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Abstract
Georges Island supports an unusually dense and morphologically diverse population of Thamnophis sirtalis pallidulus. Between May and October 1993, a total of 391 garter snakes were marked. The total male:female ratio was 0.8:1.0; population density of snakes > 1 year old was minimally estimated at 120/ha. Thamnophis sirtalis pallidulus on Georges Island were docile: 50% of encounters showed no response or only a delayed anti-predator response to human handling. The population is unusual morphologically; it contains melanistic and partly melanistic individuals and an unusually high proportion (48%) of individuals with abnormal ventral and/or subcaudal scales. Adult garter snakes on Georges Island are slightly smaller than adults in mainland Nova Scotia populations. Despite a less diverse diet than is available to its mainland counterparts, the island population appears to thrive.
Introduction
Georges Island is a 5-hectare drumlin located in Halifax Harbour, NS (44°38'N; 63°35'W; maximum elevation 23 m above sea level) (Fig. 1). The climate on the island is less extreme than on the adjacent mainland due to maritime moderation. The island, once covered by coastal forest, was cleared and is now dominated by old-field vegetation and a complex of stone walls from a former fort (circa 1794).
In response to concern that development of Georges Island as a national historic site might negatively impact the resident Thamnophis sirtalis pallidulus (Allen) (Maritime Garter Snake) (nomenclature follows Collins 1990) population, we report the distribution and seasonal patterns of activity of snakes on the island. We assess the degree of morphological and genetic divergence of this island population from conspecific mainland Nova Scotia populations. The island population is morphologically diverse and unusually dense, despite an apparently low diversity of potential prey species.
Little work has been done on garter snakes in the Maritime Provinces (Bleakney 1959, Gilhen 1984, Gregory and Larsen 1993). Preliminary research regarding Georges Island snakes revealed some apparently contradictory observations. A lighthouse keeper and his family lived on the island until the early 1960s, but have no recollection of garter snakes being present; nevertheless, garter snakes were recently reported to be very abundant (Barnes 1994). Unlike adjacent mainland populations, Maritime Garter Snakes on Georges Island are docile and have an increased frequency of melanism (John Gilhen, Nova Scotia Museum of Natural History, Halifax,...