On a remote Florida island crawling with venomous snakes, a scientist believes he has discovered an unusual truce between predator and prey.
The tiny island of Seahorse Key on the central Gulf Coast is renowned among researchers for its teeming numbers of poisonous cottonmouth snakes.
"The population of cottonmouths on Seahorse Key is large and dense—I mean a lot of snakes," said Harvey Lillywhite, a University of Florida biologist who has been studying the island.
About 600 vipers slither around the 165-acre (67-hectare) island, Lillywhite estimates—in some areas with an average of 22 cottonmouths on every palm tree-covered acre. (See photos of the island's snakes.)
Scientists have long puzzled over how so many snakes can thrive on an island with no fresh water and only a scant number of mammals to prey upon.
The secret to the snakes' success, Lillywhite believes, is Seahorse Key's other inhabitants—tens of thousands of seabirds that nest there from spring to fall.
But the snakes aren't eating the birds, the scientist says—instead they live almost exclusively on the huge amounts of dead fish that the birds drop, vomit, and excrete every year.
"There's this disgusting carrion of fish that falls down for the snakes, and the snakes essentially scavenge on it," Lillywhite said.
In return for this fishy bounty, the cottonmouths not only refrain from eating the birds, the scientist added, they also seem to deter other would-be predators from raiding the nests.
The result is a win-win for both predator and prey that Lillywhite said he has not seen on any other island.
"There are a lot of island systems where there are birds and snakes. Of all the cases I know, the snakes are predators on the birds," he said. "At Seahorse Key, it's totally different. Here the snakes do not eat the birds, and the birds are providing food for [the snakes]. So it's a pretty cool system."
Seahorse Key is a centerpiece of the Cedar Key National Wildlife Refuge, a network of protected islands near the mouth of the Suwannee River (see an interactive map of the Suwannee River region).
The island is home to one of central Florida's biggest rookeries—a nesting site for more than a hundred bird species, including pelicans, ibis, and egrets.
Lillywhite believes it's no accident that the birds prefer this dry, viper-ridden island to other, more hospitable sites in the refuge.
"There's a lot of other nesting habitats for the birds, but the birds don't use them," he said. "They come to Seahorse Key. Why is that? We think the key is the snakes."
To examine this theory, Lillywhite and colleagues began by mapping the locations of both bird nests and snakes. Results showed that most cottonmouths stayed close to the rookery, often directly under nests.
Even without the maps, the team was usually able to tell where snakes had been, Lillywhite said.
"The snakes, which are normally almost jet black, can be almost white, because they curl up under the bird rookery and get pooped on."
A coating of excrement may be a small price to pay, he added, because research so far has revealed that the snakes are getting a steady diet of predigested fish.
Lillywhite has seen cottonmouths foraging for fish firsthand, and has even seen baby snakes taking part in the regurgitated feast.
"I was with a faculty member showing him around [the island], and there was this wonderful example of a plop of half-digested fish," he said. "There were two babies and about four or five other [snakes] … that had been attracted to it. So babies actually may get into this system fairly early."
In addition to field observations, Lillywhite's team is studying chemical signals called isotopes in cottonmouth tissue to find clues to what the snakes are eating.
"We haven't analyzed all the data yet, but based on our observations and limited isotope data, we know that [the snakes] have been largely feeding on fish," Lillywhite said.
What his team has not found, he added, is any sign—from the field or in the lab—that the cottonmouths are preying on birds, no matter how young or defenseless.
"Sometimes chicks fall out of the nests for various reasons, so we see chicks on the ground. But the snakes aren't eating them," he said.
"I think [that's] probably [because the snakes] are full on fish. It's a simple way to look at it, but that seems to be the key."
Mutualism
Lillywhite stressed that his research is ongoing and his findings are "a progress report."
One of the remaining issues to explore, he said, is the degree of mutualism—or shared benefit—that the cottonmouths and seabirds derive from this distinctive dynamic.
Here, Lillywhite suggested, the key may be one of the island's smallest players: the brown rat.
The rats are an invasive species and are "notorious bird-nest predators," he said.
His team has found that cottonmouths near the rookery—while presumably full on fish—are eating enough of the rats to keep them at bay.
"What we have found is, where the cottonmouths are dense, there are fewer rats. And the snakes are largest in numbers where the birds are," he said. "So that's part of the mutualism."
Alan Savitzky is a snake biologist at Virginia's Old Dominion University who is not involved with Lillywhite's research.
"The association between cottonmouths and bird rookeries is unusual but not unique," he said.
"But the situation at Seahorse Key is very interesting because [there's] the deterrence of predators. So you have a mutualism of sorts in which there's a benefit to both species."
He and Lillywhite agreed that the findings on Seahorse Key, preliminary as they are, help burnish the cottonmouths' image as an indiscriminate predator.
"I think the real strength [of the research] is in revealing the flexibility of what we normally regard as rather stereotyped predators," Savitzky said.
"It also suggests that there's probably a greater diversity of interactions between predator and prey across the landscape than we normally recognize."
For Lillywhite, the findings highlight the snakes' crucial role in a vital Gulf Coast ecosystem.
"[The cottonmouths] actually are an important, integral part of the system and probably are the reason that the birds keep nesting—and do so successfully—on Seahorse Key," he said.
"This is unusual, but kind of cool, PR for the lowly snake."
