Size matters for sex cheat frogs

Calling male frog

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Smaller treefrogs are more likely to "cheat" their way to a mate, French scientists have found.

The team studied the response of differently sized European treefrogs to a chorus of mating calls.

They found that smaller males lurked near the sound of an attractive call, rather than calling more frequently than their larger rivals.

Research suggests that "cheat" tactics are mainly caused by the intrinsic disadvantage of being small.

The study, which is published in the journal Animal Behaviour, was carried out by a team from the University of Lyon, France.

The scientists wanted to learn more about what causes animals to use "parasitic tactics" rather than "bourgeois tactics" during the mating season.

"Bourgeois" or "classic" tactics involve a high-energy investment and competition for reproduction, whereas "parasitic" or "sneaking" tactics involve exploiting the energy invested by another male.

Freaky frogs

Resting male frog

In treefrogs these tactics manifest themselves in either calling for a mate - a bourgeois tactic - or loitering near an attractive calling male - the sneaky approach.

The scientists wanted to discover whether it was food deprivation or body size that caused the frogs to resort to this behaviour.

"The most important finding of this work is that the energetic constraint imposed by a relevant starvation period does not impact significantly the probability to switch from one tactic to another when compared to the inherent disadvantage of being small," said Loic Brepson, PhD student in behavioural ecology at the University of Lyon.

"For the first time in this study, thanks to an experimental approach, we can compare quantitatively these two effects and assert that one is completely negligible in front of the other," he said.

The study took 100 differently sized male European tree frogs from ponds outside Lyon, France.

After having their stomachs flushed, half of the frogs were fed bluebottles and house crickets, while the other half were starved for a week.

They were then placed in a makeshift pond and the sound of both "attractive" and "unattractive" calls from their own population was played to them through loudspeakers.

"Attractive" calls have a different amplitude and were more frequently made during each round of calling.

The frogs responded to the chorus by calling back, remaining near the sound, or doing nothing.

A couple of mating frogs The results may affect our understanding of the mating systems of other frogs and toads according to Mr Brepson

As well as finding that smaller males being more likely to "cheat" the scientists also found that "males were more likely to act as a satellite if confronted with an attractive competitor than if confronted with an unattractive one".

The food-deprived males were not more likely to use "sneaking" tactics than the frogs that were well fed, according to the study.

In many species, "extravagant" male attributes have been linked with attracting females or competing with other males.

Examples of extravagant attributes are the luminescent plumage of peacocks or the ornate antlers of deer stags.

According to Mr Brepson there has been much scientific interest in what role alternative reproductive tactics such as these "parasitic tactics" might play.

"Some individuals exploit the investment made by other individuals in reproduction, sometimes leading to great behavioural and/or morphological differences between individuals using the two tactics," he said.

"The adoption of one or the other tactic basically depends on the costs/benefits balance that each individual can expect when using one or the other tactic."

He believes that his study may have important implications for study in this area.

"This result probably applies to many anurans," he said referring to the animal group that includes frogs and toads.

"And probably also to many other taxa with this kind of mating system."

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