and looked
about cautiously for a short time, with body and limbs still relaxed
in the feigning posture. Failing to detect any sign of danger, it
gradually shifted to a sitting position, and then to a standing one,
from which it began moving away with many short pauses at first, and
then more rapidly.
Upon being released, some opossums scrambled for shelter immediately;
others stood their ground defiantly with back arched, hair bristling
and fangs bared. One that was put on the defensive would usually
maintain its stance for less than a minute if not further disturbed by
movements of the trapper. It would then slowly turn its head and begin
walking away with deliberate gliding movements, often pausing abruptly
in the middle of its stride with one or two feet off the ground in a
pose reminiscent of that of a bird dog making its "point." After
moving away a few yards, it would gradually accelerate its pace in a
scramble for shelter, but an occasional individual moved away
unhurriedly, even foraging as it went.
[Illustration: FIGURE 2. Half-mile-square area on Reservation,
showing dates and successive sites of capture for two subadult
male opossums; one opossum on upper half of map and other
opossum on lower half. Arrows from circles show courses taken
by released opossums that were followed to dens (crosses).]
On the few occasions when opossums were seen at night, their relative
alertness and speed of movement contrasted with the sluggishness and
seeming stupidity of those observed in daylight. Several were seen on
roads in the beam of automobile headlights. These were quick to
escape, running into thick roadside vegetation or woods to elude
pursuit. Others were found in woodland, with the aid of a powerful
flashlight as the investigator moved about on foot. They did not
permit close approach, and escaped by running. One hid in a blackberry
thicket. Several that were chased climbed trees when hard pressed. One
that was overtaken, and others that were shaken out of trees and
caught, showed fight, standing on the defensive, and slashing at the
pursuer with a rapidity and vigor never encountered in those removed
from traps in the daytime.
[Illustration: FIGURE 3. Half-mile-square area on Reservation,
showing dates and successive sites of capture of an old adult
male in upper half of map and an adult female in lower half.]
Nocturnal tendencies of the opossum were emphasized by the infrequency
with which undisturbed individuals were seen in the daytime. In more
than a thousand days of field work on the Reservation, opossums were
found out on only four occasions. These occasional daytime forays seem
to occur almost always in animals driven by hunger on winter days,
when the temperature has suddenly risen after periods of severely cold
weather that have imposed inactivity and fasting.
MOVEMENTS
Earlier field studies of the opossum have produced somewhat
conflicting evidence and conclusions regarding the extent and manner
of the opossum's travels. Lay (1942:158) live-trapped and marked 117
opossums on an 86-acre study area in eastern Texas over a two-year
period and caught 29 of them at three or more different trapping
stations. He found that "The average minimum area between the stations
in these 29 home ranges was 11.5 acres. The mean of the greatest
distances traveled between stations was 1460 feet, which would form a
theoretical circle of 38.4 acres.... Separate individual territories
are not important to opossums as home ranges overlapped in every
instance." Reynolds, in central Missouri, concluded that: "The
subsequent recovery of only 5 of 68 released animals, the reported
capture of one individual 7 miles from the point of release nine
months later, and the rapid repopulation of an area devoid of opossums
at the close of the hunting season indicate that most opossums are
nomadic." In southeastern Iowa, Wisemann and Hendrickson (1950:336)
found that: "Recaptures, in 1942, of three opossums tagged in 1941
indicated a yearly mobility of one-fourth mile; four tagged in 1942
were recaptured within one-half mile from sites of tagging."
Opossums, like other animals, obviously make various types of
movements. Ordinarily one tends to keep within a relatively small area
that is familiar to it and that satisfies all its ecological
requirements. This constitutes its home range. Many other animals,
including various mammals, are characterized by territoriality;
individuals, pairs or groups occupy definite areas, defended as
territories, to the exclusion of other members of their species. Like
Lay (_loc. cit._) we found no evidence of territoriality in the
opossum. In general, opossums are unsocial but not intolerant in their
behavior. In the present study numerous individuals of both sexes and
various sizes and ages were found to be occupying the same area
simultaneously, with overlapping but no exact correspondence in home
ranges. Occasionally two or more opossums may use the same den, but
each goes its own way on its foraging and it seems that no sociability
is involved.
On many occasions opossums were tracked in soft snow or mud which
retained footprints. Under conditions prevailing locally, it was
difficult to follow such a trail for any great distance but trailing
did divulge information concerning the type of route followed and the
method of foraging. Opossums were found to have little inclination to
follow beaten trails, either their own or those of other animals. A
foraging opossum moved about in an extremely circuitous and erratic
route, seldom taking more than a few steps without a change of
direction, and frequently crossing its own course in a series of
loops, some only a few feet or a few inches in diameter. In moving
about, it is guided partly by the tactile and olfactory stimuli of
objects on or beneath the ground surface which are potential food
sources. Foraging consists of a succession of tests of such objects,
as the animal moves from one to another. Opossums may habitually
follow intermittent creeks or gullies or even roads when these provide
better foraging than does the adjoining habitat. Metamorphosing
amphibians may provide such a food source along a creek and the supply
of crushed insects or other small animals along a road attracts the
opossum. Food is found by turning chips and leaves, and by poking and
probing in chinks and crevices with its snout and paws. On a few
occasions short, well worn trails made by opossums were found, from
dens to near-by feeding areas where grape tangles provided an abundant
and readily available food source over periods of weeks. More often,
an opossum follows no trail in its search for food, but seems to
wander at random within its home range.
[Illustration: FIGURE 4. Quarter-mile-square areas on
Reservation showing dates and successive sites of capture
of individual opossums; (A) subadult male; (B) subadult male;
(C) subadult male; (D) adult female. Arrows from circles show
courses that were taken by released opossums that were
followed; crosses show location of dens to which they were
traced.]
Evidence of the existence and extent of home range was obtained for
those opossums that were trapped on several or many occasions. Records
of each were usually well scattered over an area hundreds of feet in
diameter. Limits of home ranges are not sharply defined and at any
time the opossum may extend its range into new areas. It may shift to
a new den from which areas beyond its original home range are readily
accessible, and may then occupy a new home range overlapping part of
the old one. Or, it may make a relatively long shift, to an area
entirely distinct from the original home range and well separated from
it. That such shifts are frequent was indicated by the brief span of
records for most of the opossums live-trapped on the Reservation.
