Female Frogs Fake Death to Escape Unwanted Male Attention

Female frogs have developed several ingenious and sneaky tactics to get away from unwanted males attempting to mate with them, one of which includes faking their own deaths. These European common frogs can often be harmed or even killed by males attempting to mate with them during the breeding season, leading them to evolve defensive mechanisms against the advances of the hordes of horny males, according to a new study in the journal Royal Society Open Science. The female frogs usually use tactics such as turning away from the grips of a male, or mimicking the release call of the male to trick him into letting her go, but occasionally, they will pretend to die.

"The most common behavior was turning, where a female tries to turn on her own axis to escape the male's grip," study co-author Carolin Dittrich, a life sciences researcher at the University of Veterinary Medicine's Konrad Lorenz Institute of Ethology in Vienna, and a scientist at the Museum für Naturkunde in Berlin, told Newsweek. "The second behavior was calling. Females produced two different calls, a deeper, low-frequency 'grunt' sound, which is an imitation of the male's release call (when a male is grabbed by another male, they give this release call, it signals that it is a male) and a higher frequency 'squeak' sound, where we are not sure what the communication signal is." "The final and most astonishing behavior was tonic immobility, where females stiffly extend their arms and legs away from their body. Tonic immobility in the context of mating is quite unusual or probably less frequently observed," she said. Other studies have seen this faking death tactic being used in a sexual context in species such as spiders and dragonflies, but it is most commonly used to avoid predators.

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"This immobility can last several minutes. In one of our videos we see that a male is dragging an immobile female until he lets her go. She stays in that immobile position for a couple of minutes until she turns and swims away," Dittrich said. Female frogs may have evolved these defenses due to the process of sexual selection, which arises when the males and females of a species have different reproductive strategies that benefit them the most: the males "want" to mate with as many females as possible during their rapid mating season, while the females don't want to die as a result of being swarmed by the males. "This species is an explosive breeder. An explosive breeding species is an amphibian species with a very short breeding season," Dittrich said. "Breeding usually takes place in early spring within a few days to two weeks, depending on weather conditions. This means that hundreds of individuals gather at the pond at this time. The males fight for access to the females, which are usually the rarer sex in these breeding aggregations. It was previously thought that the females were unable to defend themselves against this male coercion."

"The males don't seem to be very selective, at least in terms of female body size, and will randomly grab other individuals, even males or other species. This behavior leads to the formation of so-called mating balls, where several males can cling to one female, which can lead to the death of the female. So we think that these mate avoidance behaviors have evolved to protect the female from the formation of mating balls," Dittrich said. In other species, sexual conflict can arise as a result of females wanting to choose the best quality male to father her children. "Female animals (and plants) benefit from having their eggs fertilized by high-quality males (by the high quality that means good genes, or in context, are well adapted to the local environment)," David Hone, a senior lecturer in zoology at Queen Mary University of London, told Newsweek. "Not all males are equal, some will be big and healthy, some far less so. Females obviously have better offspring that are themselves more likely to survive and reproduce effectively and so this is something that would be strongly selected for in females that can do something to improve the chances of mating/fertilization from a high-quality male. By extension, being able to avoid or get rid of low-quality males who are trying the mate/fertilize your eggs is also a good thing." This can lead to males evolving tactics to appeal to female choice, such as ornamental displays and courtship behaviors, or alternatively, evolving ways to supersede female choice. This can be seen in many species of waterfowl, where unpaired males will force mating on females. "There are all kinds of strategies at play here, some of which are more than a bit problematic from a human morality point of view. Forced copulations are common as seen here in the frogs where the males attempt to cling on or force themselves on the females but you get this especially in mallard ducks too. And then there's things like lions killing cubs to bring females into oestrus," Hone said. "Many female spiders will eat males that attempt to mate with them and of course that's common for praying mantis too. And so you get these evolutionary arms races (called the Red Queen effect) where males are trying to get past female defenses and females are trying to erect new ones. So for example in ducks, males have evolved these explosively inflating genitals to force their way into female reproductive tracts but in response females have evolved these dead-ends in their tracts to try and stop males fertilizing them."

In the frogs, however, there doesn't appear to be female choice at play, merely the females evolving to prevent the males from crowding them to death. "We cannot say that they choose a particular male, as choice would imply that they benefit from choice," Dittrich said. "In this species, males do not provide any kind of resources or parental care, so we don't know if they choose, and if so, which traits would be under selection. We can only say that females show avoidance behavior, but not whether this behavior is targeted by sexual selection." "There was an interesting study in 2004 where colleagues showed a novel reproductive mechanism called clutch piracy. They described how satellite males in this species would fertilize the eggs in a clutch that had not been fertilized by the male that had clasped the female. This would override any kind of female choice behavior, as the females leave the pond after laying the eggs and thus have no control over the paternity of their eggs. This would make female choice unnecessary. This is one of the reasons why we do not believe that females choose a mate, but that they defend themselves against male coercion and the risk of being trapped in a mating ball." Do you have a tip on a science story that Newsweek should be covering? Do you have a question about frogs? Let us know via science@newsweek.com.