Comparative Ecology of Great Basin Rattlesnakes (Crotalus viridis lutosus) and Great Basin Gopher Snakes (Pituophis melanoleucus deserticola) and Their Impact on Small Mammal Populations in the Snake River Birds of Prey Natural Area

Capture data from southwestern Idaho indicate that P. melanoleucus disperse shortly after spring emergence in mid-April, while C. viridis, which emerge in late April, remain near hibernacula for several weeks. Both species reach a peak of activity in late May to early June with activity gradually diminishing throughout summer and early fall. Most daily activity of C. virdis occurs between 1000 and 1300 hours with little evidence of nocturnal activity. Daily activity of P. melanoleucus is strongly bimodal during summer with peaks in mid-morning and earlu evening. At the time of capture, most snakes for both species had a cloacal temperature of about 30° C when the air temperature was about 22° C and the subrstrate temperature was about 33° C. However, C. virdis had a narrower range of eccritic cloacal temperatures. Based on recapture data, 40% of all recaptures of C. viridid occured within 5 m of the originaal capture point. Of movements over 5 m, 70% were between 30 to 150 m. Limited recapture data on P. melanoleucus indicate they are more vagile. Both species appear to fit the concept of a total range . Male C. viridis are significantly larger than femaes, while both sexes of P. melanoleucus are essentially equal in size. The sex ratio is nearly equal for C. viridis, but male P. melanoleucus clearly outnumber females due to a supposed increaes mortalit in females. Female C. viridis grow more slowly than makes. Two-year-old P. melanoleucus show a spurt in grwoth which is believed to be associated with a change in prey utilized. Females for both species reproduce for the first time in their fourth year. Male C. viridis had sperm present in the vas deferens by the fall of their second year, while male P. melanoleucus reached this condition by late summer of their third year. Ovulation occurred in early June for both species. Limited observations indicate that young C. viridis are born from mid-September to October, while P. melanoleucus hatch in October indicating a shorter developmental period for the live-bearing C. viridis. Mean clutch sizes were 8.3 and 6.9, respectively, for C. viridis and P. melanoleucus. Annual reproductive effort for P. melanoleucus. Both species show a strong correlation between clutch size and female size (r = 0.78 C. viridis; r = 0.86 P. melanoleucus). Fat bodies as a percent of body weight are larger in C. viridis than P. melanoleucus. Also, females of both species tend to have larger fat bodies than males. Nonreproductive mature female C. viridis have larger fat bodies than gravid or pregnant females. Males of both species tend to have excess fat reserves, so that seasonal fat body cycles can not be identified. Fat bodies were used primarily for reproduction in both specis [sic.]. Based on drift fence captures, mean densities were 0.6 snakes/ha for C. viridis and 1.3 snakes/ha for P. melanoleucus. C. viridis occurred at a high density (6.9 snakes/ha) in rocky habitats such as the canyon rim and basalt outcrops, but were rare in all other habitats. The density of P. melanoleucus was between about 1 and 2 snakes/ha throughout most of the study area. More than 80% of the adult C. viridis diet was composed of Spermophilus townsendi, while P. melanoleucus took a variety of small mammals. In general, C. viridis was seen as an ecological specialist and P. melanoleucus an ecological generalist. Feeding studies with captive snakes indicated annual consumption rates of 300% body weight (3.0%/day) for young C. viridis and 160% (1.6%/day) for adult C. viridis. P. melanoleucus had corresponding annual consumption rates of 220% (2.2%/day) for young and 150% (1.5%/day) for adults. Production efficiency (total production of body tissues/total weight of ingested prey) was 30% and 28%, respectively, for young and adult P. melanoleucus. C. viridis annually take an estimated 25% of the Spermophilus townsendi, while P. melanoleucus take 10% of the S. townsendi, 20% of the Sylvilagus nuttalli and 10% of the Permomyscus maniculatus. The patchy distribution of C. viridis suggests that they would have a high impact on S. townsendi only in localized areas of high snake density, but little impact elsewhere