After the first capture and marking an individual was often caught
consistently over periods of weeks, only to drop out suddenly either
having been eliminated or having moved elsewhere. Of the 111 opossums
marked and released, 62 were caught only once and 25 others were
recaptured only within a period of one or two months. Relatively few,
only 24 (14 males and 10 females), had records extending over more
than two months. Many of the opossums trapped were probably at or near
the edges of their home ranges which barely overlapped the study area;
consequently the chances of recapturing them were poor. Those caught
well within the trapping area were much more likely to be recaptured.
Tracking of opossums suggested that having once left the home den, an
animal ordinarily did not return until it had finished its nightly
foraging, and wandered more or less at random over its home range.
Successive capture sites for any one opossum might be near together or
far apart with respect to its over-all range, but on the average, they
would be separated by approximately half the breadth of the home range
assuming the animal's activity to be evenly distributed over the whole
area. Each of twenty-two opossums was caught at only two different
trapping stations. For this group, the average distance between
stations was 761 feet (657 feet for seven males and 810 feet for 15
females) indicating home ranges of approximately 42 acres in extent.
Each of ten opossums was caught at three different stations; for these
the distances between the first and second stations, between the first
and third and between the second and third comprise three distinct
movement records, and the average of all three probably affords a
more reliable figure for the radius of the home range than does the
single movement available for each of the 22 animals captured at only
two stations. For these average individual movements the mean of this
whole group of 10 was 841.5 feet. Each of five opossums was taken at 4
different trapping stations, and for each of these a record of six
different movements was available. The average was 1016 feet. For the
37 opossums caught at two, three or four different trapping stations,
the mean distance was 817 feet; this is an indication of home ranges
of approximately 48 acres in extent. Each of thirteen opossums was
caught at five or more trapping stations. The distribution of these
stations affords a crude idea of the extent and position of each
animal's home range, but ordinarily it might be expected that the area
included between capture sites would be less than the animal's actual
home range, because relatively few of the sites of capture would be on
the margin of the home range. For this group, maximum distances
between trapping stations averaged 1954 feet suggesting a home range
of nearly 70 acres, larger than that computed for the opossums caught
at only two, three, or four stations. However, for those caught at
five or more stations, the time involved averaged longer and probably
some had altered their ranges to invade new areas. Ranges may have
been broadly oval rather than circular so that the maximum diameter
measured between stations exceeded somewhat the average range diameter
for each animal.
The opossums having home ranges entirely within the study area were
those most likely to be caught repeatedly and at different locations,
while those with ranges centering near the edge of the area, or
outside of it tended to be caught at fewer locations and less
frequently. For those animals with ranges partly outside the study
area, the captures recorded would represent only one sector of the
home range and would tend to be near together, so that many of the
radii computed for individual home ranges are too small. Each average
figure for home range is perhaps erroneously low for this reason. The
error tends to be greatest for those taken at only two locations, and
least for those trapped at the greatest number of different locations.
Approximate size of the usual home range is apparent from the several
figures although various unknown or unmeasurable factors distort the
data. The usual home range of the opossum in the area of the study is
in the neighborhood of 50 acres or a little less. With the data
available no significant differences in sizes of home ranges are
discernible between males and females nor between adults and young of
the year. Shifts occur frequently, contributing to population
turnover, which may result in almost complete replacement of
individuals in the course of a year's time, on an area of less than a
square mile.
DISPERSAL OF YOUNG
One hundred and fifteen small young of 14 different litters were
marked while still attached to the mother's teats in the pouches.
A fairly high rate of mortality probably is normal in the small
dependent young and further mortality probably resulted from the
deleterious effects of examining and handling them and the females
that carried them. At any rate, 47 of 208 young recorded, were missing
at subsequent recaptures of the females, before the young were old
enough to become independent. It is almost certain that the actual
losses were much higher, because the records for each female cover
only part of the period during which young are carried in the pouch.
Fifteen of these marked young of seven different litters were
recaptured after periods of months, when they were well grown or adult
and the locations of these recaptures afford information concerning
the animals' dispersal. Their records are summarized below. Opossums
that wandered much more than half a mile or at most three-fourths
of a mile from the place of original capture were unlikely to be
recaptured, and some originally recorded at sites near the edge of the
study area might have moved beyond its boundary with much shorter
shifts.
Date of capture and Date of Distance
Sex marking as pouch young recapture in feet
Female April 14, 1951 September 22, 1951 1870
Female May 6, 1950 February 28, 1952 1800
Female May 14, 1950 December 31, 1950 1750
Female March 28, 1951 January 23, 1952 1700
Female May 11, 1951 November 9, 1951 1700
Female May 11, 1951 March 2, 1952 1450
Female April 2, 1950 October 7, 1950 1160
Female April 14, 1951 May 19, 1952 1100
Male May 11, 1951 February 3, 1952 800
Female May 11, 1951 January 9, 1952 700
Female April 2, 1950 October 3, 1950 700
Female May 6, 1950 April 3, 1951 650
Female March 28, 1951 February 2, 1952 500
Male April 18, 1952 July 6, 1952 120
Female April 2, 1950 April 14, 1951 10
Most of these opossums were recaptured within a year of the time they
were marked as small young in the females' pouches, and on the average
they had moved a little less than 400 yards. While the sex ratio was
equal in the pouch young that were marked, it is noteworthy that all
but two of the recaptured opossums were females; and of the two males,
one was recaptured early, before it could have had time to wander far.
The young males, after becoming independent must tend to wander much
more widely, and to settle in new areas far removed from the mother's
home range. It is unlikely that this dispersal of the young males is
motivated either by rivalry and intolerance of larger males or by
sexual drive. The dispersal occurs in late summer when there is no
breeding activity, and when food is present in greatest abundance and
variety.
FEEDING HABITS
The feeding habits of the opossum in Douglas County, northeastern
Kansas, have been discussed by Sandidge (1953). His data were obtained
from stomach analysis of specimens caught in steel traps. In the
present study no stomachs were available for analysis as the opossums
on the Reservation were not sacrificed for this purpose and effort was
made to avoid mortality in those that were live-trapped. Information
concerning their feeding habits was obtained mainly by examination of
scats in the field. On this 590-acre tract maintained as a Natural
Area with human disturbance kept to a minimum, the available food
sources differed somewhat from those of other woodland areas and
especially from those of cultivated or suburban areas as reported upon
by Sandidge.
