October 8, 1950. On November 9 he had moved 220 feet, from
the lower outcrop to the upper, and he was recaptured at or
near this same site on November 10, 28 and 29, and on
January 11 and February 9, 1951. On November 21, 1951, grown
to maximum adult size, he was caught at a new location 1080
feet from the original.
(10.) This male was caught as a subadult twice at the same
place on November 30 and December 14. By the following
autumn he had shifted to a new location 180 feet south along
the outcrop, and he was caught there on September 22 and
October 18, 1951, and on January 20 and February 2, 1952.
_Females_
(11.) This female was captured 27 times over a span of 211
days. She moved back and forth considerably between two
houses 40 feet apart but made only one substantial movement
of 245 feet; at this time she was in breeding condition.
Nearly seven months after the first capture she was seen for
the last time only 16 feet from the original site of
capture. It was assumed she fell prey to spotted skunks
which were raiding traps.
(12.) First captured on March 24, 1951, she remained on the
area 105 days in which period she was live-trapped 25 times.
Sixty per cent of the total captures were at the same house
and the longest movement recorded was only 56 feet. She was
last caught in a trap 25 feet from the site of original
capture.
(13.) This young adult remained at her house at the rock
fence approximately four months. In this period she was
captured 11 times. On March 16, 1952, she had moved 410 feet
to a house at the eastern section of outcrop, probably
searching for a male. She was never seen again.
(14.) This subadult female moved from the site of original
capture to a house 253 feet away on the same outcrop. She
was probably in search of a new home when caught the first
time. She was recorded at another house 40 feet away on one
occasion.
(15.) Over a span of 90 days and 15 captures this female was
not recorded as making any movement. She was living in one
of the woodland houses. Mature males were numerous in the
area and she was visited by at least two.
(16.) This female was also living in the woodland section
and was first caught on March 30, 1952, in one of the less
favorable houses. She was trapped 17 times over a period of
85 days. One movement of 68 feet to a new home site was
recorded, but the area of foraging probably did not change.
She was caught here four times and then disappeared.
(17.) This female was first trapped as a subadult on October
5, 1948, at a house in brush on the upper part of a north
slope. On November 24 she had shifted 590 feet to the bottom
of the slope and was living in the recess beneath an
undermined honey locust on a gully bank. On November 25 she
was caught in a similar situation 100 feet farther east
along the gully bank. She was recaptured at the gully on
November 26 and 30, December 1, 3, 22, and March 8 and 9,
and in all she shifted six times between the two gully-bank
dens.
(18.) This female was first trapped as an adult on November
18, 1948, in a gully-bank den. She was recaptured at this
same place a year later, on November 18 and 30, 1949. On
February 19, 1950, she was caught at a hollow sycamore 650
feet farther up the gully, and she was recaptured there on
February 25 and April 7, and on June 15, 1951. On August 6,
1951, she was caught at a house in a thicket on the gully
bank, between the first and second locations and 150 feet
from the latter.
(19.) This female was recorded only twice; on October 15,
1948, she was at a hilltop rock outcrop. On July 14, 1950,
she had moved 1480 feet and was living in a rock pile at the
base of the slope, near the same hollow sycamore where
female No. 18 had been caught.
(20.) This female was first caught as an adult on April 5,
1950, at a large boulder of a hillside rock outcrop. On
October 7, 1950, she had shifted 110 feet to a house at an
osage orange tree on the hilltop rock outcrop. On November 9
she was back at the first location and on November 28 she
had moved 70 feet south along the hillside outcrop. On
January 11 and February 9, 1951, she was back at the
original location. On November 9, and 21, 1951, she was
again at the site 70 feet south, and was still there at her
last capture on February 3, 1952.
