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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Volume 8, No. 9, pp. 499-533, 3 figs.
June 12, 1956
Ecological Observations on the Woodrat, Neotoma floridana
BY
HENRY S. FITCH AND DENNIS G. RAINEY
UNIVERSITY OF KANSAS
LAWRENCE
1956
UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
Editors: E. Raymond Hall, Chairman, A. Byron Leonard,
Robert W. Wilson
Volume 8, No. 9, pp. 499-533
Published June 12, 1956
UNIVERSITY OF KANSAS
Lawrence, Kansas
PRINTED BY
FERD VOILAND, JR., STATE PRINTER
TOPEKA, KANSAS
1956
ECOLOGICAL OBSERVATIONS ON THE WOODRAT, NEOTOMA FLORIDANA
By
Henry S. Fitch and Dennis G. Rainey
_Introduction_
The eastern woodrat exerts important effects on its community associates
by its use of the vegetation for food, by providing shelter in its stick
houses for many other small animals, and by providing a food supply for
certain flesh-eaters. In the course of our observations on this rodent
on the University of Kansas Natural History Reservation, extending over
an eight-year period, from February, 1948, to February, 1956, these
effects have changed greatly as the population of woodrats has
constantly changed in density, and in extent of the area occupied.
This report is concerned with the population of woodrats on the
Reservation, the changes that the species has undergone, and the factors
that have affected it. Our two sets of field data, used as a basis for
this report, supplement each other and overlap little, either in time or
space. Fitch's field work which covered approximately the western half
of the Reservation, was begun in September, 1948, and was pursued most
intensively in the autumn of 1948 and in 1949, with relatively small
amounts of data obtained in 1950 and 1951 because of the great reduction
in numbers of rats. Rainey's field work began in the spring of 1951 and
was continued through 1954, concentrating on a colony in the extreme
northwestern corner of the Reservation and on adjacent privately owned
land. In actual numbers of rats live-trapped and for total number of
records the two sets of data are comparable. Fitch's field work
consisted chiefly of live-trapping while Rainey's relied also upon
various other approaches to the woodrat's ecology. Rainey's findings
were incorporated originally in a more comprehensive report (1956), from
which short passages have been extracted that are most pertinent to the
present discussion. Our combined data represent 258 woodrats (153
Fitch's and 105 Rainey's) caught a total of 1110 times (660 Rainey's and
450 Fitch's). Rainey's records pertain, in part, to woodrats outside the
Reservation but within a few hundred yards, at most, of its boundaries.
_Habitat_
In the autumn of 1948 the population of woodrats was far below the level
it had attained in 1947 or earlier, but the rats were still abundant and
distributed throughout a variety of habitats. Almost every part of the
woodland was occupied by at least a sparse population. Also, many rats
lived beyond the limits of the woodland proper, in such places as
deserted buildings, thickets, roadside hedges, and tangles of exposed
tree roots along cut banks of gullies. All these situations are
characterized by providing abundant cover, a limiting factor for this
woodrat.
In 1947, when the population of woodrats was especially high, plant
succession on the wooded parts of the Reservation may have been near the
optimum stage for the rats. For some 80 years, since the time the land
was first settled and prairie fires were brought under control, woody
vegetation has been encroaching into areas that were formerly grassland.
About 1934 the University changed its policy with regard to treatment of
the tract that was later made the Reservation. Up to that time, most of
the area had been used as pasture and subjected to heavy grazing, but
several fields had been fenced and cultivated. Under the new policy the
hillsides and hilltop edges with open stands of various deciduous trees
were enclosed with stock fences and protected from grazing. Successional
trends were greatly altered. Woody vegetation, already favored by
protection from the prairie fires originally important in the ecology of
this region underwent further development as a result of protection from
browsing. Thickets of shrubs and saplings sprang up throughout the
woodland, forming a dense understory layer beneath the discontinuous
canopy of the relatively scattered mature trees. The composition and
density of the undergrowth varied markedly in different parts of the
woodland. The parts that were formerly most open acquired the most dense
understory. Blackberry, honey locust, osage orange, and prickly ash
formed in places thorny tangles almost impenetrable to humans. This
thicket stage reached its peak in density in the middle to late forties
coinciding approximately with the time of maximum abundance of the rats.
In the past eight years, under continued protection from burning,
cutting and browsing, the forest has developed further; sizable trees 20
feet or more high and up to eight inches in trunk diameter have grown
from seedlings during the period of protection. An almost continuous
canopy of foliage has developed, shading the understory and thinning it
by killing shrubs and saplings. In those situations where the canopy is
most dense, as on north slopes having stands of young hickory averaging
twenty feet high, the understory is now largely lacking, but in other
situations, particularly on south slopes, the understory thickets are
still dense. On the whole, however, habitat conditions have become less
favorable for the woodrat.
