[Pg 307]

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

[Pg 308]

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

[Pg 309]

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, racoon 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
[Pg 310]
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.

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
[Pg 311]
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.

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
[Pg 312]
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,
[Pg 313]
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.

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.

[Pg 314]

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.

[Pg 315]

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.

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
[Pg 316]
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
[Pg 317]
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
[Pg 318]
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.

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
[Pg 319]
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
[Pg 320]
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.

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.

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
[Pg 321]
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.

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.
[Pg 322]
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.

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
[Pg 324]
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.
[Pg 325]
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.

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
[Pg 326]
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.

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.

[Pg 327]
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.

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
[Pg 328]
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.

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.

[Pg 329]
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,
[Pg 330]
were taken. Dates of capture, weights in grams and sexes were as
follows:

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.

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,
[Pg 331]
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.

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.

[Pg 332]
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.

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.

[Pg 333]

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
[Pg 334]
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
[Pg 335]
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.

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
[Pg 336]
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.

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
[Pg 337]
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
[Pg 338]
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.

Transmitted May 4, 1953.