Ordinarily each house that is in use harbors only a single woodrat. To a
greater degree than any other kind of mammal on this area woodrats show
intraspecific intolerance. On various occasions when captives were
placed in the same or adjacent cages, they focused their attention on
each other with evident hostility, the more powerful or aggressive
individuals attacking or pursuing. Several times the confinement of two
rats in the same live-trap or cage resulted in the death of the weaker
individual, and seemingly this is the normal outcome unless the attacked
rat is able to escape. On various other occasions two or more rats have
been caught in the same trap simultaneously but in every instance these
were either: a pair of adults, the female appearing to be in oestrus; a
lactating female and one or more of her young; or young less than
half-grown, that were obviously litter mates. Older woodrats, especially
males, often have their ears torn and punctured from fighting.
Territoriality involves, primarily, defense of the house itself. An
individual that ventures into an occupied house may be quickly routed by
the occupant even though the latter is smaller. Chasing has been
observed occasionally, but it is doubtful whether any individual is able
consistently to defend the entire area over which it forages. Because
each rat spends most of its time within the shelter of its house, an
intruder might venture onto its home range unchallenged and undetected,
so long as it did not enter the nest cavity.
An adult female was live-trapped on October 14, 1951, beside her house
at the outcrop. As soon as she was released, she disappeared within the
house. After approximately two minutes, a soft, high pitched whine was
heard and immediately another woodrat dashed into view closely followed
by the female. The chase continued for several seconds in the vicinity
of the house, but the woodrat being chased soon left the area _via_ the
outcrop. Probably this intruder had moved into the house in the night
while the female was in the trap.
On June 17, 1952, an adult male was found in a live-trap set at one of
the brush pile houses in the woodland area. This house was occupied by
an adult female. He ran into the house after release, and immediately
there was a loud squeal. He ran outside and paused under some limbs
approximately 15 feet from the house, and remained there for 15 minutes
before clipping off an ironweed 12 inches long, which he carried to the
house. He did not enter the house but stopped beneath overhanging sticks
at the edge, eating leaves from the plant. He made another attempt to
enter the house but loud squeals and rustling followed and he returned
to the ironweed plant and was still eating when observations were
halted. In another instance, squeals and rustling indicated that the
occupant and intruder were in combat.
[Illustration: FIG. 2. Diagram illustrating spacing (due to
territoriality or intolerance of the rats) in twelve woodrat houses in a
hedge row extending south from south boundary of the Reservation at the
middle.]
Although home ranges may overlap to some extent, intraspecific
intolerance tends to maintain a certain minimum interval between houses.
The arrangement of twelve houses along a hedge row 1170 feet long is
diagrammatically represented in Figure 2. The average interval was 78.5
feet (minimum 42; maximum 171). The habitat was uniform. Home ranges
probably overlap somewhat, and the spacing is the expression of the need
for an otherwise unoccupied area in which there is sufficient space to
live. Because individuals tend to fight whenever they meet, there is
probably a psychological tendency for sequestration which results in
spacing of houses and reduces social contact thereby avoiding a
depletion of energy that would be detrimental to the population. Whereas
condition of the hedge row determines whether or not it will be
inhabited by woodrats, length determines the number of occupants. The
spacing of houses in a hedge row must be attributed to something other
than restriction of sites because the number of sites available always
exceeds the number that are in use. Although rock outcrops situated in
areas of uniform habitat have not been observed to the extent that hedge
rows have, a similar spacing seems to exist and the sites available for
houses always exceed the actual number found. This behavior pattern
limits the number of houses and is probably advantageous to the species
through preventing overcrowding and possible critical depletion of the
food supply.