Within the woodland the population of woodrats was not evenly
distributed even at its maximum density; only those situations that
provided sufficient overhead shelter were occupied by woodrats. The
hilltop limestone outcrop, which was the refugium of the survivors when
the population was at low ebb, also supported the greatest concentration
when the population was high. The number of individuals living along any
particular stretch of ledge could be determined only by intensive
live-trapping, whereas residences of individuals could be more readily
identified in most other situations away from the ledge. Stick houses of
woodrats are, characteristically, large and dome-shaped in woodland, but
along the ledges they usually lacked this typical form and consisted of
a much smaller accumulation of sticks, often merely filling a small
crevice. Sticks carried into such places where they were partly or
wholly protected from moisture and sunshine were much less subject to
decay than those in more open situations, and remained long after the
rats themselves were gone. Accumulations of droppings in depressions in
rock surfaces beneath overhanging ledges likewise have lasted for many
years. The rock outcrop provided a continuous travelway along the
hilltops, and even parts that were not permanently occupied usually had
some sign. The following types of situations were found to be especially
favorable for occupancy: deep crevices beneath overhanging projections
of the ledge; large flat boulders broken away from the main ledge; thick
clumps of brush (usually fragrant sumac, _Rhus trilobata_) providing
shelter and support for the house; logs fallen across the ledge
providing support and protection for the house structure.
A second outcropping limestone stratum approximately 20 feet below the
level of the hilltop was just as extensive as the upper outcrop, but it
was little used by the rats because the exposed rock surface was more
regular, lacking the jagged cracks and deep fissures of the hilltop
outcrop; and it lacked the overhanging projections which provided
overhead shelter for the rats along the upper outcrop. More than ninety
per cent of the rats that were recorded as associated with the outcrops
were at the hilltop stratum.
Second in preference to the hilltop outcrop as a house site was the base
of an osage orange tree in thick woods. This tree occurs throughout the
woodland of the Reservation, having become established when the leaf
canopy was more open, and the whole area was subject to grazing, with
less development of the understory vegetation in the woodland. Houses
were most often situated in those osage orange trees that had been cut
one or more times, and had regenerated with spreading growth form, the
multiple branching stems offering substantial support. Occasionally
houses were built in crotches from two to six feet above ground.
Blackberry thickets also are favorable locations for houses. These
thickets grew up mostly in fenced areas from which livestock were
excluded, but where there was not dense shade--hilltop edges and level
or gently sloping ground adjacent to creek banks. The houses were
usually in densest parts of the thickets where they were almost
inaccessible. Mats of dead canes more or less horizontal, with the live
canes growing up through them, provided effective overhead protection,
while the ground beneath was relatively open. Houses built in the
thickets were so well concealed that they were usually not detected
until after leaves were shed in autumn. In most cases the blackberry
thickets were small and well isolated. Houses of the rats were sometimes
unusually near together suggesting that these thickets provided
especially favorable habitat conditions.
Hollow trees are often utilized, the accumulation of sticks for the
house being largely inside the cavity. To be suitable for a house site,
the snag must have an opening near ground level, and another higher on
the trunk, providing emergency outlets in two directions. Most of the
hollow trees utilized were black oaks (_Quercus velutina_).
In 1948 there were many houses in cut tops of trees left from small
scale lumbering operations a few years earlier. The densely branched
tops of elms, oaks and hickories had satisfied the requirement for
support of the house and nearby shelter. The houses built in them were
in open woodland well separated from otherwise favorable situations. By
1948 the tops were disintegrating and no longer provided effective
shelter. The houses built in them were falling into disrepair and were
not permanently inhabited but were often used temporarily by wandering
individuals.
Along cut banks of gullies where trees were partly undermined by
erosion, the exposed, tangled root systems provided sites for occupancy.
In these situations the accumulations of sticks were small and lacked
the typical domed shape, consisting essentially of a lining to the
cavity beneath the roots.
Two small buildings at the Reservation headquarters were accessible to
woodrats and were utilized off and on throughout much of the period of
this study, despite the fact that most other sites of occupation away
from the hilltop outcrops were deserted in the same period. One small
building used as a laboratory had an enclosed wooden box five feet
square housing an electric water pump. The interior of this box was
accessible to the rats from beneath the floor. Litter of sticks and
stems and various food materials were carried in by the rats. The nest
thus protected and enclosed was not surrounded by the usual accumulation
of sticks. An old garage 30 feet from the laboratory building was also
occupied, sometimes by a different individual. The nest and food stores
were behind boards propped against the wall.
In October, 1948, live-trapping was begun on a heavily wooded slope
facing northwest, and a ten-acre area was trapped rather thoroughly in
the succeeding weeks. Because few traps were then available, this was
the only area that was well sampled in 1948, although diffuse trapping
was carried on over some 200 acres. On the ten-acre tract a total of 17
adult and subadult woodrats were caught, four along the hilltop rock
outcrop, six along the gully at the bottom of the slope, and seven at
intermediate levels on the slope. Judging from the many unoccupied
houses, the population on this tract had been much higher before the
study was begun. On the basis of this sample it seems that in 1947 a
population of several hundred woodrats lived on the wooded parts of the
square mile where the Reservation is located.