The feces or "scats" of the opossum are not liable to be confused with
those of other mammals except possibly with those of the striped skunk
or raccoon, both relatively uncommon on the Reservation. Favorite
sites for deposition of opossum scats were at the bases of large
trees, usually honey locusts or elms, near the animal's den.
Accumulations of several dozen scats may collect in such situations.
Often the opossums live-trapped were found to have deposited scats and
many of these were saved for examination, although they were usually
trampled, broken and mixed with earth and hair. Few scats were seen in
the field throughout the summer. Their disintegration is rapid at that
time of year because of the high temperature, frequent heavy rains,
and abundance of dung-feeding insects. Scats were seen in greatest
abundance in the fall, partly because the opossum population was then
at its annual high point. During fall, wild fruits made up the greater
part of the diet and were represented in almost every scat that was
seen. Wild grape (_Vitis vulpina_) is an abundant woodland vine on the
area and often forms dense tangles both in deep woods and in edge
situations. Grape was the most abundant single item, and a large
number of scats consisted exclusively of grape seeds and skins. In
November and December opossums could be trapped most effectively by
making sets in or near grapevine tangles where the animals were
attracted by the abundant ripe fruits. The crops of wild grapes were
especially heavy in 1948 (before live-trapping was begun) and in 1949,
and scats containing them were noticed in those years especially.
Opossums, too, were more numerous on the Reservation in 1948 and 1949
than they were in 1950, 1951, and 1952.
Hackberry fruit (_Celtis occidentalis_) was second to grape in
importance and large numbers of scats were found to be composed mainly
or entirely of the skins and seeds of this fruit. In the fall of 1951,
these fruits were especially important and were the principal food
source.
Wild plum (_Prunus americanus_) and wild crabapple (_Pyrus ioensis_)
also are important in fall and winter and are present in many scats.
In summer, blackberry, abundant on some parts of the Reservation, is
an important food. Other wild fruits noticed in scats include those of
cherry (_Prunus virginiana_) and climbing bittersweet (_Celastrus
scandens_), and mast (acorn ?). In the fall of 1948, corn made up a
large part of the contents of scats noticed. Crops of corn were grown
on two fields of the Reservation in that year. In following years,
corn was noticed less frequently in scats but still continued to be
one of the important food items. Several cornfields adjoined the
Reservation, and the scats containing the grain were observed mainly
along the borders of these fields.
The crayfish is evidently the most important animal food, at least
during the cooler half of the year when scats are seen in greatest
numbers. Remains of crayfish were far more conspicuous than those of
other invertebrates, and often made up the greater part of the scat.
The sample of scats examined in the field, as noted below, are thought
to be representative of the much larger number noticed but not
examined in detail.
August 19, 1951, 16 scats. Food items in their approximate
order of importance were: blackberry in six (100% in 5, 95%
in 1); grape in five (100% in 2, 97% in 1, 95% in 1, 50% in 1);
crayfish in three (100% in 1, 60% in 1, 40% in 1); wild plum
in two (85% in 1, 5% in 1); wild crabapple in two (100% in
both); insects in three (scarabaeid beetle 10% in 1, cicada 2%
in 1, unidentified insect fragments in 5); fox squirrel in one
(15%); unidentified plant fibers in one (40%).
September, 1951, 16 scats. Grape in seven (all or most of 5
scats and small percentages of 2 others); cherry in seven
(all or most of 5 scats and small percentages of 2 others);
crayfish in seven (all or most of 5 and small percentages of
2 others); rabbit in two, making up most of both; insects
(grasshopper, and large black beetle) in two making up small
percentages.
October, 1951, 8 scats. Hackberry in three, making up nearly
all of them; grape in two (all of 1 and most of the other);
wild plum in one (100%); mast (acorn?) in one, making up 100%;
crayfish in one making up about half; fox squirrel in one
making up the remainder of the scat containing crayfish;
rabbit in one making up a small percentage.
November, 1951, 12 scats. Hackberry in five, making up all or
most of four and a small part of the fifth; grape in five,
making up all or most of four and a small part of the fifth;
wild crabapple in three, making up all of two and most of the
third; and cottontail in one, making up all of it.
January, 1952, 3 scats. Hackberry in all, making up all of two
and most of the third; copperhead (scales of medium-sized
adult) making up a fraction of the third scat. Pile of more
than a dozen scats not individually separable, nearly all
consisted mainly or entirely of hackberry fruits estimated at
2000; other contents chiefly crabapple and corn.
September, 1952, 8 scats. Grape in all, making up all of six
and 90% of the seventh, and about 20% of the eighth; wild plum
seeds in one making up 40%; blue feathers, evidently of a jay,
in one, making up a trace; carabid beetles in one making up a
trace.
October, 1952, about 14 scats, two separate (both consisting
exclusively of grape) and the remainder mixed in two
approximately equal piles, one pile consisting of grape,
except for small quantity of fine fur; second pile consisting
mainly of grape (about 90%) with small percentages of
yellowjackets (_Vespula_, about 6 individuals, all in one
scat), toe bones and fur of cottontail rabbit; a few scales of
immature copperhead; and a snail.
November, 1952, 2 scats. Grape in both, making up all of one
and about 90% of the other.
Sandidge (_loc. cit._) found remains of cottontail rabbit in some of
the stomachs he examined, but followed Reynolds (1945) in regarding
these as carrion since the opossum was considered to be too
inefficient a predator to catch and kill cottontails--prey
approximating its own size and much superior in speed. Adult
cottontails seem to be secure from opossum predation under ordinary
circumstances. However, the opossum obtains some of its food by
raiding the nests of small animals, including those of rabbits. At the
Reservation, on May 21, 1951, at 9:00 P. M., distressed squealing of a
rabbit was heard in high brome grass. Investigation revealed that a
large male opossum had killed a young cottontail, weighing
approximately 150 grams, and had started to eat it. This young rabbit,
about the minimum size of young wandering outside the nest, evidently
was pounced upon as it hid beneath the high grass.
