- University of Sydney
Low frequency of reproduction among iteroparous organisms is most often observed among female ectothermic vertebrates and is thought to be a strategy used to defer reproductive costs. We assessed reproductive costs of male water snakes (Nerodia sipedon) to determine why half of adult males abstain from reproduction each year. There was no evidence of a short‐term energetic cost of reproduction. Change in mass did not differ between reproductive and non‐reproductive males during the one‐month mating season or during the entire four‐month activity season. Changes in mass of reproductive males were similar at two sites in which the spatial distribution of females differed. However, there were size‐specific differences in growth and survival between reproductive and non‐reproductive males. Among reproductive males growth rate decreased with body size at a lower rate than among non‐reproductive males. Survival increased with body size for reproductive males, but decreased with body size among non‐reproductive males. Most of the differential survival between reproductive and non‐reproductive males did not occur during the mating season but rather during hibernation. Size‐related differences between reproductive and non‐reproductive males may reflect selection having eliminated low quality males from the larger size classes. Overall our results appear most consistent with there being high variance in male quality, such that the best males can bear the cost of reproducing and still grow and survive as well or better than low quality males that abstain from reproduction.
The study was carried out at two marshes approximately 1 km apart, located 10 km from the Queen's University Biological Station in eastern Ontario, Canada (45°37′N, 76°13′W). Barbs Marsh and Beaver Marsh are 4 ha and 3 ha in area, respectively, and have been illustrated and described elsewhere (Brown and Weatherhead 1999a,c). A difference between the marshes important to this study is the spatial distribution of females during the mating season. At Beaver Marsh the majority of mating occurs on two beaver dams located at either end of the marsh. At Barbs Marsh, reproductive females are more dispersed (mating occurs throughout the marsh) and male home ranges during the mating season are significantly larger than in Beaver Marsh (Brown and Weatherhead 1999c). Estimated mean population sizes are 101 and 84 individuals age one or older in Beaver and Barbs Marshes, respectively (Brown and Weatherhead 1999a).
Most data for this study were collected from 1994–1996, although long‐term marking and monitoring of the population bracketed those years. Each year we searched both marshes intensively at least once each day beginning in mid April and continuing until after all mating activity had ceased. Snakes were captured by hand or in minnow traps and brought to the lab to be sexed, weighed, measured for snout to vent length (SVL) and tail length and have a 100 μl blood sample taken from the caudal vein. We estimated condition using the approach of Weatherhead and Brown (1996), where fat‐free wet mass is estimated from SVL. Subtracting this value from total wet mass estimates fat mass, which is then expressed as a percentage of total mass.
Snakes were permanently marked using subcutaneous passive integrated transponder (PIT) tags. During the mating season we also marked snakes with spots of non‐toxic, acrylic paint. Use of unique color combinations allowed us to identify individuals at a distance in the field without disturbing mating aggregations. Most adults were measured and marked soon after emergence from hibernation, before the mating season began. By the latter half of the mating season almost all individuals observed in aggregations were painted and could be identified. If paint marks faded before the end of the mating season, individuals were captured and repainted, reweighed but usually not remeasured.