Female common frogs are not as helpless as expected when confronted with the “intrusive” behavior of males during the mating season: two biologists have discovered previously unknown defense techniques used by the harassed female frogs. If they can't free themselves from the clutches through stubborn movements or feigned masculinity, they apparently even play dead, the scientists have observed.
They hop through the meadows, forests and gardens of Germany in many places: grass frogs (Rana temporaria) are among the most common and best-known representatives of our native amphibian world. But not everything is known about these prominent amphibians. In the current case, a special aspect of the mating behavior of grass frogs was the focus of biologists Carolin Dittrich and Mark-Oliver Rödel from the Museum für Naturkunde Berlin: They wanted to test the previous assumption that the females of these frogs were only passive “victims” in the wild reproductive struggle in the pond are.
Female frog in a stranglehold
As the biologists explain, the common frog is a so-called explosive spawning species. This is the name given to amphibians that gather together over a period of time, sometimes only a few days, in order to produce masses of offspring in one fell swoop. In the spring, the grass frogs sometimes come in thousands from their diverse habitats to the nearby waters.
Since female common frogs have to reach an older age to reach sexual maturity, their ranks are more thinned out compared to those of males. This means that the males are clearly in the majority and compete fiercely for the chance to fertilize the females' spawn while they hold them tightly. Accordingly, they act reflexively and brutally - often to the detriment of the females: sometimes many males even clasp a "victim" at the same time and a "mating ball" is formed. This onslaught, in which even misguided male common toads sometimes take part, can be fatal for the females.
Fake masculinity and rigidity to death
In order to examine the mating behavior of grass frogs in more detail, Dittrich and Rödel filmed the amphibians as part of their study and then analyzed the behavior and sounds of the amphibians. They discovered that the females can definitely defend themselves against unwanted suitors - using various strategies. Most often they show defensive movement behavior in order to escape the male stranglehold: they rotate around their own body axis. The researchers also found that distressed females sometimes make a low-frequency “grunt” sound. This is apparently a clever trick. Because it means "Let go of me - I'm a male!" As biologists explain, frogmen usually produce it in order to free themselves from the clutches of overconfident rivals.
The most surprising thing, however, was the third strategy that the team discovered: heavily harassed female frogs feign lifelessness in order to drive the males away from desire. They stretch their arms and legs stiffly away from their body. They then maintain this state, known in technical language as tonic immobility, until the disappointed male lets them go. “We suspect that this defensive behavior evolved to protect the female from forming mating balls, which often lead to her death,” says Rödel. “By calling, the females can show that they are not ready to mate and if this does not help, stressed females can fall into this tonic immobility,” says Rödel.
Finally, Dittrich emphasizes the biological significance of this discovery: “Making dead in connection with mating is exceptional and is very rarely observed. I know of only a few studies that have found tonic immobility in connection with mating, for example in spiders or dragonflies. It is generally assumed that this strategy is used last to avoid being eaten by predators,” says the scientist. But the female grass frogs apparently also use this drastic strategy on their “predatory” admirers.
Source: Museum für Naturkunde Berlin, specialist articles: R. Soc. OpenSci. doi: 10.1098/rsos.230742