Live-traps for mice, in lines or grids of 100 or more, often were set
on the Reservation, and predators, including opossums, disturbed them
on many occasions. Attacks sometimes resulted in release and escape of
the trapped animal, and in other instances resulted in its being
caught and eaten. In many instances identity of the predator could not
be determined, but it is believed that such attacks by the opossum
were relatively infrequent and inefficient. Steel traps set beside the
mouse traps after consistent raids, to catch or discourage the
predator, caught opossums on several occasions. These opossums usually
had overturned mouse traps without opening them and when the trapped
mouse was missing from the trap no evidence of its having been eaten
was obtained. On other occasions raccoons were caught in the steel
traps, and their raids were characterized by systematic and dextrous
opening of the mouse traps and, frequently, by predation on the small
mammals inside them.
Wire funnel traps set for reptiles along rock ledges also were often
disturbed by predators, mainly skunks and opossums, both of which were
caught on several occasions, when steel traps were used as a
protective measure. The opossums often were attracted to the funnel
traps by large insects such as camel crickets, grasshoppers and
beetles, but also by trapped lizards including the skinks (_Eumeces
fasciatus_ and _E. obsoletus_) and the racerunner (_Cnemidophorus
sexlineatus_). Both Sandidge (1953) and Reynolds (1945) recorded the
five-lined skink (_E. fasciatus_) in opossum stomachs. On the
Reservation this common lizard probably is one of the most frequent
items of vertebrate prey of the opossum. Flat rocks a few inches in
diameter frequently have been found flipped over; larger flat rocks
and those solidly anchored in the ground often have been found partly
undermined by opossums scratching away the loose dirt at their edges.
Flat rocks similar to those found disturbed by opossums are the
favorite resting places of the skinks, which, in cold or wet weather,
are sluggish when beneath such shelters; this is especially true of
female skinks that are nesting. The shape and size of some of the
excavations suggested predation on skink nests. Other possible food
sources in the same situation, in loose soil beneath flat rocks,
include narrow-mouthed toads, lycosid spiders, beetles (mainly
carabids such as _Pasimachus_ and _Brachinus_) and occasionally,
snails, centipedes and millipedes.
A pond, a little more than an acre in size, was a focal area for
opossums and more were caught there than on any other part of the
Reservation. Opossums that were trapped and marked on other parts of
the Reservation were likely to be caught here sooner or later. Tracks
in the mud showed that the edge was patrolled almost nightly by one or
more opossums and this activity was especially noticeable when the
pond was drying. Frogs were obviously the chief attraction inducing
the opossums to forage there. Of the 8 kinds of frogs and toads
breeding at the pond, the bullfrog (_Rana catesbeiana_), leopard frog
(_Rana pipiens_) and cricket frog (_Acris gryllus_) were most
abundant, throughout the season and especially when drying occurred.
All three probably are important foods of the opossum locally.
WEIGHTS
Opossums were weighed in the field, with small spring scales of
2000-gram capacity, graduated in 25-gram intervals. Weights recorded
were accurate within a margin of about 10 grams. After other data were
recorded, the opossum was offered the hook at the base of the scale,
and usually bit and held fast. Then it could be suspended off the
ground and a reading taken.
When the same opossum was trapped two or more times within a few days,
weight was usually found to fluctuate sometimes more than 200 grams,
or more than 10 per cent of the animal's body weight. Opossums
recaptured soon after their original capture and toe-clipping were
generally found to have lost weight, reflecting the deleterious effect
of marking by this method. The temporary laming of the animals
prevented them from traveling as far or as fast as they normally would
have; consequently they probably obtained correspondingly less food.
They were also handicapped in digging, grasping and climbing. Nineteen
such animals taken within a month of the original capture and marking,
averaged 94 per cent of their original weights. The minimum was 82 per
cent. Only 2 of the 19 had gained.
The stumps of amputated toes did not heal rapidly in
opossums--contrary to experiences with many other kinds of mammals,
reptiles, and amphibians also marked by toe-clipping. For many weeks
the toes remained unhealed, sore and swollen. In several instances
after periods of months the clipped toe stumps were unhealed. This was
observed even in some of the opossums that were marked as pouch young
and recaptured when grown to nearly adult size.
Some adult opossums trapped were heavier than the 2000-gram capacity
of the spring scale usually used in the field, and no definite weights
were recorded for most of these animals. Some of them that were caught
near the laboratory were brought there for weighing.
Even within the same age- and sex-group at any one time, opossums
varied widely in general condition and in weight. Some were emaciated
and sickly in appearance with sparse, ragged pelage, while others were
in excellent condition, fat and with thick, glossy pelage. Seasonal
trends are partly obscured by these differences in individuals, by the
tendency to lose weight in those recently marked, and by the irregular
fluctuations that occur in each animal.
[Illustration: FIGURE 5. Weight changes in opossums
live-trapped; lines connect successive weight records of the
same individual, showing, in most, a downward trend throughout
the winter and early spring, and an upward trend in late
spring.]
The few opossums caught in summer were thin and appeared to be
suffering from infestations of ectoparasites, especially chiggers
(_Eutrombicula alfreddugesi_) and ticks (_Dermacentor variabilis_).
Those trapped in October and November were mostly fat and in good
condition. For individuals caught at different seasons, maximum
weights were generally recorded in these two months. The maximum
weight record of the study was one of an adult male weighing 5000
grams on December 23, 1950. The weight records of this individual were
more complete than most and are recorded below to illustrate seasonal
trends for adults. May 10, 1950, 1925 grams; May 14, 1830 grams; May
17, 1940 grams; November 5, 4540 grams; November 28, 4540 grams;
December 23, 5000 grams; February 18, 1951, 3300 grams; March 6, 3080
grams; March 28, 3080 grams; May 28, 3080 grams; June 18, 2620 grams.
Of opossums that were trapped alive, the weight ranged from the
maximum of 5000 grams to a minimum of 126 grams. The maximum in males
was higher than in females. In fall, three rather poorly defined
age-size groups were discernible in each sex: adults more than a year
old and including all the largest individuals; large young born late
the preceding winter and approaching small adult size; smaller young
born in early summer and still less than half-grown. After November,
young cease to gain, or gain slowly and irregularly through the winter
and spring and adults tend to decline in weight, as food becomes
scarce and frequent fasting is enforced by cold or stormy weather. The
smaller young probably are subject to drastic reduction in numbers as
a result, directly or indirectly, of severe winter weather. Many of
these smaller young, weighing considerably less than 1000 grams, did
not survive overnight when caught in live-traps in cool autumn
weather, whereas adults and well-grown young generally survived
exposure even for several successive nights in various extremes of
weather conditions.