_Reduction of Population_
The abrupt reduction in the population of woodrats on the Reservation
cannot be explained conclusively with available data. Probably weather
played a major part, but other unknown factors must have been important
also. It is certain that the population of woodrats was high, if not at
an all-time peak, in 1947. In late February, 1948, when one of us
(Fitch) first visited the area on a preliminary inspection trip (not
concerned primarily with woodrats), houses of these rats were found to
be unusually numerous and those seen seemed to be occupied and well
repaired. Possibly the population was drastically reduced within the
next few weeks, as unseasonably cold and stormy weather occurred in
early March. For the first 12 days of March, 1948, temperature averaged
20 deg. below that of average March weather, and even colder than the
average for January or February. A reading of -5 deg.F. on March 11 set a
new low locally for the month since records were begun in 1869. The
record low temperatures were accompanied by 12.8 inches of snow. This
spell of unusually severe weather in early March coincided with the
period in which first litters of young usually are born, as most females
breed in early February and the gestation period is in the neighborhood
of five weeks. That most of these first-litter young may have been
eliminated by the unfavorable extreme of weather at the most critical
stage in the life cycle may be readily imagined although definite proof
is lacking. However, the mortality must have extended beyond newborn
young. Loss of first litters ordinarily would be compensated for by the
end of the season, since a female usually breeds more than once in the
course of a season. In any case, by autumn, when the actual field study
of woodrats was initiated, many houses were already deserted and in
disrepair. Although the rats were still moderately abundant, they were,
seemingly, much below the population peak of the preceding year.
Further drastic reduction of adults and subadults took place in the
winter of 1948-49. In the course of live-trapping operations from
mid-October into early December, 51 individuals were caught and marked.
Chiefly because of unfavorable weather conditions, field work was
discontinued in mid-December, and live-trapping was not resumed until
early March. Subsequently, only 12 of the woodrats previously marked
could be recaptured, and the population had become noticeably sparse.
Seemingly, more than three-fourths of the population present in late
autumn had been eliminated in the interval. In January, weather was
exceptionally severe; on the ninth and tenth the worst sleet storm in
twelve years occurred. Sleet fell in small granules, while the
temperature remained several degrees below freezing. Partial thawing on
January 12, 13 and 14 was followed by a steady drizzling rain on the
fifteenth. On the following day the temperature dropped to -7 deg.F. Ice
still remained from the sleet storm, and the slush again froze. On the
night of January 18, there was one of the worst snow storms on record
and temperature reached a low of 2 deg.F. Exceptionally low temperatures
persisted through January 24, with more sleet on January 25. Ice from
the earlier storm still remained. On January 30, the temperature dropped
to -7 deg. and a three-inch cover of snow still remained over the coat of
ice. The month of January, 1949, had the heaviest precipitation in 81
years (5.09 inches) and a cover of ice remained for at least 21 days.
There were other sleet storms of lesser proportions on February 2 and
again on February 21.
Ordinarily sleet would not seriously damage woodrats living in houses in
woodland habitats and less suitable hedge rows because it usually
freezes as it falls and coats only the surface of the house. Gradual
thawing would allow normal runoff without much penetration. Because the
sleet during the storm described above did not form a glaze as it fell,
the ice particles penetrated many houses. It has been observed many
times that captive woodrats refused food that was frozen or were unable
to eat it. Woodrats in live-traps in winter rapidly weaken unless a
large supply of food is available. If food supplies became sealed over
by ice, woodrats would have died by starvation or by falling an easy
prey to predators. The rats were more accessible to several predators
than were smaller mammals such as meadow voles which were difficult to
obtain because of the coating of ice over the fields.
The decimated population surviving into the breeding season of 1949
failed to make substantial gains. In fact, during the following
four-year period the general trend of the population over the
Reservation as a whole seemed to be one of gradual further decline.
In November, 1949, the rats were almost gone from the area of north
slope and hilltop in oak-hickory-elm woodland where the most intensive
live-trapping and other field work had been done the previous year. The
following descriptions of houses remaining on the area at that time give
some idea of the habitat, and of the course of events correlated with
the fluctuations in numbers of woodrats.
No. 1. At the hilltop outcrop, partly on a substrate of
limestone boulders, built around an elm of two-foot DBH,
which lent support to one side. A hackberry sapling one inch
in stem diameter grew through the middle of the house,
providing further support. The house was two feet high and
six feet in diameter, and was in obvious disrepair, with a
hole several inches in diameter in its top. It had been
occupied in the autumn of 1948. It was constructed mainly of
sticks, ranging in diameter from approximately one inch to
straw size. Many of the sticks, from .4 to .5 inches in
diameter and one to two feet long, seemingly would have been
heavy burdens for a rat, although they were of light-weight
wood, sumac and elm. Mixed with the sticks were quantities
of dry leaves, bark, and chips of wood, all material
appearing old and weathered. This house was in
elm-oak-hickory woods 50 feet from a cultivated field on the
hilltop to the east and south. To the north and west the
escarpment sloped away abruptly. There was a coralberry
thicket beneath the trees on the adjacent hilltop.
