For the common rain frog, mating begins with a tight embrace.
The male, a patchy brown-and-white creature, hops over to its chosen partner and hangs on to her behind. Its short arms—unable to wrap around the far more rotund and ball-like female—rest on her backside. As the pair bury themselves into the soil and begin the reproductive process—releasing eggs and sperm at the same time—the male remains attached to the female as if with glue.
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Though it’s long been known that common rain frogs, scientific name Breviceps adspersus, secrete an adhesive while they mate, scientists had not been able to determine which of the sexes possessed the special glue. New research from Japan has found the answer: The fixation is mutual.
Atsushi Kurabayashi, who leads the study at Nagahama Institute of Bio-Science and Technology in Japan, told VICE World News he artificially aroused the frogs to get their skin secretion using a device called a Transcutaneous Amphibian Stimulator. The animals were anesthetized first before getting prodded by the machine, which applies a weak electric current to stimulate glands that produce the secretion.
After testing 20 frogs—10 for each sex—Kurabayashi found that the males secreted the glue on their chest and arms, while the females produced an adhesive on their back, where the male latches on.
Using a tensile strength indicator, the scientists also found out how sticky the frogs’ glue is: very. Their bond becomes as strong as velcro after sticking together for about an hour, he said, and the secretion naturally loses its adhesive properties only after three days. (Frogs are known to latch on to each other sometimes for months to lay eggs.)
“So if humans wanted to use this secretion, maybe the way would be as post-it notes—ones that you’d write for 3-day tasks,” he joked. The findings were published in February in Salamandra, a German journal of herpetology—the study of amphibians and reptiles.
Aaron Bauer, a biology professor at Villanova University who has published papers on the common rain frog species, said the research clarified assumptions about the frogs.
“It suggests that the animals are pretty good at not wasting the secretion, that they’re only going to produce it where it’s actually going to be of use to them,” Bauer, who is not affiliated with the study, told VICE World News.
There are over 5,000 known frog species. The genus Breviceps, simply known as rain frogs, are mainly found in barren areas across Eastern and Southern Africa. All known species of this genus are subterranean, meaning they burrow underground—where they spend a majority of their time—during the dry season and pop out during the wet months to breed. The amphibians also usually mate at night.
That means scientists only have a narrow window of opportunity to study their mating habits. “If you want to see the moment at which they mate, you have to stay up all night,” Kurabayashi said.
What the researchers in Japan haven’t been able to find out, however, is why the females also secrete the adhesive, given that just the males’ is sticky enough.
A case in point: When Kurabayashi and his team were doing fieldwork in South Africa, they stumbled upon a male common rain frog attached to the back of a female frog of a different species, one that doesn’t produce an adhesive.
“We didn’t aim for that to happen, but we just accidentally found that male frog cheating on his species,” he said. That’s another mystery the animal bioscientist wants to solve.
