The Cottonmouth Moccasin on Sea Horse Key, Florida

Abstract

From 1954 to 1957 the author studied the cottonmouth moccasin (Agkistrodon piscivorus) population of the islands near Cedar Key, Levy County; Florida. Sea Horse Key, one of the outer islands, approximately 1 mile long and 7 miles from the mainland, then supported a total population of roughly 600 cottonmouths. The cottonmouths aggregate under the breeding colonies of cormorants and herons on the outer Cedar Keys and scavenge the fish the birds drop accidentally from their nests. After post-nidal feeding stops in August, they eat birds, rats, and squirrels. An abundant skink is the principal food of young cottonmouths. The bird rookeries control distribution and food habits of three-fourths of the island's snakes, serving snakes up to 150 meters distant. Activity ranges are remarkably small (males average 0.43 acre, females 0.35 acre). Although the main ridge lacked rookeries, ranges were similar. No evidence of territoriality was found. Snake Key rookeries also support a large population of cottonmouths. Atsena Otie and North Keys have no rookeries and few cottonmouths. Island snakes den in shallow stump holes and under debris. Frequent warm winter periods make emergency demands on stored fat, and 77 per cent of Sea Horse Key snakes are in danger of starvation during winters with a mean temperature of 16.3° C, compared to 36 per cent of the fatter snakes on Snake Key. The fat bodies of snakes apparently function as reserve food during periods of high temperature. Fat-body weights, length/weight ratios, and feeding behavior suggest that Sea Horse snakes are at a critical survival level. When removed from their winter dens, cottonmouths apparently shun all holes for periods of up to 2 years; a persistant site memory is postulated. At lowest cloacal temperatures (4.0°C) cottonmouths are passive; they are able to strike at 4.5°C, crawl at 12.5°C, and feed at 14.5°C. Variation in observed rates of heating and cooling suggests some physiological control. Up to 36 per cent of cottonmouths showed aggressive behavior at low temperatures contrasted to 4.1 per cent at high readings (21°C and above). The major cause of mortality is starvation. Adults have no enemies other than linguatulid parasites and man. Two healthy and apparently genetically eyeless snakes indicate the relative importance of olfaction in the island environment. The ambithermal cottonmouth, with its endogenous biennial sexual cycle, its vagile nature, and its keen olfaction seems well adapted to preempt the island niche of a terrestial carnivore-scavenger.