In order to gain access to females, male European frogs harass, intimidate and force them to copulate. These efforts can cause reproductive failure in both individuals and cost females their lives. Previously thought to be passive and unable to resist male coercion, recent research published in the Royal Society Open Science journal shows that females have different strategies for avoiding males they do not want to mate with.
When male and female reproductive interests or strategies differ, it can lead to sexual conflict. During the short breeding season, which lasts two weeks in spring, the sex ratio often skews heavily male. This causes many individuals to congregate and fight each other over a female. “Females end up losing [in this scenario], as they often die; [they are] drowned by the group of up to eight frogs that are on top of them,” explains Iñigo Martínez-Solano, of the Biodiversity Department at Spain’s National Museum of Natural Sciences. Such pile-ons are known as mating balls.
Scientist Carolin Dittrich of the Konrad Lorenz Institute of Ethology in Vienna, Austria, collected 96 female and 48 male common frogs during the breeding season. She placed a male together with two females of different sizes in a box with five centimeters of water and allowed them to move freely for an hour while videotaping their behavior. The males did not pick up on the rejection cues, but the researcher found that the females used evasive maneuvers to push them away.
Among these frogs’ mate-avoidance strategies, the most common was rotation, whereby the female attempts to turn on her own axis to escape the male’s grasp. The second is protesting. Dittrich describes this behavior as a “deep, low-frequency” growl, which she believes is an imitation of the release calls that males produce when fighting each other. They may also emit a higher frequency sound described as a “chirp.”
The final and “most surprising” behavior was tonic immobility, that is, they played dead. Females rigidly extend their arms and legs away from their body to appear dead for several minutes. In one of the videos analyzed, a male is observed dragging a female that remains motionless. After releasing her, the female holds the position until the male turns around and then she swims away. This is very unusual behavior.
Tonic immobility is typically associated with a strategy to avoid predation, but in a mating context it has only been observed in spiders and dragonflies “as a defense of last resort,” the researcher notes. Often used in combination, the maneuvers worked. Of 54 females grabbed by a male, 25 managed to shake him off and escape. Smaller females have a higher success rate because it is easier for them to escape the male’s grasp. Although this study was conducted in a laboratory, Dittrich believes that female frogs would exhibit similar behavior in the wild.
Rejection by females is a matter of preference. Many times, they simply choose not to mate with the first male that grabs hold of them and instead mate with the one that has the deepest song or the largest one. After being rejected, some males intercept the females by surprise to try their luck again, but this clinging does not guarantee fertilization. If the female doesn’t accept his advances, she activates her escape strategies. “We have seen cases in which females carry a male on their back for days or even weeks, waiting for a larger male or another one they prefer,” Martínez-Solano observes. Unlike the females, males don’t seem to be very selective; they capture mates randomly and show no preference in terms of female body size. Their aggressive behavior is the result of the species’ short mating season.
Dittrich believes that these strategies had not been detected because previous research tended to focus on male reproductive behavior. “But [research] is slowly changing to include the female perspective as well,” she says. Martínez emphasizes that research like Dittrich’s is a positive thing, because it contributes to our understanding of the biology of amphibians, of which “there is an enormous lack of knowledge.” Better understanding them can help us discover their demographic aspects and apply that knowledge to effective conservation measures.
Dittrich explains that the behaviors she detected do not threaten the reproduction or survival of the species in any way. But climate change does. Although it is a common species, European frog populations are declining. The ones that will survive are those that are best adapted or can adapt to a changing environment. In sum, Dittrich quotes her PhD advisor, Mark-Oliver Rödel: “There is nothing frogs can’t do.”
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