‘Tonic Immobility,’ Female Frogs Play Dead To Avoid Unwanted Male Advances

. ‘Most astonishing behaviour,’ an evolutionary and behavioral ecologist, Carolin Dittrich, gives details of scientific findings published in Royal Society Open Science

Female frogs aren’t hopping to mate with every interested male frog, scientists have found. Instead, they are faking their deaths to escape unwanted attention.

Female European common frogs were observed engaging in “tonic immobility,” essentially feigning their own death to avoid mating, according to a study published in Royal Society Open Science.

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The phenomenon seems to have evolved in order for females to survive an intense and potentially dangerous mating season, Carolin Dittrich, an evolutionary and behavioral ecologist who conducted the research as part of the Natural History Museum Berlin, told ABC News.

European common frogs engage in an “explosive” breeding season, a short season in which males fiercely compete for access to females, which results in scrambling and fighting. Males also may harass, coerce or intimidate females into mating, according to the study.

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Amid the chaos, female frogs are at risk of getting trapped in “mating balls,” in which several males cling to them to vie for their attention, which could lead to their death, Dittrich said.

Dittrich’s research began when trying to determine whether male frogs were choosing female mates with larger bodies, because larger female bodies tend to have more eggs, therefore producing more offspring, she said.

The results from that study showed that the males were not choosing females based on body size, and instead seemed to be interested in all of the females, Dittrich said. The researchers also observed that the females were showing some avoidance behaviors toward the males — a behavior not expected to occur in this species because “explosive” breeders typically have a short timeframe for mating season, Dittrich said.

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Among the avoidance behaviors the females exhibited included a turning motion, in which they turn and twist their bodies to get out of the grip of the males — a technique used more successfully by smaller females — as well as engaging in a call that is similar in the frequency and structure to the calls males make.

However, the “most astonishing” behavior females exhibited to avoid male attention was tonic immobility, or feigning their own death, Dittrich said.

Female European common frogs do not have many opportunities to increase their fitness because they reproduce once a season, which is what likely led to the evolution of the avoidant behavior instead, Dittrich said.

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The researchers observed female European common frogs stretching their arms and legs straight from the body, in a way that could appear similar to rigor mortis, Dittrich said.

There is very little literature to support other vertebrate species feigning their own deaths to avoid mating, Dittrich said.

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While faking death has previously been observed in amphibians, spiders and dragonflies, the purpose is typically to avoid being detected by a predator, she added.

Credit: abc NEWS

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