Eleven of the young that weighed 100 grams or less when originally
captured and were presumably still living at the mothers' houses, were
recaptured repeatedly over periods of weeks or months, providing a
limited amount of information regarding dispersal. They followed no
definite pattern. In seven instances (five males and two females) the
young stayed on at the house beyond the age when they were completely
independent of the female. In at least two instances the female was
known to have moved away while the young remained. One female shifted to
a house 58 feet from the one where she had reared her litter of two, and
was accompanied by the young male, while the young female stayed on in
possession of the maternal house. Two months later this young female was
caught at a house 90 feet away, and an adult male was in possession of
her former house. One young male shifted to a house 220 feet from his
original home and remained there several months, but was recaptured once
back at the original location. Another male made a series of moves over
a period of weeks and finally settled in a house 490 feet from his first
home. One male who stayed in the maternal house all summer, and reached
adult size there, later moved several times, and was last recorded 900
feet away. One young female shifted 110 feet. In several instances
juveniles appeared abruptly in houses known to have been unoccupied
previously, and some of these houses were in poor repair. These young
had wandered from their maternal houses, for unknown reasons. On one
occasion a young woodrat was caught in a mouse trap set in a meadow, a
habitat into which adult woodrats would scarcely be expected to
venture.
_Feeding_
Rainey (1956) has listed 31 food plants that are used by the woodrat in
northeastern Kansas. He has emphasized that each rat usually obtains its
food from plants growing in the immediate vicinity of its house, and
that individuals thus differ greatly in their feeding, according to the
local vegetation. Therefore, with a sufficiently large number of
observations, the list of food plants might be greatly expanded, to
include most of the local flora, with the exception of the relatively
few kinds that have developed strongly repellent properties rendering
them unpalatable to herbivores in general.
At the quarry where one or more woodrats usually lived beneath metal
strips, as described previously (under the heading of "Commensals"), the
situation seemed to be especially favorable, despite the fact that the
metal offered no insulation from extremes of heat in summer and cold in
winter. Perhaps the rat had an alternative nest among nearby boulders,
to use when temperature was unendurable beneath the metal.
The rat itself, the stored food, and other details of its home life,
could be observed with a minimum of disturbance by raising one side of
the metal strip momentarily, then carefully lowering it into place. The
following observations made in the summer and autumn of 1948 give some
idea of the range of food plants stored at any one time and the change
as the season progresses.
July 12: Bundles of leaves of carrion-flower (_Smilax
herbacea_); 15 green pods of honey locust (_Gleditsia
triacanthos_) with seeds eaten out; several green fruits of
osage orange (_Maclura pomifera_), and several seeds of
coffee-tree (_Gymnocladus dioica_).
July 24: Bundles of green leaves of osage orange and
carrion-flower; many pods of honey locust.
August 30: Three large clusters of the fruits of pokeberry
(_Phytolacca americana_).
October 20: Many small clusters of grapes (_Vitis vulpina_)
judged to weigh perhaps one pound in all; several old pods
of coffee-tree and a few berries of dogwood (_Cornus
Drummondi_) and of pokeberry; a pile of small acorns of
chinquapin oak (_Quercus prinoides_); dry seed heads of
grass (_Bromus inermis_ and _B. japonicus_).
December 22: Many twigs of bittersweet (_Celastrus
scandens_) with fruits still attached; several seed heads of
sunflower (_Helianthus annuus_); a few acorns of chinquapin
oak; fragments of the fruit of osage orange; cured bundles
of trefoil (_Desmodium glutinosum_), carrion-flower, and
tickle grass (_Panicum capillare_).
Although the eastern woodrat is relatively unspecialized in its feeding
habits, a few species of favored food plants probably make up the
greater part of its diet. In northeastern Kansas, at present, osage
orange probably is by far the most important single species. Despite the
fact that its aromatic leaves and fruits are somewhat repellent to
insects and some other animals, they are well liked by woodrats, and
provide a year-round food supply to those individuals having houses in
or near the trees. Honey locust similarly provides thorny shelter for
house sites, while the foliage, the seeds, and the bark of twigs and
trunks are eaten. In houses that are situated near honey locusts, the
large, heavy seed pods are sometimes stored by the hundreds. Old pods
are often used in substitution for sticks as building material in the
house. Nevertheless, honey locust is used relatively little as compared
with osage orange. Other plants that figure most importantly in the diet
include bittersweet, fox grape, pokeberry and horse nettle (_Solanum
carolinense_).
