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The Starving Snakes of Seahorse Key

UF zoologists discovered a strange harmony between snakes and birds in the Cedar Keys. Now they're studying how it's fallen apart.

Story by UF College of Liberal Arts and Sciences November 9th, 2017

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. Mark Sandfoss, UF zoology doctoral student, is among the latest of several generations of scientists studying the unique symbiosis between cottonmouths and the island’s waterbirds.

Mark Sandfoss places a cottonmouth in a 5-gallon bucket. He tags and tracks some and brings others back for examination. (B. Brzezinski)
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Cottonmouths are the world’s only semi-aquatic viper and also one of the few snakes that eat carrion. 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, in April 2015, 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 Sandfoss, who was two years into his research on waterbird–cottonmouth mutualism in the Cedar Keys, only to discover that one party was vanishing. 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.


The Cedar Keys are nesting and feeding locations for great egrets and cormorants, among others.
A great egret with three hatchlings.
An amazing variety of waterbirds nested on Seahorse Key — until 2015. (Andrea Westmoreland)

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.

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Cottonmouths are fond of water. (Brent Anderson)
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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.

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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.



Read the full story on our publications website.




Footnote: Story by Rachel Wayne. Portraits of Sandfoss by Bernard Brezinski, UF Photography.