Ecological Observations on the Woodrat, Neotoma floridana Part 2

(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 s.h.i.+fted 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.

[Ill.u.s.tration: FIG. 2. Diagram ill.u.s.trating s.p.a.cing (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 s.p.a.cing is the expression of the need for an otherwise unoccupied area in which there is sufficient s.p.a.ce to live. Because individuals tend to fight whenever they meet, there is probably a psychological tendency for sequestration which results in s.p.a.cing 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 s.p.a.cing 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 s.p.a.cing 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 s.h.i.+fted 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 s.h.i.+fted 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 s.h.i.+fted 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 cl.u.s.ters of the fruits of pokeberry (_Phytolacca americana_).

October 20: Many small cl.u.s.ters 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 gra.s.s (_Bromus inermis_ and _B. j.a.ponicus_).

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 gra.s.s (_Panic.u.m 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 th.o.r.n.y 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 subst.i.tution 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 circ.u.mstances 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 sc.r.a.ps, canned dog-food, bacon grease, or carca.s.ses of small vertebrates. In fact, such baits seemed to be even more attractive than the grain, seeds, peanut b.u.t.ter 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 carca.s.s of the snake was found to have been dragged a short distance and gnawed; a quant.i.ty 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 carca.s.s 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 v.a.g.i.n.al orifice was used as an indication of s.e.xual 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 v.a.g.i.n.al orifice is prominent and gaping. By February most females born the previous season have matured, and breeding involves the entire population, except possibly for r.e.t.a.r.ded 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 v.a.g.i.n.al 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 s.e.xual maturity at an age of five to six months unless this age is reached in the winter period of s.e.xual 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 s.e.xual 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 v.a.g.i.n.al orifice. Of 35 adult and subadult females examined by Fitch in October, eleven had a v.a.g.i.n.al 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 s.e.xually quiescent, with the possible exception of one having a partially open v.a.g.i.n.a on November 10. All females taken in December, and most of those taken in January, also were s.e.xually quiescent. January 20 was the earliest recorded date for a female with a v.a.g.i.n.al 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.

[Ill.u.s.tration: 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 circ.u.mstances, 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 quant.i.ties 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.

June 16, 1949 96 gms.

September 26, 1949 230 gms.

September 27, 1949 230 gms.

October 18, 1949 260 gms.

October 27, 1949 250 gms.

October 29, 1949 220 gms.

November 8, 1949 235 gms.

November 15, 1949 245 gms.

November 24, 1949 240 gms.

November 26, 1949 240 gms.

November 30, 1949 240 gms.

December 20, 1949 260 gms.

February 18, 1950 230 gms.

April 5, 1950 290 gms.

April 7, 1950 300 gms.

October 7, 1950 320 gms.

November 29, 1950 345 gms.

March 23, 1951 340 gms.

Another example, showing winter cessation of growth in a male at even smaller size is shown below. This was in the winter of 1950-1951.

November 9 145 gms.

November 28 175 gms.

November 29 165 gms.

January 10 180 gms.

January 11 175 gms.

March 1 225 gms.

March 23 200 gms.