Rainey (_op. cit._) mentioned that captive woodrats would eat meat, both
cooked and raw, and on one occasion he found remains of a cicada on a
house under circumstances suggesting that this insect had been eaten by
a rat. In the course of trapping for opossums and small carnivores,
woodrats were caught on many occasions by Fitch in traps baited with
animal material exclusively--miscellaneous meat scraps, canned dog-food,
bacon grease, or carcasses of small vertebrates. In fact, such baits
seemed to be even more attractive than the grain, seeds, peanut butter
and raisins that had been used customarily to bait the traps set for
woodrats. However, such meat baits could be used effectively only in
cold weather, because of rapid spoilage and interference by insects at
higher temperatures.
On one occasion an adult pilot black snake found dead on the road, a
recent traffic victim, was brought to the Reservation headquarters for
examination and was left overnight in the garage. On the following
morning the carcass of the snake was found to have been dragged a short
distance and gnawed; a quantity of flesh was eaten at an exposed wound
on the neck. Woodrat tracks were thickly imprinted on the dusty soil
around the snake. The adult male woodrat that lived in the garage had
evidently spent much time moving about the carcass and over it, and
feeding upon it. It seemed remarkable that this individual was not
deterred from feeding on the snake by an instinctive fear of one of its
chief natural enemies.
Although the eastern woodrat's food consists mostly of vegetation, the
strong tendency noted to feed upon flesh when it is available suggests
that these rodents may, occasionally at least, prey upon helpless young
of small vertebrates that are readily available to them. Nestling birds,
either on the ground or in low trees, and young mice in nests that are
accessible, might tempt the rat to indulge in predation.
_Breeding_
Reproductive activity continues to some extent throughout the year
except in late autumn and early winter. Presence of a vaginal orifice
was used as an indication of sexual activity. In most instances the
orifice was not indicative of actual oestrus, as it persisted through
the preceding and following stages of an oestrus cycle. In anoestrus the
orifice is sealed, the genitalia are reduced in size and the skin in the
genital region is white. Immature females, and adults during most of the
winter, are in this quiescent condition. Onset of the breeding season in
late winter is relatively abrupt, and seemingly is a photoperiodic
response. Breeding may begin in late January, and most females are in
breeding condition within the first half of February. In oestrus the
genitalia are enlarged and discolored and the vaginal orifice is
prominent and gaping. By February most females born the previous season
have matured, and breeding involves the entire population, except
possibly for retarded young and individuals suffering from disease,
injury or malnutrition. Rainey (1956) recorded an average of 2.3 young
per litter.
Number of litters normally produced in the course of a season by an
adult female is unknown, but most mature females examined within the
period February to September inclusive were in some stage of the
breeding cycle. It is obvious that the females which are successful in
rearing their litters produce at least two litters annually, and
probably some produce three litters. When entire litters are lost at an
early age, to predation, or other causes, productivity is much
increased, with perhaps only short intervals between pregnancies.
The smallest female having a vaginal orifice weighed 160 grams, but in
most instances somewhat larger size is attained before the onset of
oestrus. Judging from the average growth rate of immature females (Fig.
3), most probably attain sexual maturity at an age of five to six months
unless this age is reached in the winter period of sexual quiescence.
Rainey (_op. cit._) found no clear cut instances of young maturing in
time to breed before their first winter. He concluded, tentatively,
that in most instances sexual maturity is not attained until the spring
of the year following that in which the rat is born. However, the
evidence was inconclusive because few of the young marked survived to
maturity. In late summer and early autumn, the latter third of the
breeding season, newly matured young of the year, born in early spring,
may be the most productive group. Young conceived at the beginning of
the breeding season, and born in early March, would normally reach adult
size and breeding maturity in August. For example, a young female first
caught on June 15, 1951, weighed only 150 grams, but by August 10 she
had gained to 220 grams (probably in pregnancy) and had a vaginal
orifice. Of 35 adult and subadult females examined by Fitch in October,
eleven had a vaginal orifice, the latest on October 18. Of these eleven
showing signs of breeding, four at least had not yet produced litters,
judging from the undeveloped condition of their mammae, and others that
showed evidence of recent lactation probably included young of the year
that had bred in August or September. One female gave birth to a litter
in a trap on the night of October 6, 1950. Of 32 adult and subadult
females recorded by Fitch in November, all were sexually quiescent, with
the possible exception of one having a partially open vagina on November
10. All females taken in December, and most of those taken in January,
also were sexually quiescent. January 20 was the earliest recorded date
for a female with a vaginal orifice. Females examined in February mostly
were perforate and many of them appeared to be in oestrus. One female
trapped on February 19, 1950, weighed only 140 grams and was still
imperforate. Another, weighing 200 grams on February 3, 1952, still was
imperforate, but by February 27 she was perforate and appeared to be in
oestrus. An adult female that appeared to be in oestrus on February 3,
1952, was imperforate on February 10.
