Male frogs often force females to mate with them, but in one species, a croak from the female seems to persuade males to leave her alone. The sound may be an “honest” signal that she isn’t fertile and mating would be a waste of the male’s sperm and energy.
“In general, male frogs are known to jump on even inorganic objects if they are female-like,” says at Nagoya University in Japan. “However, this species [stops attempting] such holding behaviour at the sound of a female’s call. I consider it rather gentlemanly.”
Most frog calls are produced by males, especially as they try to attract mates. A few years ago, Itoh discovered that female black-spotted frogs (Pelophylax nigromaculatus) , although their calls are simpler, shorter and quieter than those of males, he says.
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Curious about the purpose of these female calls, Itoh captured 15 males and 27 females from ponds in Kyoto during the frogs’ annual breeding season. Fourteen of the females had maturing eggs in their ovaries, suggesting they were probably fertile.
In his laboratory, he introduced each female to up to six males, one at a time, in a test tank. All but one of the females without eggs and half of those with eggs called out at the males – especially when the males took a hop towards them. The calls were low volume and seemed to be meant as a communication to the approaching male. “They do not appear to be intended to be heard by other individuals further away,” says Itoh.
Sometimes, the males called back – and when that happened, half of the females called back at them again.
Following these calls, none of the males attempted mating, says Itoh. For the most part, they turned and hopped away in the opposite direction. Successful mating only occurred when the female remained silent.
Although some of the vocal females had eggs, Itoh says the calls may have been an honest signal about the females’ fertility status, as the eggs might not have been mature.
“The results do definitely suggest that females emit calls to deter male coercion, but I do not think the data are quite robust enough to say that with 100 per cent confidence,” says a frog communication researcher.
Considering that scientists currently know relatively little about how females cope with attempted coercive mating, the findings are “extremely interesting and novel”, says , who researches frog mating behaviour.
“The counter-strategies female frogs use to avoid unwanted matings in mating systems that are dominated with intense male-male competition remain unclear,” says Kelleher. “This paper presents evidence for a novel female strategy: using calls to ward off males. I am not aware of another study that has reported this.”
Further research with a larger number of frogs, more clearly defined settings and more robust statistical analyses would strengthen the findings, say Kelleher and Coss.
Behavioural Processes
