Growth and Sexual Size Dimorphism in Northern Water Snakes (Nerodia sipedon)

Copeia ◽  
1999 ◽  
Vol 1999 (3) ◽  
pp. 723 ◽  
Author(s):  
Gregory P. Brown ◽  
Patrick J. Weatherhead
Keyword(s):  
Sexual Size Dimorphism ◽  
Nerodia Sipedon ◽  
Size Dimorphism ◽  
Northern Water Snakes ◽  
Water Snakes

Demography and sexual size dimorphism in northern water snakes, Nerodia sipedon

1999 ◽  
Vol 77 (9) ◽  
pp. 1358-1366 ◽  
Author(s):  
Gregory P Brown ◽  
Patrick J Weatherhead
Keyword(s):  
Sexual Maturity ◽  
Sexual Size Dimorphism ◽  
Survival Rates ◽  
Seasonal Patterns ◽  
Nerodia Sipedon ◽  
Size Dimorphism ◽  
Mortality Cost ◽  
Northern Water Snakes ◽  
Water Snakes ◽  
Better Than

We used data from a 9-year mark-recapture study to determine whether demographic factors could explain female-biased sexual size dimorphism in northern water snakes (Nerodia sipedon). Most males reached sexual maturity at 3 years of age, while most females delayed maturity for an additional year. Female survivorship was not significantly lower than that of males, despite the fact that females grow as much as four times faster than males. Among females, survivorship increased until maturity and decreased thereafter, suggesting a survival cost to reproduction. Life-table calculations indicated that the increase in both survival rates and fecundity with body size made 3 years the optimal age for females to reach sexual maturity. However, if females were not large enough at 3 years of age, their best strategy was to mature the following year. Seasonal patterns of mortality suggest that mating imposes a high mortality cost on males. Intermediate-sized males survived slightly but not significantly better than small and large males. This slight survival advantage of intermediate-sized males was not sufficient to explain why males are so much smaller than females. Therefore other selective factors must be responsible for males retaining a small size. A reproductive advantage associated with small size seems the most likely possibility.

The role of temperature in microhabitat selection by northern water snakes (Nerodia sipedon)

1992 ◽  
Vol 70 (3) ◽  
pp. 417-422 ◽  
Author(s):  
Ian C. Robertson ◽  
Patrick J. Weatherhead
Keyword(s):  
Experimental Manipulation ◽  
Time Of Day ◽  
Microhabitat Selection ◽  
Nerodia Sipedon ◽  
Leopard Frogs ◽  
Water Snake ◽  
Northern Water Snakes ◽  
Water Snakes ◽  
Circadian Patterns

Using field observations and laboratory experiments we examined the role of temperature in microhabitat selection by an eastern Ontario population of northern water snakes (Nerodia sipedon). From 1349 random transects through a marsh we found that basking activity peaked at 09:00 and then declined steadily until 14:00 before increasing again. Our ability to detect snakes depended upon the microhabitat they occupied, and to the time of day when the snakes were encountered in water. In the field, temperatures of basking snakes averaged (±SE) 26.3 ± 0.7 °C (n = 36), while captive snakes in a thermal gradient showed a narrower selectivity, averaging 27.7 ± 0.4 °C (n = 21). The temperatures of basking snakes never exceeded 33 °C, even though a model snake placed in the sun reached 48 °C, suggesting that the snakes were thermoregulating to prevent overheating. In both the field and enclosures, water snakes basked more frequently as the temperature of the air increased relative to the water. Experimental manipulation of water temperature relative to air temperature revealed that temperature influenced microhabitat selection independently of circadian patterns. Finally, when in water, snakes tended to frequent habitats where leopard frogs (Rana pipiens), a common prey species, were most abundant, suggesting that prey distribution may also be an important component of water snake habitat selection.

Variation in offspring sex ratios in the northern water snake (Nerodia sipedon)

1998 ◽  
Vol 76 (12) ◽  
pp. 2200-2206 ◽  
Author(s):  
Patrick J Weatherhead ◽  
Gregory P Brown ◽  
Melanie R Prosser ◽  
Kelley J Kissner
Keyword(s):  
Sex Ratio ◽  
Sex Ratios ◽  
Birth Date ◽  
Nerodia Sipedon ◽  
Water Snake ◽  
In Captivity ◽  
Northern Water Snake ◽  
Northern Water Snakes ◽  
Gravid Females ◽  
Water Snakes

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). Because neonate size did not vary with maternal size, these sex-ratio patterns were independent of each other. These patterns were more pronounced for wild females than for females maintained in captivity while gravid, but rearing conditions did not have a significant effect on sex ratio. Also, because sex ratios were similar between captive and free-living females despite captive females giving birth 16 days earlier, on average, and because sex ratios did not vary with birth date within the two groups of females, gestation appeared not to affect sex ratio. If females vary sex ratios adaptively, only the relationship between sex ratio and neonate size was consistent with our predictions. Limited evidence from other snake species also indicates variation in neonatal sex ratios that is nonrandom but not necessarily adaptive. A better understanding of these patterns will require information on the factors that affect the fitness of male and female neonates differently. An unexpected sex-ratio pattern that we found was that 14 of 19 stillborn young were male. We speculate that this pattern could be a result of male embryonic sensitivity to temperature. Thus, the need for gravid females to maintain a high body temperature so that their young are born with enough time to find hibernation sites may conflict with the need for embryos to develop at a safe temperature.

Sex ratios, mating behavior and sexual size dimorphism of the northern water snake, Nerodia sipedon

1995 ◽  
Vol 36 (5) ◽  
pp. 301-311 ◽  
Author(s):  
Patrick J. Weatherhead ◽  
Frances E. Barry ◽  
Gregory P. Brown ◽  
Mark R. L. Forbes
Keyword(s):  
Mating Behavior ◽  
Sexual Size Dimorphism ◽  
Sex Ratios ◽  
Nerodia Sipedon ◽  
Size Dimorphism ◽  
Water Snake ◽  
Northern Water Snake
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