It is doubtful whether any animals live in a truly commensal relationship with spectabilis, but of not unfriendly associates there are a great number. It is the experience of Bailey, corroborated by observations of Vorhies on living animals, that these kangaroo rats are active in defending their caches of food, and will even fight individuals of the same species savagely and to the death. One moonlight night a strange individual was liberated on a mound. It deliberately entered one of the openings, but after about two minutes' time made an exceedingly rapid exit, running rapidly out of sight as if pursued, though the owner of the home did not appear outside of the burrow. There can be little doubt that the stranger was precipitately ejected by the owner. We suspect, though this is a point difficult to prove satisfactorily, that merriami does not always store food supplies for itself, but visits the burrows of spectabilis regularly to pilfer the seed stored therein. The observed facts thus far recorded which suggest this are that in no merriami burrow examined has a store of food been found, and also that in trapping for spectabilis on its own characteristic mounds one catches a large percentage of merriami.

On two separate occasions Vorhies has observed the smaller species running over the mounds of the larger, actually carrying away the grain which had been placed to entice the larger when it might appear. (In these cases the larger species did not put in an appearance until near morning.) Furthermore, the dens of merriami are often connected by distinct runways with those of spectabilis, indicating much traveling or visiting. That this is probably not friendly visiting is suggested by the certainty with which an individual of the larger species will strike and kill one of the smaller when they are placed together in the same inclosure. The word "thief" expresses this suspected relationship better than would the term "parasite."

It is not to be expected that such obvious shelter retreats as the mounds of spectabilis should fail to attract the attention of other animals. We have found a small gecko (Coleonyx variegatus), scorpions of two or three undetermined species, and certain insects (of the Order Orthoptera) to be very common inhabitants of the dens. With the exception of the parasitic insects the most common are wingless locustids (Ceuthophilus spp.) and the peculiar wingless females of a species of cockroach (Arenivaga erratica). These two are seldom absent when a burrow is excavated, the female cockroaches being abundant, although the winged males have never been taken in the burrows.

Cary's observations at Monahans, Tex., and those of others at numerous localities, combined with our own, show that at various times the dens furnish protection and shelter for various species of cottontail rabbits (Sylvilagus), ground squirrels (Citellus and Ammospermophilus), wood rats (Neotoma), grasshopper mice (Onychomys), rattlesnakes (Crotalus), and most of the common lizards. Of these the ground squirrels Citellus tereticaudus and Ammospermophilus harrisii are most often noted on the Range Reserve using the dens as a retreat, the Ammospermophilus seldom being observed to enter any other kind of burrow. It should be added that the total observations include dens which have been deserted by their rightful owners.

NATURAL CHECKS.

The enemies of the kangaroo rat are not determined in detail, or as to relative importance, but the badger (Taxidea taxus berlandieri) and the kit fox, or swift (Vulpes macrotis neomexicana), may well be foremost. Dens which have been deeply excavated by badgers are frequently seen, and sometimes two or three badger tunnels penetrate one burrow system. Dens thus despoiled are probably soon reoccupied even if the original owner is captured, and in the course of a few months the reworking of the abode obliterates the signs of destruction.

Droppings of the kit fox show an abundance of bones of small mammals of kangaroo rat size, among them those of spectabilis.

Bobcats (Lynx baileyi) and coyotes (Canis mearnsi) probably are a prejudicial factor. Skunks may sometimes be able to surprise the kangaroo rats, but probably not often. The western horned owl (Bubo virginianus pallescens), the barn owl (Tyto alba pratincola), and perhaps others may well be among the most feared enemies, but no special investigation of owl pellets on the reserve has been possible. In 592 barn-owl pellets from California were found remains of 230 kangaroo rats, only one other rodent being represented by a larger number (McAtee, 1921, 258).

Much more information on enemies is needed. The relatively low rate of reproduction (see p. 18) indicates comparative freedom from inimical factors.

PARASITES.

Dipodomys s. spectabilis is regularly infested with a species of flea, Ctenophthalmus sp. Seldom or never is a specimen taken in reasonably fresh condition without some of these parasites present on its body, though of course they desert the body of the host after it becomes cold, and hence dead specimens left too long may be free from them. The den conditions are ideal for the breeding of this parasite, because of the great quantities of fine, dusty, organic refuse littering the tunnels and furnishing food and refuge for the larvÆ. As demonstrated to us by F.C. Bishopp, of the Bureau of Entomology, a handful of this refuse taken from the floor of a burrow within arm's length of the entrance is almost certain to contain these larvÆ.

Less regularly present, perhaps because of its different life history, is a small tick, Trombicula sp. At times this parasite is very common, being present on nearly every individual rat, and at other times specimens are difficult to find; it appears to be more commonly present in summer and fall than at other seasons, and is found attached chiefly to the ears.

No internal parasites have been detected. The nocturnal and fossorial habits of the animal seem to give complete protection against a form of parasite which is very common among some other rodents of the Range Reserve, notably Lepus and Sylvilagus. Nearly all rabbits are infested with "warbles," the larvÆ of a species of bot-fly, Cuterebra (family Oestridae). Other small mammals also are occasionally parasitized by the Cuterebra, but in the handling and examination of perhaps 200 or more individuals of spectabilis and merriami, we have yet to find a single case of infestation by an oestrid fly.