Turtles communicate to synchronize hatching, socialize
Turtles comprise one of the oldest living groups of reptiles, with hundreds of species found throughout the world. Many have been well-researched, and scientists know very specific things about their various evolutionary histories, metabolic rates, and the ways in which their sexes are determined. But there was one very obvious thing that has been largely left unknown by science until very recently. Turtles can make sounds.
Two new studies published recently in Chelonian Conservation and Biology and Herpetologica find that two turtle species vocalize when they reproduce and during some social interactions, and that their vocalizations are many and varied.
A leatherback sea turtle (Dermochelys coriacea) digs a nest in Suriname. Photo by Tiffany Roufs.
But why exactly did researchers go so long without discovering this aspect to turtle behavior? According to Richard Vogt, a herpetologist and turtle conservationist with the Brazilian Institute for Amazon Research and Director of the Center for Amazon Turtle Conservation, dogmatic assumptions are to blame.
“Because no one studied it, because some of the literature on reptiles published back in the 1950s claimed that turtles were deaf as a stump and did not vocalize, and everyone just believed [it] without investigating it,“ Vogt, who is a coauthor of both studies, told mongabay.com.
However, Vogt wasn’t very surprised when he and his colleagues discovered that turtles were making sounds. He had long suspected this to be the case, but circumstance prevented him from studying it.
“While filming courtship behavior of false map turtles (Graptemys pseudogeographica) in captivity in the mid 1970s for part of my PhD thesis at the University of Wisconsin – Madison I noticed that the males were opening and closing their mouths while they were titillating the females with their vibrating foreclaws, and not trying to bite,” Vogt said.
“At that time only the Navy had hydrophones and since the war protesters had blown up the army math research center at the UW. I did not think it a wise idea to be associated with the military, so my ideas laid fallow until 2005 when an inquisitive Australian student had access to underwater recording equipment and dropped a hydrophone in an aquarium with side necked turtles and found out they were vocalizing.”
But Vogt held on to his curiosity, and later investigated the phenomenon with one of his students while studying turtles in the Amazon River. He attributes part of the reason for the knowledge lag about turtle vocalizations to technological limitations and the fact that the noises turtles produce are low and quiet.
“One of the reasons the sounds were not detected before was the lack of proper recording equipment,” Vogt said. “The sounds are at the lower end of the human audible range, so hard for people over 40 to hear, but young people can hear hatchlings turtles as them emerge from the nest. The sounds besides being low frequency are also low in volume and infrequent, thus easy to overlook. Just the movement of someone swimming in the waster is enough noise to block out the sounds of turtles to the swimmer.”
The studies respectively looked at two very different species: giant Amazon river turtles (Podocnemis expansa) and leatherback sea turtles (Dermochelys coriacea). They found that the river turtles vocalized in all sorts of situations, from interactions between adults to hatchling communication. Leatherbacks, less social turtles, were also found to vocalize as when hatching and as they dispersed from their nests into the sea.
The giant Amazon river turtle is one of the largest freshwater turtles in the world, often reaching one meter (more than three feet) in length and 65 kilograms (143 pounds) in weight. It is found in the Amazon Basin and is quite social, with females gathering in large groups to lay eggs along the riverbanks. Because of this, it can be vulnerable to poaching, with reports of reductions in some populations due to over-harvesting of eggs and hatchlings. While is not currently considered threatened by the IUCN, it is listed as Conservation Dependent.
A giant Amazon river turtle (Podocnemis expansa). Photo by Whaldener Endo.
The leatherback sea turtle is the largest turtle alive today, with lengths averaging 2.2 meters (7.2 feet). It is also the fourth-heaviest reptile species, with an average weight of about 380 kilograms (850 pounds). The largest leatherback ever found was more than three meters (9.8 feet long) and weighed 916 kilograms (2,019 pounds) – more than some small cars. Leatherbacks are currently listed as Vulnerable by the IUCN, with adults threatened by the fishing industry and plastic garbage in the ocean. They may mistake plastic bags and balloons for jellyfish and consume them, potentially causing intestinal blockages. Hatchlings are also sometimes collected and eaten by people.
The studies found both species vocalized when hatching, with sounds described by Vogt as “chirps, clicks, meows, and clucks.” He believes many other species also make sounds as they hatch, and that they do so to coordinate their hatching.
“I think all turtles are vocalizing underwater to some extent. Some species of turtles are more social than others and thus communicate more,” he said.
“Hatchlings of several species of marine turtles as well as the giant Amazon river turtles (Podocnemis expansa) begin making sounds in the egg up to 10 days before they hatch to stimulate synchronous thatching and afterwards synchronous digging out of the nest, and continue vocalizing as they enter the water.”
But why would they evolve this behavior? According to Vogt, emerging from a nest en-masse allows more hatchlings to survive the perilous first minutes of their lives. Without any other defenses, the babies are easy pickings for hungry predators.
“If the hatchlings all leave the nest at once there is safety in numbers, a swamping of predators, thus a few turtles will make it though to the sea,” Vogt said. “There they keep communicating to migrate off in cohesive groups, which again should be safer than trying it alone.
Giant Amazon river turtles have an additional incentive to vocalize.
“The hatchlings of the giant Amazon river turtle respond to the vocalizations of their mothers and migrate with the large adult females away from the nesting beaches to feeding grounds 70 to hundreds of kilometers away,” Vogt said.
But with these new findings come new questions and concerns. Noise pollution at beaches may interfere with nestlings’ abilities to hear each other and synchronize their hatching, thus making them more vulnerable to predators. With many turtle populations declining around the world, this could further hurt many species’ chances of long-term survival. It also is not known how exactly turtles are producing sounds.
According to Vogt, there is “a lot yet to do.”
Leatherback sea turtles are the largest turtles living today. Photo by Tiffany Roufs.
Citations:
- Ferrara, C. R., Vogt, R. C., Sousa-Lima, R. S., Tardio, B. M., & Bernardes, V. C. D. (2014). Sound Communication and Social Behavior in an Amazonian River Turtle (Podocnemis expansa). Herpetologica, 70(2), 149-156.
Chicago- Ferrara, C. R., Vogt, R. C., Harfush, M. R., Sousa-Lima, R. S., Albavera, E., & Tavera, A. (2014). First Evidence of Leatherback Turtle (Dermochelys coriacea) Embryos and Hatchlings Emitting Sounds. Chelonian Conservation and Biology, 13(1), 110-114.
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Credits
Morgan Erickson-Davis Editor - Ferrara, C. R., Vogt, R. C., Harfush, M. R., Sousa-Lima, R. S., Albavera, E., & Tavera, A. (2014). First Evidence of Leatherback Turtle (Dermochelys coriacea) Embryos and Hatchlings Emitting Sounds. Chelonian Conservation and Biology, 13(1), 110-114.
