BRITT, E.J.; HICKS, J.W.; BENNETT, A.F.; Univ. of California, Irvine; Univ. of California, Irvine; Univ. of California, Irvine: Production Efficiency in prey specialist and generalist populations of the garter snake (Thamnophis elegans).
The Western Terrestrial Garter snake, T. elegans, exists in two geographically isolated populations in northern California: a coastal population with a specialized diet of slugs and an inland population with a generalized diet of fish, anurans, mice and leeches. Since some species of garter snakes are slug specialists, the intraspecific variation in T. elegans presents a unique opportunity to study digestion of different prey types in two diverse populations of conspecifics with different niches and selective pressures. The difference in prey preferences between the two populations is congenital, heritable, and ontogenetically stable. In addition, newborn and adult snakes from the inland populations have a high frequency of slug-refusing morphs, in which animals will refuse to ingest slugs (Arnold, 1981). To test whether or not the coastal snakes have an energetic advantage over the inland generalists when digesting slugs, we measured production efficiency of each adult snake population on both slug and fish. The production efficiency was measured upon digestion of the meal and continued until each snake reached a post-absorptive metabolism. By standardizing the meal and determining its caloric content using bomb calorimetry, the amount of energy remaining in the feces gave an estimate of total assimilation energy. The assimilation energy minus the cost of respiration resulted in an estimate of the production energy available for growth, storage and activity. Our experiments show the coastal population has higher production efficiency on both slug and fish diets than does the inland population. Coastal snakes were able to utilize more energy towards production from the ingested food mass by reducing the costs of digestion (SDA). Supported by NSF IBN-0091308 awarded to A.F.B and J.W.H.
