Summary
The cornerstone of life-history theory is the expectation that current reproduction will have a detrimental effect on survival and (or) future reproduction. When fecundity increases with body size, the cost to future reproduction arises through decreased growth of reproductive individuals. We investigated the effects of reproduction on aspects of survival and growth in female northern water snakes (Nerodia sipedon). We did not find a decrease in survival associated with mating despite the conspicuousness of mating aggregations, and pregnancy did not impair locomotor ability. We found evidence of a decrease in over-winter survival of reproductive females related to their emaciated state following parturition. Reproductive females grew less in length than nonreproductive females, but increased similarly in mass. Following parturition, reproductive females weighed less than in the spring, indicating that mass gain prior to parturition was invested in the litter and that most foraging occurred prior to ovulation. Captive reproductive females given food ad libitum grew in length at a rate similar to free-living reproductive females, but increased more in mass. Captive females weighed more after giving birth than in the spring, indicating that unlike that of females in the wild, some of their mass increase was due to energy storage, and also that they continued to feed after ovulation. Consistent with the prediction that smaller females would benefit more than larger females from reproducing less and growing more to increase future fecundity, we found that smaller females participated less in mating aggregations and reproduced less often.
Methodology
Materials and methods We conducted the study at the Queen's University Biological Station in eastern Ontario (45 "37 'N, 76" 13 'W). The water snake populations in two beaver ponds, Barb's Marsh and Beaver Marsh, have been marked and studied for the past 8 and 5 years, respectively (Weatherhead and Robertson 1992; Barry et al. 1992; Weatherhead et al. 1995). We studied mating activity at Barb's Marsh from 1990 to 1995 and at Beaver Marsh in 1994 and 1995. We captured as many snakes as possible during late April before mating began (mating generally occurs throughout May). Each snake was sexed by cloaca1 probing and total length, tail length, and mass were measured. A unique colour combination of acrylic paint allowed us to identify individuals participating in mating aggregations from a distance without disturbing their activity. During the mating season we walked transects throughout the marshes at least twice per day and noted the locations and activity of marked snakes. Throughout the active season unpainted snakes were captured whenever they were encountered and then measured and marked. We intermittently used minnow traps set in shallow water as a means of capturing snakes throughout the summer. Details of mating activity and general study methods have been reported previously (Barry et al. 1992; Weatherhead et al. 1995). In early August 1994 and 1995 we made intensive searches to capture adult (i.e., sexually mature) females from Barb's and Beaver marshes. In our study population, female size at sexual maturity, based on the minimum size of females observed in mating aggregations, is 55 cm snout-vent length (SVL) (Weatherhead et al. 1995). Captured females were held in captivity until parturition (late August or early September). Females that did not give birth were kept for 10 days after the last female gave birth, palpated to ensure that they were not gravid, and then released. Females were also captured in early May 1994 and 1995 from sites other than Barb's Marsh or Beaver Marsh, and were mated and then kept in captivity for the entire active period. Mating was achieved by housing females with several males captured from the same population as part of a study investigating paternity (M. Prosser, unpublished data). We refer to the females that were captive only during August as our free-living sample and females that were captive all summer as our captive sample. All captives were held indoors in heated rooms in glass, plastic, or fibreglass tanks. Room temperature varied with ambient conditions but was maintained above 22°C. Each cage was lined with artificial grass carpeting or wood shavings and contained a water dish large enough to allow the snake to submerge. In addition, several of the large fibreglass tanks also had heating rocks. Snakes were fed live or frozen minnows ad libitum 1-3 times per week. For nine of the captive females we recorded the number and mass of minnows eaten between 5 June and 1 August 1995, to allow us to estimate the amount of food required for a given increase in female mass. Additional data on maternal size and reproductive frequency were obtained from a sample of 20 females taken into captivity in 1990 (see Barry et al. 1992) and from 2 snakes kept in captivity in August 1993. Growth data were not available for these individuals.