_Growth_
At birth woodrats weigh approximately 10 grams or a little more. In a
litter born in captivity and kept by Rainey, the average gain amounted
to a little more than 1.5 grams per day during the first two months, but
in the third month it was somewhat less. As this was an unusually large
litter, of five young, one more than the female's teats could
accommodate, their growth may have been a little less rapid than in most
of those under natural conditions. At an age of three months they
averaged approximately 120 grams. The three males consistently exceeded
the two females.
[Illustration: FIG. 3. Typical growth curves for male and female
woodrats; early stages are based on the litter of a captive female,
later stages on average gains of recaptured juveniles and subadults,
excluding those that seemed to be stunted. Solid line represents males
and broken line represents females.]
Young weighing less than 100 grams are rarely caught in live-traps. Four
young, all males, first caught at an average weight of 80 grams, gained
on the average, 1.39 grams per day over intervals that averaged 44 days.
Six other young males first caught while in the weight range of 100 to
149 grams, were recaptured after intervals of 17 to 45 days and they had
gained, on the average, .92 grams per day. The corresponding figure for
four young females in the same size range was .71 grams per day. In
seven young males in the weight range 150 to 250 grams, that were caught
after intervals averaging 66 days, the gain in weight amounted to .83
grams per day. In seven females in the range 150 to 199 grams, gains
averaged only .68 grams per day. Fully grown females that are not
pregnant weigh, most typically, a little less than 250 grams while fully
grown adult males average a little more than 300 grams. Growth rate and
adult weight both are influenced to a large extent by season and even
more by individual differences. The underlying causes are obscure in
most instances, but individual rats that are still short of adult size
may stop growing for periods of months, and some individuals grow much
more rapidly than others. One male that weighed 108 grams when he was
first caught on July 3, 1951, was estimated to have been born in early
May. He was last captured 152 days later on December 2, 1951, and by
then his weight was 300 grams, representing an increase of 1.2 grams per
day. Another male that weighed only 75 grams when he was caught on
October 8, 1950, may have been less than two months old then. By
November 21, 1951, at a probable age of 15 months, he weighed 350 grams
having attained almost the maximum size. Other exceptionally large
individuals were known to be less than two years old, while those rats
that survived longest on the study areas did not much exceed average
adult size. These records seem to show that exceptionally large woodrats
are usually not those of advanced age, but are individuals which have
grown most rapidly through fortuitous circumstances, probably depending
upon both innate and environmental factors.
None of the woodrats handled was excessively fat, nor were any
emaciated. The habit of keeping on hand stores of food at all seasons
perhaps obviates the necessity for storing quantities of fat. Seasonal
trends in weight vary among individuals, and are not wholly consistent
from year to year. Rainey found that in late autumn and winter, rats
steadily gain weight reaching a peak in late February or March. However,
in the winters of 1948-49 and 1949-50, Fitch found that most rats lost
weight and hardly any, even those that were short of adult size, made
gains.
The following records of a male born in the spring of 1949 show rapid
growth and attainment of adult size in his first summer, cessation of
growth during the winter, and resumption of growth, with attainment of
near-maximum size the following spring.