BREEDING SEASON
Hartman (1928:154) stated that there were at least two litters of
young per year in the southern states with a small percentage of
unusually fecund females producing a third litter. Lay, in eastern
Texas, concluded (1942:155) that "The present investigation
substantiates Hartman's deduction of two litters being normal, but
fails to disclose any evidence of a third litter." He found females
carrying young in the pouch only within the seven-months period
January to July with definite peaks in February and June, and stated
that second litters appear in the pouch from early April to as late as
May 20 to 23. Reynolds (1945:362) found that the breeding season in
central Missouri in 1941 and 1942 began about the first of February,
with known or calculated birth dates of 42 litters rather evenly
distributed throughout the periods February 12 to April 2, and May 16
to June 4. Eight of these females had given birth to young between
March 16 and April 2, approximately six to nine weeks after the
beginning of the breeding season. Reynolds assumed that these were
individuals that had failed to find mates during the first oestrus of
the season and that after completing the regular dioestrus of about 28
days they had then mated and borne young. Wiseman and Hendrickson
(1950:333) in southeastern Iowa recorded a female with a litter no
more than two days old on February 23, and several other females with
young were estimated to have borne litters at approximately this same
date, while still others bore litters as late as early March. Two
lots of small young found in early June may have been second litters.
For the region represented by the present study, the data indicate a
breeding season with later onset and sharply circumscribed limits as
compared with an earlier onset and less circumscribed limits in Texas,
central Missouri, and even southeastern Iowa, which is a little
farther north. The available data indicate that there are two distinct
and well-defined breeding seasons in the course of the annual cycle on
the University of Kansas Natural History area. The whole population,
including young of the preceding year, some still far below average
adult size, breeds from about the middle of February into early March,
and first litters are born mainly in early March. Individual females
may vary as much as two to three weeks in the time of breeding, and
varying weather conditions from year to year may hasten or delay onset
of the breeding season. Data are recorded below for all females caught
in March that were carrying litters.
Weight of
Date female Number of
in grams young Development of young
March 1, 1952 2000 9 Newborn
March 2, 1952 1450 6 Newborn
March 2, 1952 1230 7 Newborn
March 5, 1950 1200 10 About 16 mm. snout to vent
March 5, 1950 1300 1 About 14 mm. snout to vent
March 6, 1951 1110 4 Newborn
March 18, 1952 1930 8 Not present when female
was trapped on March 1
March 18, 1952 1520 6
March 18, 1952 1230 12 About 40 mm. snout to vent
March 19, 1951 1000 8 Estimated 1 week old
March 22, 1950 1040 9 About 34 mm. snout to vent
March 24, 1950 1280 10 74 mm. snout to vent
March 24, 1950 1480 8
March 27, 1950 965 8 Total length 26 mm.,
weight .8 g.
March 28, 1951 820 7 20 mm. crown to rump; born
since previous capture of
female on March 7
March 30, 1950 1325 9 Total length 33 mm.
March 31, 1952 1930 8
March 31, 1952 1630 5 Total length 73 mm.
None of the females trapped in February was carrying young in the
pouch, but probably some early litters are born in the last week of
February or even earlier. By late March most of the females are
carrying young in their pouches, and those which do not have young,
have their pouches enlarged and vascularized for accommodation of the
young. Presumably such females have already borne young and then lost
them. Nearly all the litters seen in the latter half of March had
young that were much larger than at birth.
Of 13 females examined in April, 12 were carrying young, and the
remaining one was known to have been carrying a single young on March
1, but had lost it. Eleven females were examined in May, four of which
were the same ones examined in April. Eight of the eleven females were
carrying young; of the remaining three, one had lost the litter of
young that it had been carrying when trapped in April. Two had empty
pouches on May 19 and 20, but probably had successfully reared the
litters of young which they had been carrying when trapped in April.
The young of all those females trapped on different dates in April and
May were in stages of growth indicative of birth about the first week
in March. The latest date on which a female was recorded with
first-litter young in the pouch was May 22, 1951, and these were the
largest pouch young observed. Their eyes were recently opened, they
were estimated to weigh 60 grams each with hind feet 20 mm. long.
Young continue to grow rapidly after leaving the female's pouch. A
young female caught on June 16, 1949, weighed 126 grams. For seven
young caught on July 5 and 6, 1952, weights and hind-foot measurements
were, for males: 660 grams, 52 mm.; 560 grams, 46 mm.; 550 grams,
48 mm.; 450 grams, 44 mm.; 370 grams, 44 mm.; 330 grams, 37 mm.; and
for the one female: 430 grams, 46 mm.
The wide variation in size in this small group of young of nearly the
same age is noteworthy. Size and condition of the females carrying
them, number of competing litter mates, and early success or handicap
in independent life causes so much divergence in size that at the age
of four months some young are twice as large as others.
By late fall the young grow to small-adult size. For example, the
female that weighed 126 grams when first caught on June 16, 1949, was
recaptured on November 29, 1949, and on that date weighed 1710 grams.
A second breeding season ensues soon after the young of the first
litter leave the pouch, and these young probably soon learn to shift
for themselves. Second litters are usually born in early June. On June
14, 1952, a female was taken with young only a few days old in her
pouch. On July 5, 1952, two females last taken on May 19 and May 20,
with their pouches recently vacated by first litters, were found to
have young the size of half-grown mice, evidently two to three weeks
old. In the months of October, November, December and January, a total
of 11 young, thought to represent second litters, were taken. Dates
of capture, weights in grams and sexes were as follows:
Oct. 3, 1950 400 grams male
Oct. 6, 1950 510 grams female
Oct. 8, 1950 260 grams female
Oct. 8, 1950 350 grams female
Oct. 18, 1950 350 grams[A] female
Dec. 5, 1951 630 grams female
Dec. 30, 1950 710 grams female
Jan. 1, 1951 660 grams female
Jan. 1, 1950 700 grams[A] male
Jan. 9, 1950 550 grams male
Jan. 11, 1950 550 grams male
[A] estimated
The hind foot measured 48 mm. and 51 mm., respectively, in the young
weighing 630 grams and 660 grams. These young, born in early summer
have grown, by October, to a size comparable with that attained in
July by young of the early spring litters. The variation in size is
also similar but with a little wider range. The summer breeding season
may be somewhat more protracted than the breeding season in early
spring.