[Illustration]
FIGURE 1
(A) Map of part of University of Kansas Natural History Reservation,
showing first-capture sites for all woodrats live-trapped in the autumn
of 1948. Because of the short time involved and the few traps available,
much of the area shown was not thoroughly trapped. Woodrats were
abundant, though much less so than in 1947, as shown by the large number
of deserted houses.
(B) Map of woodrat study area, same as shown in (A), showing
first-capture sites for all woodrats live-trapped in 1949. Woodrats were
still moderately abundant, but much below the level of the previous
year. Triangles indicate those capture sites not sampled in 1948.
(C) Map of woodrat study area, same as shown in (A), showing
first-capture sites for all woodrats live-trapped in 1950. Numbers were
medium-low, having undergone drastic reduction from the peak level.
Triangles indicate those capture sites where trapping was not done in
earlier years.
(D) Map of woodrat study area, same as shown in (A), showing
first-capture sites for all woodrats live-trapped in 1951. The
population was low, but had not yet reached its lowest ebb.
(E) Map of woodrat study area, same as shown in (A), showing
first-capture sites for all woodrats live-trapped in 1952, when the
population had declined to relatively low numbers and disappeared from
much of its former habitat.
(F) Map of the 590-acre Natural History Reservation, showing the area
where woodrats were studied.
No. 2. On gently sloping hilltop edge 15 feet from the
outcrop and escarpment, built around a forked walnut sapling
having both trunks approximately five inches in diameter.
The sapling, coming up through the center of the house at a
45 deg. angle, evidently had been bent by the accumulated weight
of the debris at an early stage of its growth, many years
before. Trees were small in this part of the woods, with a
well developed understory thicket of coralberry and sumac.
This house approximately one foot high and six feet wide,
was constructed mainly of sticks and was similar in
composition to No. 1, but appeared considerably older with
all the sticks blackened and rotten. In the autumn of 1948
this house was used by woodrats, but probably only as a
temporary stopping place, because it was already in
disrepair then.
No. 3. At edge of escarpment, 25 feet from No. 2, on a flat
boulder approximately six feet long, three feet wide and one
foot thick. The decaying and much flattened mass of sticks
was mainly on top of the boulder, but also spilled over its
edges. Fresh sign was noted at this house in the autumn of
1948, but the house was already in disrepair then, and
seemingly it was used only as a stopping place.
No. 4. At the hilltop outcrop where an elm had fallen across
it. The decaying log remaining was approximately 12 feet
long and 15 inches thick. This log passed diagonally through
the house, providing its main support. The house was
approximately 39 inches high, its summit extending a little
above the level of the top of the outcrop. The house was
approximately seven feet wide along the outcrop. This house
was somewhat intermediate between the typical dome-shaped
stick piles that the rat builds in open situations and the
formless accumulations of sticks with which some rats living
in deep rock crevices line the entrances. Part of the
accumulation was beneath the limestone boulders and
outcropping slabs. Approximately half of the material used
in the house consisted of sticks and the remainder of pieces
of bark and chips of wood, mostly gathered from the fallen
elm. This house had shrunken noticeably from decay and
settling in the months since it was occupied, in the autumn
of 1948. The house was surrounded by a thicket of fragrant
sumac, dogwood, and hackberry saplings.
No. 5. At edge of a protruding boulder one foot thick at the
hilltop outcrop of the west facing escarpment, and 100 feet
back in the woods from the edge of a corn field, in
undergrowth of dogwood, wild currant, and coralberry. The
house consisted of a pile of rotten twigs, 3 inches deep and
30 inches wide on the upper side of the boulder, and a
lining of similar material at the lower edge of the boulder,
partly blocking the crevice beneath it. The twigs composing
the house were old and rotten. However, a few dry but still
green hackberry leaves were stored in the crevice beneath
the boulder. In a bare space atop the boulder were several
recent woodrat droppings, small and obviously produced by an
immature individual, which, perhaps, had recently settled at
this old house site.
No. 6. In hilltop woods, 30 feet from a corner adjoining a
pasture and a corn field, at the base of an osage orange
tree of one foot DBH, and also over a hollow cottonwood log
one foot in diameter and three feet from the osage orange
tree. Suspended mats of grape and smilax vines, and the
thorny, dead, lower branches of the tree provided additional
shelter. The house was composed of sticks and twigs, mostly
of osage orange, with spines still present; slabs of bark,
wood chips, and dry leaves also made up part of it.
Materials on the exterior of the house appeared old and
weathered, but the house was conical and solid. Seven fresh
corn cobs were on the house or near its base, suggesting
that corn from the nearby field had figured importantly in
the diet of the occupant. A well beaten path led from the
base of the house alongside the log, to a large cottonwood
tree 15 feet from the house. This evidence that the house
was occupied was verified by live-trapping the occupant.
