A regular Source column, Undercurrents explores issues, ideas and events developing beneath the surface in the Virgin Islands community.
From the “croak” to the “coqui,” frog calls can make for some noisy nights in the tropics.
No one knows that better than Renata Platenberg, a terrestrial wildlife biologist who has been studying frogs and recording their calls for years, listening for signs of changes and hints about the health of the Virgin Islands environment.
This month she’s asking for help via a citizens’ science campaign that relies on volunteer’s own cell phones. She launched the first Great VI Frog Count in 2015 and is repeating the effort this year. See below for details.
Her regular research utilizes more specialized equipment.
“I have seven recorders. They all have names so I can keep track of them,” Platenberg said. “I try to have some out all the time.” Some of the recorders are acoustic, some ultrasonic, and some are both, though not simultaneously. The acoustic are for frogs. She uses the ultrasonic for bats, which she also has studied for many years.
To record frogs, she sets up the devices at randomly selected survey points. The next day, she collects the recordings, listens, downloads and analyzes the sounds.
“What I’m looking for is whether all the species (normally found in a given area) are present and calling … If it’s a loud wall of sound, or if it’s just one or two calling,” she said.
The recordings tell when a frog starts calling, whether it takes a break, whether there’s a responding call, whether the activity is robust or feeble and whether there are competing sounds – perhaps from other species or even from humans or other animals.
There are six types of frogs in the territory, four of them native, as well as one kind of toad.
“We share a lot of our species with Puerto Rico,” Platenberg said. At one time Puerto Rico, Vieques, Culebra, St. Thomas, St. John and all the British Virgin Islands were all part of the same land mass, now known as the Puerto Rican Bank.
Some of the native species are rain frogs, or Eleutherodactylidae. They don’t lay their eggs in ponds or other standing water; they have adapted to live in regions where there is little standing water but plenty of moisture in the air. They lay their eggs on leaves and the animals develop into tadpoles within the eggs, not in water.
“They don’t have an aquatic stage in their life cycle,” Platenberg said.
While other frogs lay thousands of eggs, rain frogs lay a fraction of that. But one parent – generally the male – sticks around to protect those few eggs until they hatch into little frogs, so the mortality rate is low. Conversely, frog eggs and tadpoles in ponds are subject to a wide range of predators, so few make it to adulthood. Things even out.
In a variation, the so-called ditch frog, Leptodactylidae, lays its eggs not in standing water, but in anticipation. It can lay its eggs in a crevice in a rock and then whip up a foam to act as a moist medium until there’s a good rainfall that creates a little bit of standing water for tadpoles.
“These are the guys who are in the puddles,” Platenberg said.
One species native to the U.S. and the British Virgin Islands is found nowhere else in the world, not even Puerto Rico. That has led to speculation that the “mute frog” did not originate on Puerto Rican Bank, but rather may have evolved on St. Croix where there were no other species to compete for attention.
“The mute frog is a quiet little guy,” Platenberg said, but it is not actually – as originally believed – silent. Rather it has a very soft call.
Most frogs start up around sundown and keep up the racket until about 9 p.m. when they finally shut up, she said. Later, when it’s nice and quiet in the middle of the night, the mute frog takes its turn.
Action across the board is strongest from May to December, Platenberg said.
One of the loudest frogs is named for the sound it makes, and its voice is beginning to dominate in some parts of the territory. The tiny Puerto Rico coqui was introduced on St. Thomas sometime after 1950, Platenberg said. It probably came in with construction materials. It likes to live in crevices and is often found in pots in garden centers. It needs a moist environment in order to reproduce, and so, she theorized, any males that landed on St. Thomas’ dry East End would have set up futile calls for a year or two and eventually perished without offspring.
But it was a different story in the western and northwestern part of the island where the perpetual damp made a welcoming home. Even so, the coqui remained pretty much confined to the Crown Mountain area, she said, until 2005 and the next year or so when rainfall spiked. With that, the coqui began to travel across the Northside of the island.
“They’re slow to move, but once they get a hand-hold, they really pull the freight,” Platenberg said. Now, in some parts of St. Thomas, “Coquis are kind of pushing out the other frogs.”
Maintaining a balance is important because most frogs live on the same food supply – beetles, weevils, spiders, mosquitoes and other “bugs.” They keep insects – and diseases they may carry – in check.
“They’re the first line guard against Zika,” Platenberg said, referring to the virus carried by mosquitoes.
There are more than 6,000 species of frogs known in the world. The six in the Virgin Islands are, natives: the Antillean frog or Churi Coqui, Eleutherodactylus antillensis; the Whistling Frog, Eleutherodactylus cochranae; the Mute frog, Eleutherodactylus lentus; and the White-lipped frog, Leptodactylus alibilabris; and non-natives or introduced: the Cuban treefrog, Osteopilus septentrionalis and the Puerto Rican coqui, Eleutherodactylus coqui.
Platenberg launched the first Great VI Frog Count via social media. She asked people with a GPS app and audio to make a two-minute recording of outdoor noises in the evening and, with another app, document the location of the recording and send the information on to her. The idea was to get frog calls, but, as she assured volunteers, documenting the absence of frog calls is also important.
The response for the first VI Frog Count was significant.
“There were 300 two-minute recordings,” she said. “I listened to all of them.” She also made a rudimentary map of what species were heard in what areas.
She wants to perfect the map and eventually post it on her website, where it can become a baseline tracking tool. She also is working on designing an app specifically for a future Great VI Frog Count in order to streamline the process both for the volunteers and for the researcher.
This year’s Frog Count started last weekend and will continue until Oct. 15. Volunteers are asked to make their recordings between 7 p.m. and 9 p.m. For more information and to learn how to be a part of the event, visit firstname.lastname@example.org.