Three miles off Florida's west coast, the Milky Way blazes over Seahorse Key. Thousands of stars reflect in the glassy surface of the Gulf of Mexico, but Mark Sandfoss keeps his eyes fixed on the ground. He’s watching for snakes.
Checking the path ahead for cottonmouths is a good idea for anyone visiting this uninhabited island in the Cedar Keys National Wildlife Refuge. But Sandfoss isn’t hoping to avoid snakes. He’s hoping to find them.
And lately, they’re getting harder to find.
His headlamp rakes the wrack piled up at the high-tide line. At night, some of the island’s cottonmouths leave the wooded interior and head for the beach, possibly to forage for dead fish. Fish used to rain down on them from the trees courtesy of the nesting birds above, whose regurgitated offerings often missed the open beaks of their young and fell to the snakes below. For decades, the snakes feasted on these leftovers, growing up to a meter long and thick as baseball bats, proportions unheard of in the rest of the state.
Nearly everything about these cottonmouths, in fact, is unheard of. What is a snake that lives in swamps and rivers doing three miles out in the Gulf? How did a creature armed with venom for killing live prey come to rely on carrion? Why are the snakes not eating the birds, and vice versa? How do they survive on an island with no fresh water source? In the 1930s, famed ecologist Archie Carr wrote that "there is little about these island cottonmouths that I do understand." University of Florida researchers like Sandfoss have been studying these questions ever since, and the university has maintained a lab on the island since 1951. But now a new question has supplanted all of the others: What will the snakes do next? Because in April 2015, the birds roosting on Seahorse Key left, and they haven’t come back.
Sandfoss sees something in the wrack, or rather he sees nothing where something ought to be. After three years of stalking the island after sundown, he's developed a knack for spotting areas that are darker than the rest of the night. Nine times out of ten, those patches are cottonmouths, whose brown and tan coloration darkens to black as they age.
Sandfoss sees something in the wrack, or rather he sees nothing where something ought to be. After three years of stalking the island after sundown, he's developed a knack for spotting areas that are darker than the rest of the night. Nine times out of ten, those patches are cottonmouths, whose brown and tan coloration darkens to black as they age.
He carries a five-gallon paint bucket and an aluminum snake hook topped with a golf club grip emblazoned with a pattern of black widow spiders. His only other tools are a microchip reader and a handheld GPS. There is no antivenin on the island. He’s never needed it: These cottonmouths defy the reputation of their species to stand their ground. Maybe life on Seahorse made the snakes more docile: Instead of hunting prey, they lazed under the trees and waited for a meal to fall to them. Scientists had theorized that the relationship held mutual benefit: the snakes got easy meals, while the birds – herons, egrets, pelicans, ibises and others – got a carpet of snakes to scare off climbing predators like raccoons and rats. It was an intriguing theory that needed some hard data to back it up. A doctoral student in zoology, Sandfoss began his dissertation trying to quantify the benefits the birds provided to the snakes.
Then the birds left.
Just why the birds stopped nesting on Seahorse Key remains uncertain after more than a year of study and widespread inquiry from the local community to the New York Times, with theories from Russian subs to government helicopters (which at least one leading scientist considers a real possibility). But while everyone was wondering about the birds, only Sandfoss and his collaborators are researching the snakes.
"The birds leaving is a big event, and showing how this has affected the snake population is very interesting. Getting the public and others to become interested in the natural world is one of the best services we can provide as scientists for conservation," he says. “People that care about animals tend to want to conserve them.”
He knows caring about snakes is a tough sell, but he also sees that they're emblematic of the widespread fallout that can happen when some species are removed from an ecosystem.
"The natural world is complex," he says. “There are so many interactions between species that are unexpected, often unknown, and incredibly strong.”
As Sandfoss attempts to pass the hoop-shaped microchip reader over its body, its only response is to try to slither away. He gently lifts it with the hook, lowers it into a bucket and reaches down with gloved hands to press the lid on.
The snake isn’t microchipped, so it’s going back to the UF campus to be the first recipient of a new radio tag that will let him track its movements around the island. It’s not as emaciated as some he’s seen – the worst cases are knobby as driftwood and speckled with the ticks that plague them as they get weaker. Still, he’ll need to feed it for a few weeks — dead fish, naturally — before implanting the tag.
The radio tags will enable Sandfoss to monitor the snakes’ home range and activity to see how they have changed since the birds departed. One of the main questions is how – and what – they’re eating. They might be surviving on lizards, skinks and frogs. Some might be turning their attention to the island’s rat population. They might be scavenging for fish on their nighttime forays to the beach. One thing they’re definitely eating is each other.
"It’s getting kind of sad on Seahorse," Sandfoss says.
He reaches the west end of the island, where Hurricane Hermine has left bare-swept sand where palm fronds used to blanket the shore.
“I’m worried about my snakes, especially the little ones,” he says. He wonders if they’ve been swept into the Gulf with the ground cover.
As he walks back along the waterline, his rubber boots ignite green bursts of luminescent plankton that flare like tiny fireworks beneath the surface. Then he forges on to the east end of the island, looking for more snakes.
He doesn’t find any.
There are hopeful signs, though. On Snake Key, where some of the nesting birds relocated after leaving Seahorse, the cottonmouths are eating so well that their scales barely overlap. The Seahorse snakes will have to survive some leaner years, but Sandfoss thinks the population will endure.
“The snakes will figure it out,” he says. “The question is, will I figure out the snakes?”
Find out what Sandfoss' research uncovered in the College of Liberal Arts and Sciences' story, "The Starving Snakes of Seahorse Key"