Late in 1948, also, the house was occupied by another
individual, but seemingly was deserted for a period of
months thereafter.
No. 7. On upper part of north slope where a hickory seven
inches in diameter had fallen across an old sunken log
approximately one foot in diameter. The house, composed
mainly of hickory twigs 1/4-inch to 1/2-inch in diameter,
mixed with bark, wood chips, and leaves, was partly decayed,
with no fresh sign and was in a thicket of greenbrier,
saplings of hickory and hackberry, and cut tops of
hickories. The top was flattened to less than four inches
above the level of the supporting hickory log. There were
large cavities in the side of the house. When first
discovered in the autumn of 1948, this house was occupied by
a subadult female rat, but she moved away permanently, and
the house had been deserted for approximately a year when
these observations were recorded.
No. 8. In middle of northwest slope, in thick branches of
broken top of a black oak. This house had become flattened
by decay and settling to form a mound approximately one foot
high and five feet in diameter. Only the top protruded
through the carpet of dry leaves. Once well protected and
partly concealed by the branches and twigs of the oak top,
this house was now fully exposed by the disintegration of
the top. The house consisted chiefly of oak twigs. In
October, 1948, a woodrat was live-trapped at this house, but
probably it was a wanderer. The house had then already
undergone much deterioration.
_Natural Enemies_
Some 56 species of animals that regularly prey on small vertebrates live
on the Reservation. Many of the larger kinds may take woodrats
occasionally. Because of size, habitat preferences and the time and
manner of hunting, five species stand out as the more formidable
enemies--the horned owl (_Bubo virginianus_), prairie spotted skunk
(_Spilogale putorius_), long-tailed weasel (_Mustela frenata_), pilot
black snake (_Elaphe obsoleta_) and timber rattlesnake (_Crotalus
horridus_).
Throughout the study horned owls were common on the area, but their
numbers were highest in 1948. Samples of pellet collections have shown
that the cottontail is the staple food, being represented in almost
every pellet. Various rodents also are important in the diet, the cotton
rat, prairie vole, or white-footed mouse being most prominent according
to the time and place of collection. The woodrat is approximately
optimum size for prey, and it constitutes one of the most preferred food
sources. Remains of only two woodrats were found in the pellets
examined, but at times when the pellets were collected woodrats were so
scarce that they constituted only an insignificant percentage of the
biomass of potential prey. On several occasions woodrats in live-traps
were attacked by horned owls, as shown by the overturned and displaced
trap and quantities of fine down adhering to them and to nearby objects.
The horned owl lives in the same habitat as does the woodrat. In other
regions woodrats are known to figure prominently in the diet of the
horned owl. At the San Joaquin Experimental Range in California, for
instance, _N. fuscipes_ was found 240 times, more frequently than any
other kind of prey, in 654 pellets of the horned owl, and this owl was
shown to be the one most important natural enemy of the rat, although
many kinds of carnivores, raptors and snakes also took toll from its
populations. On the Reservation the population of horned owls has been
fairly stable from year to year, with roughly one pair to 100 acres of
woodland. Some territories have been maintained continuously throughout
the eight-year period of observation, though changing to some extent in
size, shape and area included. In 1948, when livestock grazed on the
area, and the ground cover of herbaceous vegetation was relatively
sparse, cottontails were much less abundant than they were later when
the vegetation was protected. Small rodents including voles, cottonrats,
and deer mice, were also less abundant then, and the numerous horned
owls may have been supported in part by the high population of woodrats.
The spotted skunk may be an even more important enemy of the woodrat,
although the evidence is circumstantial. No records of these skunks
preying on woodrats have been found in the literature, nor were any such
instances recorded by us except for attacks on woodrats confined in
live-traps. This skunk is a formidable enemy of small and medium-sized
rodents, as it can climb, dig, and squeeze through small openings. That
it may prey on rat-sized rodents and may even be a limiting factor to
their occurrence is well shown by Crabb's (1941:353) studies in Iowa. He
found that Norway rats (_Rattus norvegicus_) ranked third in frequency
(cottontail, mostly carrion, ranked first) in the winter food of the
spotted skunk. Crabb observed that about farmyards and farm buildings
where the skunks had been eliminated by persistent persecution, rats
were abundant, but that about others where the skunks were present, the
rats were scarce or absent. On several occasions he noted that heavy
populations of rats about farm buildings in summer and autumn nearly
disappeared in winter if a skunk was in residence.
Sign of spotted skunk was noted frequently on various parts of the
Reservation, especially along the hilltop ledges which were the best
woodrat habitat. On several occasions skunks released from live-traps
took shelter in woodrat houses which appeared to be unoccupied.
