When you think of an animal playing dead, especially in North America, you probably picture the Virginia Opossum (Didelphis virginiana), commonly known in the region simply as a possum. It’s such an iconic scene that anyone or anything feigning death can be said to be “playing possum.”
Biologists call the behavior “tonic immobility,” or sometimes “thanatosis,” and until recently, it was associated almost exclusively with prey animals playing dead to escape predators. But Carolin Dittrich, a former post-doctoral staffer at the Natural History Museum in Berlin, noticed that some of the European common frogs (Rana temporaria) she was studying exhibited the behavior during mating, too. She published her findings in the journal Royal Society Open Science on Oct. 11, 2023.
Dittrich told Snopes via Zoom that Rana temporaria is an “explosive breeding” frog, meaning that they reproduce in a brief window of the year, and thousands of frogs gather in one area at the same time. In these species, the males often significantly outnumber the females and harass and coerce them into amplexus, which is the technical term used for mating in amphibians. Research prior to Dittrich's study suggested that female frogs in explosive breeding species are passive during the mating process, but she found the exact opposite.
In fact, Dittrich wasn’t initially studying the behavior of the females at all. The experiment she ran was to determine whether male frogs had a size preference when selecting mates — that is, did they prefer a larger- or smaller-sized mate? In the experiment, one male was placed in a box filled with 5 cm (2 inches) of water and two females of different sizes, and their behavior was recorded via webcam for an hour.
“But they didn’t show any size preference,” Dittrich said.
Instead, Dittrich noticed three behaviors that the female frogs repeatedly used when they wanted to avoid mating: physically rotating away, mimicking a mating call and tonic immobility. Rotating away from the male was the most common strategy — since amplexus must occur with the male behind and on top of the female, the females would flip onto their back to push the male frogs underwater. Dittrich recorded that behavior in 83% of amplexed females, and it was almost always the first attempted strategy.
If rotation was unsuccessful, 48% of the amplexed females began mimicking mating calls, as if to communicate, “I’m a male frog, not a female frog.” The tonic immobility was a last resort — 33% of amplexed female frogs stiffened their limbs, with “arms and legs outstretched from the body after being amplexed by a male.”
Interestingly, smaller frogs were more likely to go into tonic immobility than larger frogs. Dittrich theorized this might be because tonic immobility is not a conscious condition, but a response to high levels of stress. Smaller females could be younger, less experienced and more prone to high levels of stress than larger females, and thus more likely to show tonic immobility.
Lindsey Swierk, an assistant research professor at New York’s Binghamton University who was not involved with the study, told Snopes in an email that she thought Dittrich's study was a great look into how female frogs of explosively mating species avoid potentially dangerous situations.
“Mating balls of multiple males and a single female are nasty and can be fatal for females,” she wrote. “Part of why I like this study is because it brings attention to the fact that females aren't just 'sitting ducks' in explosive mating systems."
Swierk said she hasn’t seen tonic immobility in the species of frog she works with (Rana sylvatica), but she has seen females avoiding and escaping their mates in other ways. She also wondered about the efficacy of tonic immobility as an avoidance strategy, pointing out that some other species of frogs still attempt to mate with clearly dead females. Dittrich also pointed out limitations of her research, explaining that her experiment's laboratory setting is a far cry from a natural pond.
The findings placed the European common frog in a small group of animals who have gone into tonic immobility during mating. Previous research has described similar responses in a spider (Pisaura mirabilis), a newt (Pleurodeles waltl) and a dragonfly (Aeshna juncea). But some people were drawn to the behaviors Dittrich described for another reason: humans are always more similar to animals than we like to admit.
“I saw comments online of people saying things like 'feels like home,'" Dittrich said.
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