Feeding Habits of the Pacific Rattlesnake

URING 1938 and 1939 we had an opportunity to study the feeding habits of the Pacific rattlesnake, Crotalus viridis oreganus (Holbrook), at the San Joaquin Experimental Range near O'Neals, Madera County, California. Other wildlife and range management studies in progress at the same time supplied information on populations and seasonal behavior of many plant and animal species. As part of a general program for wildlife studies on range lands of the Sierra Nevada foothills, this work was directed by Mr. Everett E. Horn, Biologist, of the U. S. Fish and Wildlife Service, in collaboration with the California Forest and Range Experiment Station. Jesse W. Nelson, Ben Glading, Charles A. Kaley, Freeman Swenson, and Raymond Sharp cooperated in securing data. Gathering of field data and examining of stomachs were done by Fitch; Twining aided in preparation of the manuscript and analyzed most of the scatological material, some of which was analyzed by John E. Chattin. The animal life of the region is typical of the Upper Sonoran Life Zone in the blue oak and digger pine belt on the west flank of the Sierra Nevada. Eleven of the sixteen rodent and lagomorph species known to occur on the area were recorded in the food of the snakes. All these species vary in abundance on different parts of the Range, and are subject to fluctuations in numbers seasonally and over longer periods. The pocket gopher (Thomomys bottae) is the most generally abundant species, and may occur in populations up to twenty or more per acre over extensive areas. The ground squirrel (Citellus beecheyi), kangaroo rat (Dipodomys heermanni), woodrat (Neotoma fuscipes), white-footed mouse (Peromyscus maniculatus), rock mouse (Peromyscus truei), brush mouse (Peromyscus boylii), San Joaquin pocket mouse (Perognathus inornatus), and cottontail (Sylvilagus auduboni) generally are present in populations of one to many per acre. The meadow mouse (Microtus californicus), and the California pocket mouse (Perognathus californicus), are scarcer and more localized. Those species not recorded among the food items are all relatively scarce, and the adult gray squirrel (Sciurus griseus), and jackrabbit are too large to be eaten by the Pacific rattler. The house mouse (Mus musculus) is here confined to the vicinity of buildings, where it is relatively safe from snakes. The harvest mouse (Reithrodontomys megalotis) is probably preyed upon but we obtained no records. The species is rare and localized here. The chipmunk (Eutamias merriami) was not recorded among the prey, and it is also one of the less common rodents. Several kinds of lizards were recorded in the snakes' diet. The brownshouldered lizard (Uta stansburiana), is the most abundant vertebrate species on the area, and the fence lizard (Sceloporus occidentalis) and whiptail (Cnemidophorus tessellatus) also are common. The alligator lizard (Gerrhonotus multicarinatus) and the horned toad (Phrynosoma blainvillii) are much less common, and neither was found among the food items. The spadefoot toad is abundant, and it is the only one of the seven amphibian species occurring locally that was preyed upon. It remains inactive