We used data from 88 litters of northern water snakes (Nerodia sipedon) to test predictions about how mothers would adaptively vary the sex ratios of their offspring. Larger mothers produced significantly more daughters (r2 = 0.04, P = 0.05), and mothers producing larger offspring produced significantly more daughters (r2 = 0.06, P = 0.02).
We conducted this study at the Queen's University Biological Station in southeastern Ontario, Canada (4537N, 76o13'W), during 5 years (1990 and 1994-1997). We had two categories of adult females (>55 cm snout-vent length (SVL); Weatherhead et al. 1995) as follows. (1) Captive females: females that were caught in late April or early May of each year (except 1990) immediately after emergence from hibernation and prior to mating (early May to early June). These females were housed with adult males during the mating season, following which the males were released. Captive females were kept in captivity until they gave birth or we were certain that they were not gravid, after which they were returned to the wild. (2) Free-living females: females that were not captured until early August just prior to parturition and were returned to the wild following parturition. When in captivity, all snakes were held in heated rooms in tanks lined with artificial grass carpeting or wood shavings. Each tank contained a water dish large enough to allow the snake to submerge and most had heating rocks or coiled heating cables. Room temperature varied with ambient conditions but was maintained above 22 deg C. Snakes were fed minnows (live or previously frozen) ad libitum one to three times per week. Additional methodological details are available in Barry et al. (1992) for 1990 and Brown and Weatherhead (1997) for 1994-1997. The snakes used in this study were cared for in accordance with the principles and guidelines of the Canadian Council on Animal Care.