BY PHIE JACOBS
Spring is a dangerous time to be a female European common frog. After a winter-long hibernation, these amphibians congregate in shallow ponds to mate and lay eggs. The gatherings can turn ugly fast; male frogs, which vastly outnumber females, will regularly harass, intimidate, and coerce their counterparts into mating.
Scientists have long assumed the females have little means of defending themselves. But they may be less helpless than previously thought. Today in Royal Society Open Science, researchers report that female European common frogs (Rana temporaria) have a few tricks to escape unwanted mating, including duping a ribbiting Romeo into thinking he’s encountered another male, wriggling out of his grasp, or even playing dead.
When it comes to breeding, frogs and toads tend to use one of two reproductive strategies. They are either “sitters” or “scramblers,” says David Green, an expert in amphibian ecology at McGill University who wasn’t involved with the new study. Most species are sitters—the male squats in one place, lets loose a mating call, and waits for females to come find him. The European common frog, by contrast, is a scrambler. During breeding season, males actively seek out females and attempt to mate with as many as possible during a short period of time—a reproductive strategy vividly termed “explosive breeding.” Often, multiple males will pile onto a single female, forming a tangled heap known as a “mating ball” that can seriously injure the female or even drown her.
Carolin Dittrich, an evolutionary and behavioral biologist currently based in Vienna, didn’t originally set out to investigate the behavior of female common frogs. While pursuing a Ph.D. at the Berlin Natural History Museum, she was interested in finding out whether males of the species choose their mates based on body size. To find out, she placed male frogs in a water-filled box together with two females—one large, the other small. Large would seem to be the logical choice, Dittrich says, because bigger females can lay more eggs, but the males showed no clear preference for one size over the other, Dittrich says. They may choose mates based on other criteria, but when it comes to size, “They are not choosy.”
The females, however, are. When she and her colleagues reviewed footage of the experiment, they noticed a female frog could employ three main tactics to evade unwanted males. In most cases, she will attempt to simply slip out of his arms by rotating her body beneath him in the water. Females also exploit the males’ lack of choosiness. During mating season, male common frogs will grab hold of “whatever is in front of them,” Dittrich says. That list can include other males, the similar-looking European toad (Bufo bufo), and even completely different types of amphibians. In one instance, Dittrich notes, a male common toad was seen attempting to mate with a brightly spotted fire salamander (Salamandra salamandra).
When a male common frog tries to mount another male, the second animal will signal the error by letting out a grunt known as a release call. In this new study, Dittrich observed female frogs mimicking these sounds in attempts to fool undesirable mates into perceiving them as males.
In some cases, females take their acting abilities to the next level by pretending to be dead. Dittrich recalls a “super weird” clip that showed a female frog lying in the water with her arms and legs stretched out as a male attempted to mate with her. Unmoving and stiff as a board, she appeared to be deceased. When the male eventually lost interest, she perked up and swam away. This behavior known as “tonic immobility,” is usually viewed as an antipredator tactic, so observing it in the context of mating is “surprising,” Dittrich says. “The females have adapted,” she adds, noting that this type of “evolutionary arms race” has been observed in other species where sexual coercion is common. Female ducks, for example, have evolved increasingly complex genitalia to thwart attempts at forced copulation, and male genitalia has evolved in response.
Green doesn’t believe any major conclusions about female mating behaviors can be drawn just yet. As he notes, this research took place under artificial laboratory conditions, which could “perturb” the animals’ natural behavior. A female frog out in the wild, for example, would rarely encounter just one male at a time. It’s also unclear what factors might influence a female to choose one mate over another or not breed at all. “That’s the beauty of science,” Green says, “there’s always another question to ponder.”
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