Monday, October 4, 2010

Snake Life Spans, Body Size and Metabolism

Snakes have the ability to adapt their body size relatively quickly to their environments. This has been demonstrated with Australian Tiger Snakes, and is also suggested by island dwelling pit vipers in China and Brazil.

The Western Terrestrial Garter Snake (Thamnophis elegans) has two ecomorphs, one is long-lived, and the other is short-lived. The long-lived ecotype inhabits grassy meadows at 1.6 to 2.1 km in elevation, is exposed to summer day time temperatures of 15-30ºC, and prefers 28ºC, it is preyed upon by medium sized raptors, feeds on frogs and leeches, has a mean adult body size of 538 mm, females sexually mature in 5 to 7 years at a length of 400 mm, litter sizes average 4.3, and it has a median life span of 8 years. The short-lived ecomorph inhabits rocky shorelines at 1.5 km in elevation, is exposed to summer day time temperatures of 20-35ºC, and prefers 28ºC, it is preyed upon by large sized raptors, feeds on fish and leeches, has a mean adult body size of 660mm, females sexually mature in 3 years at a length of 450 mm, litter sizes average 48.8, it has a median life span of 4 years. The ecomorphs also have different coloration and patterns. 

Now, Anne Bronikowski and David Vleck (2010) at Iowa State University show that the adult, short-lived ecotype of the garter snake has a higher mass-specific resting metabolic rates at any given body mass at normal activity temperatures (15–32.8C). The short-lived snakes reaches a larger body size, and has life-history traits that essentially cause the snake to live fast and die young, but they find that this is a continuum (fast growth, early maturation, high reproductive output) relative to individuals of the small-bodied, long-lived ecotype. The authors suggest the difference between ecotypes in metabolic intensity, even after acclimation to identical conditions in the laboratory, may reflect evolutionary divergence and genetic differences between ecotypes. The difference in metabolic intensity is not present at birth, so an alternative hypothesis is that the developmental environment may permanently influence metabolic rate and life history. And, that the developmental path leads to altered gene expression via environmental influences on the epigenome and results in altered metabolic trajectories in the snakes’ natural habitats.

Full Citation: Bronikowski, A.  and D. Vleck. 2010. Metabolism, Body Size and Life Span: A Case Study in Evolutionarily Divergent Populations of the Garter Snake (Thamnophis elegans) Integrative and  Comparative Biology doi:10.1093/icb/icq132

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