The ecology of the northern Pacific rattlesnake, crotalus viridis oreganus, in British Columbia
Date
1985
Authors
Macartney, James Malcolm
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Abstract
The western rattlesnake Crotalus viridis occurs in arid regions from southern Canada to northern Mexico. The ecology of this species is well known from studies conducted in southern and central portions of the range, but less is known about the ecology of northern populations . Life-history parameters of the northern pacific rattlesnake Crotalus viridis oreganus were documented in a three year mark-recapture study of populations at 24 communal hibernacula in the Okanagan valley of south- central British Columbia.
Data collected in this study facilitated an examination of geographic variation in the life-history of this wide ranging species. Rattlesnakes in the Okanagan valley spend about seven months in communal hibernacula (September to April) . The active season is short and foraging is confined mainly to the months of June , July and August. Growth rates of juveniles are lower than reported for conspecific populations from lower latitudes . Males grow more rapidly than females and sexual maturity is attained at 3 to 4 years in males and 7 to 8 years in females.
Mating occurs in late summer or early fall and vitellogenesis is initiated prior to hibernation. Ovulation occurs the subsequent spring and parturition follows in September or October. The average litter size is 4.6 young. Females are capable of reproducing on a biennial basis, but most follow triennial cycles. The frequency of reproduction i s largely determined by a female's ability to accrue body mass (fat reserves) during nongravid years . The slower rate of growth, delayed sexual maturity, and less frequent reproduction in these populations, compared to more southerly populations, is due in part to a shorter active season.
Radiotelemetry was used to determine movements and activity ranges of individual snakes. Rattlesnakes disperse up to 2 km from hibernacula during the summer and may have large activity ranges ( > 8 ha). Gravid females, however, remain in close proximity to dens and have small activity ranges (0.16 to 0.22 ha) . Summer ranges of rattlesnakes from neighbouring dens overlap considerably, but individuals show high fidelity to hibernacula. Observations of inter-den matings suggest that den populations are not reproductively isolated demes. Population size and composition are highly variable among dens , but remain fairly stable within dens. Switching between dens is rare (mostly juveniles) and recruitment into a den population occurs mainly as a result of birth.
Annual survivorship increases with size (age) and adult males experience higher survivorship than adult females. Overwintering survivorship is high (averaging 97 % for males and 86 % for females) and energetic demands during hibernation are low (weight loss averages 5 - 7%). A life table based on estimates of age-specific survivorship and fecundity suggests, tentatively, that these populations are declining (R0 = 0.50). Additional evidence, however, suggests that populations on these sites are above historical levels (1930's - 1950's).