A recent study of Ratsnakes (Elaphe obsoleta) in Texas found that adult mortality was higher for females than males, consistent with the cost of reproduction in snakes being higher for females. To determine whether the same pattern prevailed in a northern population of Ratsnakes, we used data collected using radio-telemetry to test several predictions of the cost-of-reproduction hypothesis. Contrary to there being a cost to reproduction, mortality rates did not differ between juvenile and adult snakes and, contrary to females having a higher cost of reproduction, mortality rates among adults did not differ between males and females. The only evidence consistent with reproduction increasing mortality risk was higher winter mortality for females in poor condition following egg laying. Mortality did not vary with activity but increased with time spent basking, although group differences in basking were not sufficient to produce differences in mortality. High risk of winter mortality in this population may require all Ratsnakes to behave in ways that mask mortality costs associated with reproduction. To determine whether our results for Ratsnakes in Ontario are anomalous or reflect something more substantial about the cost of reproduction in snakes, details of mortality patterns from more species, ideally with diverse ecologies, are needed.
Study Species and Area
Data used here were collected as part of a long-term study of Black Ratsnakes (Elaphe obsoleta) at the Queen's University Biological Station in eastern Ontario (44°34′N, 76°19′W). Ratsnakes are oviparous colubrids that reach sexual maturity in our study population at 7–9 years old (Blouin-Demers et al., 2002). We defined juveniles of both sexes as individuals <1,050 mm snout–vent length (SVL) based on the size of the smallest known reproductive female and the smallest male observed mating. Paternity data indicate that some males may mature at a somewhat smaller size (Blouin-Demers et al., 2005), so our estimate of the size at maturation should be considered only a reasonable approximation for the population. Males and females follow similar growth trajectories until sexual maturation, after which females grow slightly slower than males, producing weak male-biased sexual size dimorphism among adults (Blouin-Demers et al., 2002).
We estimated survival directly from radio-tracked individuals caught at hibernacula during spring emergence. We tracked 92 adult snakes between 1996 and 2004 and 35 juveniles between 2001 and 2004 (Blouin-Demers et al., 2007). Radio-transmitters (Model SB-2T [5.5 g, 12-month battery life at 20°C] and SI-2T [12.0 g, 24-month battery life at 20°C], Holohil Systems, Carp, Ontario) were implanted surgically in snakes using isoflurane anesthesia (Weatherhead and Blouin-Demers, 2004). Transmitters represented, at most, 4% of the snake's mass. Snakes were kept overnight in the laboratory and then released at their point of capture. Following release, we located snakes using hand-held receivers and antennas every 2–3 days until the snakes entered hibernation. Tracking was resumed the following spring when snakes became active. To obtain data on reproduction from females, we tracked gravid females to nest sites. Once a female had laid her eggs (determined by palpation after she moved away from the nest), we weighed her. We did not have data to estimate male reproductive effort and assumed that all adult males were reproductive.