According to a local fur dealer, C. W. Ogle, spotted skunks reached a
peak of abundance in Douglas County in the winter of 1947-1948, and many
pelts were brought in for sale then. The concentration of skunks may
have had detrimental effect on the population of woodrats, especially
when extremes of weather had already made conditions critical for them,
as in early March, 1948, and in January, 1949, when snow and sleet made
their usual food supply unavailable.
The long-tailed weasel is considered to be a potentially important enemy
of the woodrat. Weasels have been seen on the Reservation on only a few
occasions, but they may be more numerous than these records would
indicate. Two were caught at the hilltop outcrop, at different times and
places, in funnel traps put out to catch snakes. The weasel seems to
prefer this rocky habitat, which is also favored by the woodrat. Because
of its ferocity and willingness to attack relatively large prey, and
because it is an agile climber and able to squeeze through any openings
large enough to accommodate a woodrat, it would seem to be a formidable
enemy.
The pilot black snake (_Elaphe obsoleta_) is an important enemy of this
woodrat on the Reservation and probably throughout the rat's geographic
range except for the extreme western part. Although this snake occurs in
every habitat of the Reservation, it has been found most often along
rock outcrops of wooded hilltop edges in the type of habitat most
favored by the rat. Most often pilot black snakes have attempted to
escape into crevices of the outcrop. These snakes are also skillful
climbers and often have escaped by climbing out of reach along branches
or even vertical tree trunks. On several occasions these snakes have
been found on or beside woodrat houses, or have escaped into them. Over
a seven-year period 143 pilot black snakes have been recorded, 53 of
which were adults.
On September, 1948, a large pilot black snake found basking on a rock
ledge, distended by a recent meal, was palped and contained a subadult
female woodrat. On June 19, 1953, one of us, approaching a live-trap set
under an overhanging rock ledge, saw a four-foot pilot black snake on
top of it. The snake struck repeatedly at the rat in this trap, but was
unable to reach it. At each stroke the rat would dash about the trap
frantically.
These snakes hunt by stealth, and might catch woodrats by entering their
nests, or by lying in wait along their runways, but are not quick enough
to catch them in actual pursuit. Young in the nest would seem to be
especially susceptible to predation by the pilot black snake. These
snakes hunt by active prowling, either by night or by day, and much of
their food consists of the helpless young of birds and mammals found in
the nests. While only well-grown or adult pilot black snakes would be
able to swallow an adult woodrat, any but first-year young probably
would be able to overcome and swallow the small young. The female
woodrat's habit of dragging the young attached to her teats as she flees
from the house at any alarm must save many litters from predation by the
pilot black snake. First litters of young, born in early March, are
already well grown, and past the age of greatest susceptibility to
predation before the snakes emerge from hibernation in late April or
early May.
The timber rattlesnake is another potentially destructive enemy, but on
the Reservation, and throughout much of its original range it is now
relatively scarce. The genus _Neotoma_ largely coincides in its over-all
distribution with the genus _Crotalus_, of the rattlesnakes. For most
kinds of woodrats, the larger species of rattlesnakes are among the
chief natural enemies.
The timber rattlesnake has habitat preferences similar to those of the
eastern woodrat. Of 30 timber rattlesnakes recorded on the Reservation
over an eight-year period, all but one were at or near hilltop rock
ledges in woodland. The woodrat is probably one of the most important
prey species for the timber rattlesnake. Like the woodrat, the
rattlesnake is mostly nocturnal in its activity. Unlike the pilot black
snake, it hunts by lying in wait, striking prey which comes within
range, and waiting for it to die from the venomous bite, rather than by
active prowling. Therefore, it is probably less of a hazard to young in
the nest than is the pilot black snake. Even young rattlesnakes too
small to eat woodrats are potentially dangerous to them, as they may
strike and kill any that come within range.
_Commensals_
Rainey (1956) listed many kinds of small animals that use the houses of
the eastern woodrat and live in more or less commensal relationships
with these rodents.
A situation unusually favorable for observing woodrats and their
associates was discovered on the Reservation where, in July, 1948, two
old strips of sheet metal, each covering an area of approximately 25
square feet, were used as shelter by a lactating female with three
young. This was on a brushy slope just below an old quarry site. A rock
pile and remains of an old rock wall were nearby. Woodrats had carried
many sticks back under the metal strips, filling the spaces beneath
their edges. There was a nest and a system of runways beneath the
strips. In the following seven years this site was seldom deserted for
long and was used by a succession of individuals. The strips of metal
could be easily raised and then lowered into place with little
disturbance. Because the situation was not entirely natural, the
findings may not be typical of other rat houses. Animals found over a
period of years beneath these metal strips include: several dozen each
of the ring-necked snake (_Diadophis punctatus_), five-lined skink
(_Eumeces fasciatus_), and ant-eating toad (_Gastrophryne olivacea_);
several individuals each of cottontail (_Sylvilagus floridanus_),
white-footed mouse (_Peromyscus leucopus_), short-tailed shrew (_Blarina
brevicauda_), least shrew (_Cryptotis parva_), American toad (_Bufo
americanus_), Great Plains skink (_Eumeces obsoletus_), pilot black
snake (_Elaphe obsoleta_); and one each of bull snake (_Pituophis
catenifer_), spotted king snake (_Lampropeltis calligaster_), red milk
snake (_L. triangulum_), and timber rattlesnake (_Crotalus horridus_).
