Animal Sex: How Tree Frogs Do it
A pair of red-eyed tree frogs (Agalychnis callidryas) mating in the wild.
Credit: Brandon Alms | Shutterstock.com

It's easy to tell when it's tree frog mating time — just listen for the chorus of croaks. But aside from making noise, what else is involved in the mating behaviors of tree frogs? 

For the most part, tree frogs are solitary animals devoid of social behaviors, and usually only come together when it's time to mate. But just when breeding occurs differs between species, said Carl Gerhardt, a tree frog expert at the University of Missouri. 

Some tree frogs mate when the weather is cold, others when the weather is warm. Some tree frogs choose to mate one or two nights after it rains. "They have all kinds of mating patterns," Gerhardt told Live Science.

Tree frogs also have different breeding systems, the most common of which is called a lek. In this system, males vie for a female's attention at night, and females are largely in control of mate choice.

To begin, males will produce an advertisement call to females within their area — this call provides females with information about a caller's gender and species. At the same time, it tells other males to stay away.

Just how the male performs his advertisement call matters much to a female. Gerhardt's own research showed that female gray tree frogs (Hyla versicolor) prefer advertisement calls that are long rather than short, because producing long calls is energetically costly and requires the male to be physically fit. "The females are choosing on the basis of an honest advertisement, on something that is costly to the male," Gerhardt said. 

A female will approach a male whose advertisement call she likes. When this happens, the male may switch to a courtship call that is longer and more emphatic than the normal advertisement call. 

When the female gets close, the male will climb on her back and grasp her with his front legs, a position called amplexus. The female will then travel — carrying her mate with her — to a pond or other body of water (if they aren't already near one) to lay her eggs, which the male fertilizes as they're being laid. The pair will normally stay in this coital position for one or two hours, if not all night. 

In some cases, the male may forgo the courtship call. "You don't want to advertise to a rival male that you have a female interested in you," Gerhardt said. This is important, because some males, called "satellite" males, choose to stay silent and hang around calling males in hopes of stealing their mates at the last moment.

If the satellite male locks into amplexus with the female, the calling male will attempt to get him off by trying to wedge in between the pair, usually with no luck. "I wouldn't say they never get [the satellite males] off, but it's pretty hard to displace them," Gerhardt said.

After a pair mates, the male sticks around in hopes of mating with other females. The female, on the other hand, is done for the night, though she may mate once or twice more that season. 

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