Too few females were caught in summer to compare the summer breeding
season with the early spring breeding season, with respect to size of
litters, percentage of non-breeders, and other factors which might
affect the size of the crop of young produced. It is not clear why,
among opossums trapped in winter, the young born in early spring
outnumber those born in early summer by about four to one. Some
females are eliminated after rearing the first litter, and others,
exhausted by rearing large first litters may fail to participate in
the second breeding season. However, it seems that the young of the
summer litters must be subject to other unusual and selective
mortality factors which eliminate most of them by fall. That such
factors vary from year to year is indicated by the changing ratio of
summer-born young to other opossums in each of the three winter
seasons when trapping was carried on.
NUMBERS OF YOUNG
Hartman (1952) has summarized his own findings and those of other
authors regarding the embryology, birth, and early development of the
opossum, and has corrected numerous popular misconceptions. He states
that an average litter consists of about 21 eggs, but mentions much
larger litters of up to as many as 56. However, many of these may fail
to develop. The female normally has 13 functional nipples in her pouch
and each one accommodates a single young. Excess young beyond this
number are doomed, and soon perish from starvation if they reach the
pouch after all the nipples are occupied. None of the females examined
in the present study had a full complement of 13 young. Under
unfavorable conditions, most or all of the young may fail to make the
trip from the vaginal orifice to the pouch. Also, the pouch young are
subject to heavy mortality, but observations concerning the time and
cause of mortality are lacking.
Lay (_loc. cit._) found an average of 6.8 pouch young in 65 litters
examined in eastern Texas; Reynolds found an average of 8.9 (5 to 13)
in 42 litters from Boone County, central Missouri; Wiseman and
Hendrickson found an average of 9 (6 to 12) in southeastern Iowa. In
the present study, 28 of the female opossums examined were carrying
litters in their pouches, and all these females were caught in the
months of March, April, May, June and July. The number of young varied
from one to 12. Seven females each had seven young, six each had
eight, three had six, three had five, and there were two each with
nine, 10, and 12 young, and one each with one, four and 11 young. The
average was 7.4 per litter. On several occasions females captured with
young in their pouches and recaptured one or more times within a few
weeks, were found to have lost some or all of the young. Some of the
females examined probably had already lost parts of their litters. For
instance, the female recorded with just one small young on March 1,
probably had lost most of her litter and when recaptured a month later
she did not have any young.
Nineteen yearling opossums were taken in the fall-winter-spring season
of 1951-52; 42 per cent of the total, and 67 per cent of the females
were individuals marked as pouch young the preceding spring. In the
course of live-trapping, that spring, some first litters may have been
missed. No second litters were marked because trapping was not
continued into June and July when second litters are being carried by
females. These figures suggest that the breeding population of females
on an area consists chiefly of those born there the preceding spring.
COMPOSITION OF THE POPULATION
Sex ratio of opossums trapped was approximately 1:1; 59 males to 58
females. Age groups for opossums caught in the three seasons are shown
in the following tabular fashion. For a few individuals age status was
doubtful.
1949-1950 1950-1951 1951-1952 Total
Old adults 11(25%) 9(26.4%) 11(39.2%) 31(29.2%)
Yearlings:
Born in late winter 29(66%) 18(53.0%) 13(46.5%) 60(56.6%)
Born in late spring 4(9.1%) 7(20.6%) 4(14.3%) 15(14.2%)
Total 44 34 28 106
In the 1950-51 season, small young of the summer brood seemed
unusually numerous. In the 1951-52 period, young of both age classes
were relatively scarce and old adults made up an unusually high
proportion of the population. Excluding the 14 marked pouch young that
were later recaptured, there were only four of the total of 106 that
were trapped in each of two seasons. One young less than a quarter
grown, that was accidentally caught in a live-trap set for woodrats,
was recaptured as a breeding adult the following winter. An adult male
and two adult females each caught in the 1949-50 season were each
recaptured repeatedly in the 1950-51 season. Ninety-five per cent
replacement of the breeding population by the following breeding
season is indicated by our figures. Only 3 (or 5 per cent) of the
individuals of the population trapped and marked in the season of
1949-50, were recaptured among the 62 opossums recorded in the two
subsequent seasons. Various mortality factors including predation,
disease, and accidents account for some 70 per cent. These are
replaced by first-year young which make up the greater part of the
breeding population. The remaining 25 per cent presumably shift their
ranges sufficiently in the course of a year to have moved beyond the
limits of an area of the size encompassed by the present study.
POPULATION DENSITY
No precise measurement of the population density on the study area was
obtained. It was not practical to capture every individual present
there, and rapid population turnover, due to mortality and wandering,
obscured the trends. The information obtained concerning movements of
opossums suggest that one may habitually forage as much as 900 feet
from its home base. Assuming that 900 feet is the typical cruising
radius, the areas drawn upon by the trap lines in the three different
seasons were approximately as follows: 1949-50--400 acres;
1950-51--350 acres; 1951-52--220 acres. In these same three seasons
the numbers of opossums caught were, respectively, 46, 37, and 30. If
these figures represent the numbers actually present, densities of one
to 8.7 acres, one to 9.5 acres, and one to 7.3 acres are indicated.
However, some opossums using the area probably were missed; and on the
other hand, not all those caught in the course of a season were
present there simultaneously. Many of those present early in the
season would have moved away a few months later, and others would have
moved in, replacing them. The number present at any one time could
scarcely have been more than half the number caught in the entire
season.
