Lifetime Reproduction in a Northern Metapopulation of Timber Rattlesnakes (Crotalus horridus)

Lifetime reproduction is a fundamental demographic variable for all vertebrate species, but rarely has been examined in snakes. In the southeastern Adirondack Mountains of New York, I used a capture-recapture method over a 36-yr period to study the lifetime reproductive biology of Crotalus horridus in a metapopulation of communal overwintering dens. These longitudinal data allowed analysis of the following parameters for many females throughout their reproductive lives: age and size at first reproduction, reproductive cycle length, duration and frequency of reproduction, body mass changes, and fecundity. Within the metapopulation, first reproduction occurred at a mean age of 9.6 yr, mean length of reproductive cycles was 4.2 yr, mean reproductive life span was 9.6 yr, and average fecundity was 7.7 offspring per litter. Nonviable offspring were observed in 20% of field litters. Although several female snakes produced multiple broods (as many as seven litters over spans as long as 32 yr), most females reproduced only once-the mean number of lifetime reproductive events was 1.7 efforts per female. During their gravid year, most females lost 2-3% of their initial mass, but gains and losses as great as 15-20% indicate feeding during gestation. Microgeographic differences were evident within the metapopulation and might be correlated with (1) resource levels that could influence growth rates, or (2) human encounters that could influence survival rates. Conserving C. horridus in its northern range depends on recognizing the importance of a suite of reproductive constraints-late age of maturity, long reproductive cycles, and low number of lifetime efforts-that clearly influence the viability and persistence of local populations.