Some male frogs cannot understand the hints. In the spring, they pile on potential mates in such a frenzy of determination that the females may drown.
Therefore, females have developed tricks to escape. A study published Wednesday in the journal Royal Society Open Science shows that female frogs use evasive strategies to ward off males. They also hide their identity. They sometimes even fake their own deaths.
Carolyn Dietrich, a postdoctoral researcher at the Konrad Lorenz Institute of Ethology in Vienna, made this discovery by chance.
She was studying the mating habits of common European frogs. These amphibians live alone, except for about two weeks each spring when they gather in pools for a free reproductive event called explosive reproduction.
A male in a hurry may use the barbed pad of his thumb to grab a female that already has a partner, or multiple partners. Sometimes six or seven males can be seen attached to a single female.
“I have to say it’s disgusting,” Dr. Dietrich says. A female caught in this tangle, called a mating ball, will likely die from drowning or being crushed.
Dr. Dietrich wondered if male frogs prefer larger females. As part of her research for her PhD, she brought wild frogs into her lab and placed two females in an aquarium with each male. She turned on the video camera and gave the frogs an hour of privacy.
In one of the first videos, she saw a male dragging a stiff female with her limbs spread. “At first I thought, oh my god, this can’t be happening,” Dr. Dietrich said.
No dead frogs were found in her tank, but the female on screen was clearly not alive. “I was very surprised and a little concerned,” Dr. Dietrich said.
After a few minutes, the male gave up and moved to the other female in the tank. When her new partner bumped into the stationary frog, she came to life and swam away.
Other videos made it clear that this behavior was no fluke. One-third of her female girlfriends played dead when grabbed by the male.
Playing dead (also called mortification or tonic immobility) is widespread in the animal kingdom, but is usually used to escape from predators. Possums playing to avoid mating are much more rare. It has been seen in females of some species of newts. Male spiders may play dead to avoid cannibalizing their mates.
No one knows what a stressed animal is thinking in its mind when it is in a state of tonic immobility. “We don’t know if it’s a conscious decision to play dead,” Dr. Dietrich says. But as a result, the animal appears to be singing, at least to the human eye.
Dr. Dietrich’s experiments showed that this was not the only way females tried to escape mating. More than 80% of women rolled in the water to get rid of their partner.
Additionally, nearly half of the females produced a distinct call that sounded exactly like the sound a male makes when accidentally grabbed by another male. Dietrich said the females appear to be masquerading as males in order to lure potential suitors elsewhere.
Their workarounds, often used in combination, worked. Of the 54 females grabbed by the male, 25 succeeded in shaking him off.
Dr. Dietrich dug deep into the scientific literature and found several references to such behavior, one of which dates back more than 250 years. Still, more modern researchers describe female frogs as passive participants in breeding events, she said. But her results suggest that women are not defenseless. They have an arsenal of tactics to counter powerful males and keep themselves alive.
As for Dr. Dietrich’s first question when he played amphibian matchmaker in the lab: Do male frogs use size to choose the best mate? – the answer is no.
“They’re actually not picky at all,” Dr. Dietrich says. “They grab whatever they can.”