The Melanistic Garter Snakes of Georges Island

Authors and Copyright: Nova Scotia Museum of Natural History
http://museum.gov.ns.ca/mnh/nature
**Special thanks go to Debra Burleson for the permission to use it on Thamnophis.com**

Georges Island is a small island in Halifax Harbour. Its old fortifications are of historical interest, and the island is owned by Parks Canada but is not open to the public. In 1992, park workers cutting the grass saw lots of black snakes. They were afraid these were exotic snakes which had escaped from ships, and might be poisonous.

John Gilhen of the Museum staff investigated. John suspected melanistic Garter Snakes because he had seen them on a nearby larger island, McNabs Island. Melanism is a colour form in which individuals have abnormal amounts of black pigment. Its genetic basis is not well understood, but inbreeding is thought to be part of the cause.

The Georges Island snakes were melanistic (black) Garter Snakes, but the variety of colours in the Garter Snake population was just as interesting. "I turned over a board and there were seven Garter Snakes, each one different - one melanistic, one part melanistic, the other five with red, or grey, or no stripes, red belly or grey belly. I had all seven in my hands and they were all different".

Pockets of snakes with a distinctive colour are known, for example, a population with orange-red bellies is found near Petpezwick Inlet and an orange-bellied population at Eureka. But such a variety of colours on a small island is unusual.

The frequency of melanism on Georges island is greater than on the mainland; until that visit John had not seen 5 melanistic Garter Snales in Nova Scotia in a long career, but that day he saw three. These snakes have normal coloured (ie, spotted) young. The part melanistic ones are juveniles and will become blacker as they age.

The Island snakes are also very docile - of 21 he picked up, only one tried to bite. That is not typical of Maritime Garter Snakes. Could it be that on the island, hidden in deep grass so that even seagulls cannot prey on them, the snakes have become non-aggressive? Will this change as Parks Canada cuts the grass and considers opening the Island to public visits?

Coastal areas, including around the Great Lakes, seem to produce more melanistic populations than inland areas. This seems to hold for Red-backed Voles, mice, even melanistic Red Squirrels as well as Garter Snakes.