CENSUS WITH HALF-MONTHLY SAMPLING PERIODS
Number of Number of Number of Computed
individuals individuals recaptures population
Sampling period taken taken in in for
in following following sampling
period period period period
Early November 1949 3 7 1 21
Late November 1949 7 8 3 18.7
Early December 1949 8 11 3 29.3
Late December 1949 11 7 4 19.2
Early January 1950 7 3 1 21
Early March 1950 5 8 2 20
Late March 1950 8 6 3 16
Early April 1950 6 3 1 18
Late April 1950 3 6 2 9
Early May 1950 6 3 2 9
Early November 1950 1 3 1 3
Late December 1950 3 6 1 18
Early February 1951 4 13 3 17.3
Late February 1951 13 6 3 26
Early March 1951 6 4 3 8
Late March 1951 4 5 2 10
Early April 1951 5 1 1 5
Late April 1951 1 5 1 5
Early May 1951 5 3 2 7.5
Early February 1952 9 4 2 18
Late February 1952 4 9 1 36
Early March 1952 9 6 2 27
Late March 1952 6 5 2 15
CENSUS WITH MONTHLY SAMPLING PERIODS
Number of Number of Number of Computed
individuals individuals recaptures population
Sampling period taken taken in in for
in following following sampling
period period period period
November 1949 9 16 7 21
December 1949 16 9 3 48
March 1950 11 9 3 33
April 1950 9 7 2 32
October 1950 9 3 3 9
November 1950 3 3 1 9
December 1950 3 7 3 7
January 1951 7 14 3 33
February 1951 14 7 4 25
March 1951 7 5 3 12
April 1951 5 6 3 10
November 1951 3 6 1 18
December 1951 6 5 1 30
January 1952 5 11 3 18
February 1952 11 13 4 36
March 1952 13 9 5 23
April 1952 9 3 1 27
Crude census-figures were obtained by utilizing the Lincoln Index
and computing the total on the basis of the ratio of marked (and
recognizable) individuals to others caught in a sampling period.
A large number of census figures were obtained over the three-year
period of the study. Each separate census, however, was based on an
inadequate sample as the number of marked individuals taken at each
sampling, as recaptures from the previous sampling period, varied from
one to five. While little confidence can be placed in any one census
computation, the trends of figures from series of such computations
reveal the approximate number of opossums on the area if due allowance
is made for certain distorting factors. Presumably the differences in
figures obtained at different samplings result chiefly from the margin
of error in the data, although it is true that there is rapid change
in the actual number of opossums.
The number of active opossums in the region of the study reaches a
peak in late summer and early fall, when second litters of young have
grown large enough to become independent. At this season the
population contains a high proportion of young of the year. During the
ensuing months of fall and winter there is a steady decrease in
numbers, through various mortality factors, with no replacement until
young are born about the first week of March. These young do not
become independent until late May or early June, and during the
intervening months there is a further reduction of the adults and
yearlings, so that the active population reaches its annual low point
in late spring. At that time of year most opossums are in poor
physical condition.
The area represented by the opossums trapped totaled more than 500
acres, but not more than 400 acres were within the area drawn upon by
the trap line at any one time. Usually the area represented at any one
time by the trap line was less--100 to 350 acres, with from 25 to 45
traps. Traps were moved from time to time depending on the
distribution of opossum sign and food sources, the weather, and the
time available for this study. As a result, successive samples are not
strictly comparable and a major source of error is introduced into the
census computations. Lack of exact correspondence in the area
represented by successive samples would result in a disproportionally
small number of recaptures, and an erroneously high census
computation. While adequate adjustment cannot be made, examination of
the data suggests that census figures are too high, by as much as 50
per cent in many instances as a result of this factor, while in some
other instances when there was little or no alteration of a trap line
from one period to another, the census figure was not affected. In the
winter of 1949-50, the area covered was most extensive, from 350 to
400 acres, and the numbers of opossums taken were correspondingly
larger. In the 1950-51 season the area involved was approximately 220
acres, and in the 1951-52 season it was a little less than 200 acres.
In view of the census figures obtained and the probable errors, it
appears that the opossum population in early autumn is about one to
20 acres, and that by late spring it is reduced to not much more than
half that number.
MORTALITY FACTORS
Many of the opossums trapped were suffering from injury, disease, or
parasite infestation, and some were in critical conditions. A large
adult male trapped on April 2, 1952, seemed to be dying from disease.
It was much emaciated and the pelage was sparse and ragged, as if the
animal had been sick for a long time. The skin had numerous
light-colored pustules 1 to 2 mm. in diameter, and these were
especially prominent on the ears, lips, and penis. When released, the
opossum was too weak to move away. It was excited by movements of the
trapper, and stood erect with violent involuntary rocking movements.
After a few seconds it gradually slumped to the ground and subsided
into quiescence. On the next day no trace of it could be found.
Most of the opossums caught in summer and early fall had eye
infections, and all of them were infested with ticks (_Dermacentor
variabilis_). Sometimes ticks were attached in dense clusters of
several dozen on the animal's ears and scattered over other parts of
the body.
In March and April, 1950, seven adult opossums were found dead in the
traps. None of these showed any evidence of disease or injury and they
were normal in appearance except that they were thin. It was concluded
that death had resulted from exposure and starvation in the traps in
these animals already in critical condition as a result of winter food
scarcity and frequent fasting. Up to this time the procedure had been
to check the trap line only on alternate days and no mortality had
resulted, even in the coldest part of the winter. The implication is
that by spring, opossums are in a condition so critical that they are
unable to withstand exposure or fasting and die whenever weather
conditions are unusually severe.
After these losses in the spring of 1950, trap lines were checked
daily. However, in October, 1950, further mortality in traps resulted
in the loss of three or more opossums. All three of these were
rat-sized young of second litters. These young lacked the abundant
supply of fat characteristic of larger opossums in fall, and seemingly
were unable to withstand exposure to chilly nights. Such
susceptibility to cold might result in heavy mortality in retarded
second-litter young when cold weather of autumn is unseasonably early
or is unusually severe.
Natural enemies of the opossum on the area include the red-tailed
hawk, horned owl and coyote. Because of the opossum's nocturnal habits
it is rarely exposed to hawk predation. Food habits of the coyote on
the area have not yet been investigated. Numerous instances of horned
owl predation on opossums have been recorded in the literature. On
January 15, 1950, an owl attacked an opossum caught in a live-trap.
The trap was found overturned, and a few feet away were entrails and a
quantity of opossum hair where the animal was eaten. Low vegetation in
the vicinity had many fine down feathers of the owl clinging to it. On
December 24, 1950, the carcass of a small adult opossum was found in a
pasture near the edge of the woods. The head and tail were intact, but
otherwise little more remained than the spinal column, girdles and
larger limb bones. White excreta of a large bird beside the carcass
indicated predation by a raptor, probably a horned owl.