The snakes which were potential predators on the rats seemed to be
merely utilizing the shelter in these instances, but they may have been
lying in wait for prey there.
Among mammals, the cottontail and the white-footed mouse are the most
persistent users of the woodrat houses, especially those that are no
longer occupied by the rats. On one occasion five white-footed mice were
caught simultaneously in a trap set beside a house at the base of an
osage orange tree. Subsequent trapping showed that this house was no
longer occupied by a rat, but that the mice lived in it. Occupancy of
such an old woodrat house by white-footed mice may continue long after
abandonment of the house by the rat, even after the house has partly
decayed and settled to a small part of its original volume.
Cottontails often have their forms under the edges of houses, either
occupied or deserted. These situations offer protection overhead and on
three sides. Abandoned houses having one or more of the entrance holes
enlarged, as by predators breaking through the side of the house to gain
access to the nest, are especially well adapted for occupancy by the
cottontail. The rabbit may make its form inside the house structure.
The opossum, also, finds the type of shelter that it requires in
abandoned houses that have had the entrances sufficiently enlarged. On
various occasions opossums or their remains have been found in such old
houses, and opossums released from live-traps have been known to seek
shelter in abandoned woodrat houses.
At the old quarry on the Reservation woodrat sign was especially
abundant. A wooden bin approximately seven feet square, used to store
crushed rock before quarrying operations were abandoned, was inhabited
by one rat. At the base of a rock crusher on the top of a bank a few
yards from the bin was an accumulation of sticks and other debris
brought by woodrats. A rock wall at the top of the bank between the
crusher and the bin had many crevices providing shelter for the rats,
and projecting rocks were littered with their droppings. In the spring
of 1949 the bin and rock crusher were removed, but at least one rat
continued to live in the rock wall. In the summer of 1951 several tons
of corn ruined in the flood were dumped on the top of the bank above the
wall. By autumn, Norway rats, either brought in with the corn or
attracted by it, had taken possession of the wall, evidently displacing
the woodrats, which were no longer present. Although this Old World
murid rat is much different from the woodrat in habits, it seemingly can
compete with it and replace it where habitat conditions are otherwise
favorable for both.
_Movements_
The woodrat is dependent on the stick houses that it constructs for
shelter. For each individual the house constitutes a home base to which
it is attached, and about which its movements revolve. The area within
which routine daily movements are confined constitutes the home range,
which is variable in size and shape. An individual may, and usually
does, alter its home range over periods of time. The home range is
somewhat nebulous because the rat may at any time move far beyond the
small area to which its activities are largely confined. It may be
motivated by sexual urge or other voluntary wandering; it may be
enticed by a food supply or some other specific attraction not available
near its house; or it may be forcibly displaced by an intruder or may
abandon in favor of an offspring.
An occupied house normally has several runways radiating from it. These
are well worn paths, smoothed by use, and cleared of obstructions, and
the rat tends to keep to them in its foraging expeditions. Usually a
trail leads to a bush or tree showing evidence of heavy use by the rat.
Ordinarily such a trail cannot be traced more than 30 feet from the
house, and it seems that the most concentrated foraging occurs within
this short radius. Experience in live-trapping has indicated that the
distance covered by a woodrat in its normal foraging for food is
ordinarily less than 75 feet in any direction from the house.
Usually the rats can be caught in traps only at their houses or nearby
places that they frequent, as indicated by their sign. When travelling,
woodrats make use of overhead cover as much as possible. Storing of food
seems to be associated with the animal's reluctance to wander far from
home. When a rat is gathering preferred food for storage the home range
may be enlarged (or the animal may travel beyond the limits of its
regular home range). In any case the rat may find it necessary to
traverse an additional area in order to reach the food source. This may
involve, in part, extension vertically, as when the rat obtains food
from trees directly over the house. The home range is thus somewhat
three-dimensional; both trails and feeding places are often above
ground. Because of dependency on cover, woodrats do not forage randomly
in all directions from the house.
Although the house and its immediate environs are defended as a
territory by the occupant, possession may be soon relinquished. A
woodrat may shift frequently from one house to another, especially if
unoccupied houses are readily available. Because woodrats had undergone
drastic reduction in numbers, as discussed on p. 505, unoccupied houses
in various stages of disrepair were numerous throughout the woodland in
1948 and 1949, and the rats that were present then seemed especially
inclined to wander. Even old houses that are collapsed and
disintegrating may be used temporarily, or may be taken over and
repaired. Houses that are in sites exceptionally favorable in that they
provide food and shelter may be occupied more or less permanently, with
a succession of woodrats over many generations.
