THE MAINTENANCE OF COLOR PATTERN POLYMORPHISM IN LAKE ERIE WATER SNAKES, NERODIA SIPEDON INSULARUM (SELECTION, MIGRATION, CRYPSIS ISOZYMES)
Abstract
Populations of water snakes, Nerodia sipedon, on islands in Lake Erie are polymorphic for color pattern. In addition to the cross-banded morph which typifies this species, a high frequency of intermediate and unbanded morphs occur. Early studies report that natural selection for cryptic coloration favors unbanded morphs on exposed island shorelines and banded morphs in more overgrown mainland habitats. Migration from the mainland is thought to explain the persistence of banded morphs on islands. This study reexamines the importance of selection and migration on this polymorphism based on a long-term capture-mark-recapture program, an electrophoretic analysis of blood proteins, and a survey of museum specimens. Populations of Lake Erie water snakes have decreased in this century and now range from 25 to 500 adults per island. Adults exhibit a male-biased sex ratio during courtship in May and June. Females grow faster and attain larger body sizes than males. Young are born in August and September, and both their number and size are correlated with female size. Island and mainland populations differ in adult body size and growth rate and may differ in size of newborns, diet, and intensity of predation. No evidence of differential selection is apparent among morphs: morph frequencies do not differ among age classes, between once-captured and multiply-captured snakes, or between scarred and unscarred snakes. Furthermore, herring gulls, the commonest snake predators in the island area, appear to detect banded and unbanded model snakes with equal ease. High site fidelity of water snakes and the distribution of morphs among islands suggest that migration from the mainland is not common. However, islands close to each other are similar in morph frequency and water snakes have colonized islands elsewhere in the Great Lakes indicating that some migration does occur. Recent decreases in island population size have apparently increased the effect of migration from the mainland as reflected by increases in banded morphs on the islands. High similarities in blood proteins among island and mainland populations indicate that genetic drift and founder effects have not been important in the 4000 years they have been isolated. Selection may have been important in the establishment of color pattern polymorphism, but its effects are now apparently swamped by migration from the mainland.
Degree
Ph.D.
Subject Area
Ecology
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