SUMMARY
On a natural area, the University of Kansas Natural History
Reservation, in Douglas County, northeastern Kansas, the population
of opossums was studied, chiefly by live-trapping, in the
fall-winter-spring seasons of 1949-50, 1950-51 and 1951-52. The study
area provided a varied habitat of elm-oak-hickory woodland,
pastureland, and fallow fields. Opossums use all parts of it, but
concentrate their activities in the woodland.
Opossums being mainly nocturnal were rarely seen in the daytime,
except when caught in traps. Reactions to humans varied; some were
indifferent, some feigned death, others merely tried to escape, and
some defended themselves vigorously, snarling and snapping.
No evidence of territorial behavior was found in the opossum. Many
individuals of both sexes and various sizes, occurred together on the
same area. Successive captures of individuals revealed the usual
extent of home ranges, which averaged approximately 50 acres, and
tended to a circular or broadly oval shape. No significant difference
in size of home ranges between males and females, or between adults
and well-grown young, was found. Of 115 young marked by toe-clipping
while still in the females' pouches, 15 were recaptured after periods
of months. All but two of these recaptured young were females which
had settled down within a few hundred feet of the locations where they
were born. The young males seem to wander much more extensively than
do the females.
Feeding habits were investigated by field examination of scats found
mainly in fall and winter. These consisted mainly of wild fruits,
especially grape, blackberry, wild crabapple, wild plum, and
hackberry. Crayfish was the most important animal food. No comparable
data for spring or summer were obtained because scats deteriorate
rapidly in warm weather and were seldom found then. Clues as to the
summer food were gained from sign. On many occasions opossums
disturbed live-traps set for small animals, to obtain the voles, mice,
skinks, or insects caught in them. Evidence of opossum activity such
as digging and scratching was frequently noticed at the edges of rocks
and in crevices, where such prey as skinks, narrow-mouthed toads,
beetles, spiders and centipedes seek shelter. One opossum was observed
to catch and kill a young cottontail.
The opossums trapped ranged in weight from 126 grams to 5000 grams but
most weighed between 1000 and 2000 grams. After being trapped and
marked by toe-clipping, animals usually lost weight, up to as much as
18 per cent of the original weight. Food scarcity and enforced fasting
in cold weather caused a weight loss from November until the arrival
of warm spring weather. By late April and May some opossums were
emaciated and in critical condition.
The entire population of opossums, including the majority less than a
year old, breeds in February, and litters are born mainly in the first
half of March. The young develop rapidly in the female's pouch, and
become independent in late May, and there is a second breeding season
with young born mainly in the first half of June. By the onset of cool
fall weather, young born in early spring have grown so that most are
as large as small adults. The young born in early summer are still
less than half-grown. The young of the second litter are less
successful than those of the first litter and make up only a small
part of the breeding population the following year. In 28 litters of
young the average was 7.4, but probably some of these litters had
already sustained losses.
In each of three different winters, the largest age group in the
population of opossums was that of the newly matured young born in
early spring. The old adults were the next most numerous group, and
the second-litter young born in early summer were the least numerous.
The figures obtained from live-trapping indicate an annual population
turnover of approximately 95 per cent, with some 70 per cent
eliminated by various mortality factors and replaced by young, the
remaining 25 per cent shifting to new areas, with compensatory shifts
of individuals replacing them.
The various mortality factors which regulate the numbers of opossums
are not well known, and even less is known regarding the relative
importance of the factors. Food supply and weather are obviously of
major importance and closely interrelated in their effect on the
population. One large adult opossum that was trapped seemed to be
dying from disease and was scarcely able to stand; but others caught
near-by before and after were unaffected. The horned owl is perhaps
the most important natural enemy of the opossum on the Reservation,
and instances of owl predation on opossums were noted.
LITERATURE CITED
FITCH, H. S.
1950. A new style live-trap for small mammals. Jour. Mamm., 31:364-365.
1952. The University of Kansas Natural History Reservation. Univ.
Kansas Mus. Nat. Hist., Misc. Publ., 4:1-38, 4 pls.
HALL, E. R., and KELSON, K. R.
1952. Comments on the taxonomy and geographic distribution of some
North American marsupials, insectivores and carnivores. Univ.
Kansas Publ., Mus. Nat. Hist., 5:319-341.
HARTMAN, C. G.
1923. Breeding habits, development and birth of the opossum.
Smithsonian Report 1921:347-363.
1928. The breeding season of the opossum (_Didelphis virginiana_)
and the rate of intrauterine and postnatal development.
Jour. Morph. and Physiol., 46:143-215.
1952. Possums. Univ. of Texas Press, Austin. xvi + 174 pp.
LAY, D. W.
1942. Ecology of the opossum in eastern Texas. Jour. Mamm., 23:147-159.
LEONARD, A. B., and GOBLE, R. C.
1952. Mollusca of the University of Kansas Natural History Reservation.
Univ. Kansas Sci. Bull., 34:1013-1055.
REYNOLDS, H. C.
1945. Some aspects of the life history and ecology of the opossum in
central Missouri. Jour. Mamm., 26:361-379.
SANDIDGE, L. L.
1953. Food and dens of the opossum (_Didelphis virginiana_) in
northeastern Kansas. Trans. Kansas Acad. Sci., 59:97-106.
WISEMAN, G. L., and HENDRICKSON, G. O.
1950. Notes on the life history and ecology of the opossum in
southeast Iowa. Jour. Mamm., 31:331-337.
_Transmitted May 4, 1953._
* * * * *
Transcriber's Notes
Other than two possible typographical errors listed below, the title
and verso (second) page specifies the pages are 305-338; but the first
numbered page (the third one) is numbered "309". The content provider
examined the text at page breaks and looked for evidence of a missing
leaf; but found none. So, this appears to be a printer's error in the
pagination as the numbering sequence otherwise follows the normal format
for these scientific texts. Therefore, the numbering was changed in the
descriptions to read "... pp. 307-338, ..."
Page Correction
==== ===========================================================
316 Occasionaly => Occasionally
338 Possible typo: Didelphis Virginiana => Didelphis virginiana
Emphasis Notation
_Text_ - Italics
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