Shifts to new areas are perhaps most often motivated by a search for
mates. Such shifts are, on the average, longer and more frequent in
males. Males must range farther in search of females when numbers are
low. On the other hand, when numbers are high and most of the best sites
are occupied, newly independent young and displaced adults are forced to
travel greater distances in search of homes. Some of the larger and more
powerful males move far greater distances than smaller males. The
longest distances recorded were mostly for large adult males in breeding
condition. The average maximum distance between successive points of
capture for 27 adult males was 345 feet. For 39 females (adults and
subadults) the corresponding figure was 143 feet. The extremes for males
were 0 to 1080 feet and for females, 0 to 650 feet. Of the 27 males,
five moved the maximum distance in a single night. Most of the long
movements by males did not constitute clear-cut shifts in home range,
and many returned to their original locations.
The average distance between points of first and last captures for 72
subadult and adult males was 165 feet. A similar figure for 72 subadult
and adult females was 133 feet. Of the males 23.7 per cent were at the
same place at the first and last captures; for females the percentage
was 36.1. These figures are from the combined data of our trapping
records, but the trends differed sharply in the two sets of records. In
Fitch's records, movements averaged longer and difference between the
sexes was much less: 189 feet for 41 males and 178 feet for 42 females.
Corresponding figures from Rainey's records were: 141 feet for 31 males
and 74 feet for 30 females. In Fitch's field work, opportunities to
record exceptionally long movements obviously were better because the
trap line encompassed a larger area, approximately half a square mile,
whereas Rainey's live-trapping was concentrated on relatively small
areas. The reason for the greater vagility of females in Fitch's records
is less evident. However, the data were obtained within the period of
drastic population reduction, at a time when there were numerous empty
houses throughout the woodland, facilitating travel, and shifts from one
home range to another where conditions were, temporarily at least, more
favorable. Rainey found that the females in the small colony in woodland
where he trapped, moved much less than did those that lived along the
hilltop outcrop, which provided a natural travel route.
Following are several examples of males and females with long histories
showing individual variation in frequency and distance of movements.
_Males_
(1.) First captured October 14, 1951, and last captured 327
days later on September 6, 1952. He was taken 12 times. For
the first seven captures (October 14, 1951, to July 15,
1952), no movements were recorded. In the following seven
days he moved 367 feet. Within the next 21 days he returned
to within 114 feet of the site of original capture. Less
than one month later he was caught for the last time, at
this same site.
(2.) This large male was captured twelve times over a period
of 827 days (March 16, 1952, to June 21, 1954). He tended to
wander more than other males and was absent from the
trapping area from early 1952 to May 1953. One round trip
made in a two-weeks period, amounted to a linear distance of
1894 feet if the rat followed natural cover. The return trip
of 947 feet was the greatest distance traversed in a single
night in any of the woodrats we recorded. Other movements
between successive captures were: 722, 397, 356, 293, 253
and 144 feet (the latter shift made three different times).
Sexual urge probably motivated most of his wandering, since
numbers of females were low.
(3.) For this male the span of records was 143 days, with 18
captures. For the first eight recaptures, extending over a
period of 39 days, he was still at the original location.
Four days later he had moved 120 feet and was visiting a
female. A week later he returned. In the following month he
was recorded as making two more moves, of 115 feet and 215
feet. He was last recorded at the hilltop outcrop.
(4.) The records of this male extended over 465 days, with
13 captures. For the entire period only one movement, of 163
feet, was recorded. Twelve of the 13 captures were at the
same house.
(5.) This male was captured 16 times over a span of 130
days. After the second capture he moved 144 feet along the
outcrop and was caught there for the next 14 times, having
developed a "trap habit."
(6.) This male was in the area 210 days (13 captures) and
shifted his range. He was first captured on August 17, 1952,
at a house at the rock fence 433 feet from the outcrop.
Between this date and October 12, 1952, he moved to the
outcrop and established residence in a vacant house. He was
recorded as making six more moves, the longest of which was
only 40 feet.
(7.) This male was first caught in June, 1949, as a juvenile
probably between two and three months old (weighing 96
grams) and hence probably still at the maternal house. In
September, grown to adult size, he was caught twice, still
at this same place. In October, November, December, and in
February, 1950, he was caught 11 times at eight places all
within a 90-foot radius of his original location. In April,
1950, he was caught at points 550 feet WSW and 700 feet SW.
In October he was caught within 150 feet of the original
location. In November, 1950, and in March and April, 1951,
he was caught four times at a place 900 feet SW from his
original location.
(8.) This subadult male was first caught at the hilltop
outcrop on October 4, 1949. Two days later he had moved 160
feet north along the outcrop. A month later he had shifted
600 feet south; in three more days 1040 feet north. On
November 15 he was 105 feet south of the November 8
location; on November 16, he had moved 70 feet north. On
November 17 he had moved 900 feet back south, but had
returned on the 18th to the November 16 location. On
November 22, he had again shifted 900 feet south. All
capture sites were at the hilltop outcrop.