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The Starving Snakes of Seahorse Key

UF zoologists discover a mysterious disappearance of seabirds on a Florida key.

Contact: Mark Sandfoss
Harvey Lillywhite

Mysteriously vanished waterbirds. Cannibalistic snakes. An island with no freshwater except for rainfall. It may sound like a Crichton novel or SyFy original movie, but it’s the reality of Seahorse Key, part of the Gulf Coast Cedar Keys that University of Florida biologists have been researching since the 1930s, when the renowned late zoologist Archie Carr first began studying the unusually large cottonmouth population there.

Cottonmouths are the world’s only semi-aquatic viper and also one of the few snakes that eat carrion. Unlike many other Florida snakes that eagerly consume bird eggs and hatchlings, cottonmouths enjoy a more pescatarian diet, which made them a great neighbor for the birds that nested on the keys. Birds are notoriously messy eaters and, on Seahorse Key, pelicans, cormorants, and other waterbirds made a smorgasbord of fish scraps for the cottonmouths, that also ate the rats lurking around the nests in hopes of snatching an egg or two.

This beneficial arrangement collapsed when the birds suddenly abandoned their nests. In the western part of Seahorse Key, where most of the rookeries were, cottonmouths have struggled ever since, limited to the occasional stranded fish or forced to eat each other. So discovered Mark Sandfoss, UF zoology doctoral student, who was two years into his research on waterbird–cottonmouth mutualism in the Cedar Keys, only to discover that one party was vanishing. In April 2015, the birds abandoned their rookeries on Seahorse Key for reasons still unknown, and as Sandfoss continued to capture, tag, and track the snakes, he found that many of them were small, weak, and dying. The research was published on Nov. 6, 2017, in the Journal of Zoology.

Sandfoss is a mentee of UF biology professor Harvey Lillywhite, who was director of Seahorse Key Marine Laboratory from 1998 to 2012 and who has studied the cottonmouths for almost two decades. “I knew, and was curious about, the dense population of cottonmouths on Seahorse Key for years,” said Lillywhite. “In 1998, I became the director of the Seahorse Key Marine Laboratory and decided to conduct a research program there to better familiarize myself with the island.”

Eleven years prior to Sandfoss beginning his research, Lillywhite and his student and eventual postdoc, Coleman Sheehy, published their observations of the island cottonmouths’ intense foraging behavior, especially below the rookeries. At that time, they noticed that drought was affecting the birds, and therefore the snakes. However, decades of literature showed that the Cedar Keys experienced natural cycles of inhabitation. The total abandonment of the Seahorse Key rookery was unprecedented, but presented what Sandfoss calls a “natural experiment.” What would become of the Seahorse Key cottonmouths?

Over the years, Lillywhite and Sheehy, now the herpetology collection manager at the Florida Museum of Natural History, have continued their annual check-ups of the snakes. Sandfoss was intrigued by their work and cites Lillywhite’s lab as his primary reason for coming to UF.

Cottonmouths (genus Agkistrodon), also called water moccasins, have a broad geographic range, making them a common fright in 16 states. Despite their scare tactic— a threatening gape that shows the white interior of their mouth, hence their name — incidence of human bites is low, and they are not particularly aggressive. They present little danger to humans, said Sandfoss, and “they’re really cool — and nice.” They’re especially so in Seahorse Key, where many have become accustomed to being approached and handled. In Florida, all cottonmouths are the species A. conanti, recently upgraded from a subspecies. (Interestingly, the type specimen for the species description was found just seven miles outside Gainesville.) Fairly tolerant of saltwater, the cottonmouths were able to populate Florida’s barrier islands. Unfortunately, they are not strong oceanic swimmers, and although their island lifestyle is no longer as blissful as it was, they are not well equipped to migrate.

Many snakes are ovivoric (egg-eating) and especially so on islands, where resources are scarce, but the Cedar Keys featured a remarkable symbiotic relationship. Typically, in island ecosystems with both birds and snakes, snakes prey upon the birds, explained Lillywhite. However, at Seahorse Key, the birds feed the snakes in a different way — their table scraps, so to speak. Sandfoss was entranced by the research potential of the Cedar Keys’ denizens: “The behavior and biology of the cottonmouths that survive on these islands is fascinating. It is truly a unique system.”

Although snakes are generally evolved to handle long periods without sustenance, the nesting season had lasted March to November in Seahorse Key, providing a long-term source of energy for the cottonmouths. Despite A. conanti’s guts of steel and exploratory feeding, starvation is still a real possibility and, if not fatal, detrimental to reproductive efforts in a species that bears live young.

Those live young get a taste for their prey in the early days of life, and in the western part of Seahorse Key, where birds bestowed a regular rain of fish detritus upon them, the young quickly lived up to their scientific name, derived from Latin for “fish eater.” Previous research by Lillywhite and Sheehy shows that although cottonmouths won’t turn their nose up at unfamiliar prey, they do develop a palate. On the eastern part of Seahorse Key, where fewer birds roosted before the 2015 abandonment, cottonmouths seem to have adapted to a less fishy diet, and therefore were not as negatively affected by the birds’ departure.

The nearby Snake Key seems to have fared better, although less research has been done there. “Now that bird nesting is occurring on Snake Key and not Seahorse Key we are interested to see how Snake Key cottonmouths will fare as a population,” said Sandfoss. “We suspect that the Snake Key population will receive some positive benefits from the presence of the nesting waterbirds.” The team plans to continue monitoring the keys and tracking the cottonmouth population. Meanwhile, Sandfoss, who received the 2017–18 Seahorse Key Fellowship, is tackling the question of how snakes subsist in an environment virtually free of freshwater — another characteristic of these unusual, island-dwelling snakes. Clearly, life finds a way in the Cedar Keys.