Ecological Observations on the Woodrat, Neotoma floridana

by Henry S. Fitch; Dennis G. Rainey

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.