Today, we have a guest post from my friend – Chris. He wrote this for an assignment at school and thought my readers would like to read it.
When male green tree frogs are looking for mates, they call with advertisement calls, telling the world “I’m here, this is my territory, come get me.” These advertisement calls are mostly the types of calls that you hear whenever you go out in the woods, especially in the spring. They’re the calls most frequently used by frogs, and the ones that are the most predictable, allowing us to use them to recognize frog species the same way we do with bird calls. However, these calls have a side effect; They don’t just attract female frogs, they also bring in other males that want the females and territory that the calling frog might have.
Bringing other male frogs leads to aggressive interactions between male frogs frequently. These don’t look quite like the types of fights that we may be used to though, as they tend to result in the frogs showing off with aggressive calls to each other until one backs down. These calls do a good job acting as a proxy for a fight without the risk of injury and other costs of actually fighting, as the larger, more fit frog usually wins.
A study from Christopher J. Leary from the University of Mississippi looked at how both these aggressive calls and advertisement calls affected both frogs emitting them and frogs hearing them. It had previously been established that advertisement calls from males generated more sex hormone production in females around, as well as in the calling male, but this study addressed also how it affected listening males, and how the aggressive calls impacted both males involved. He found that winners of these fights between male frogs showed slightly higher levels of corticosterone, a stress hormone, immediately after the fight, but the losing male showed much higher levels of the corticosterone. This may change these losing frogs strategies to reproduction in the future. Changes to the frog’s body chemistry begins to explain a phenomenon called satellite males, where some frogs stop trying to compete for mates and territory, and instead try to be sneakier and intercept females for quick mating opportunities. This change was hard to understand in the context of evolution, as giving up mating opportunities or competing for mating opportunities seemed like it would never be beneficial from an evolutionary perspective, but if it is caused by lost conflicts, this may help us understand why frogs would make this change of strategy.
His results on the impacts of advertisement calls on listening males were less clear though. He found slight drops in androgen, the main sex hormone, levels among males listening to other male’s advertisement calls, which was backwards of the expected result. Previous research had suggested that frogs would produce more sex hormones when hearing other males advertising, which makes logical sense, as this would lead to them seeking out conflict with the other males looking to steal their mates and territory. Leary’s result is suggested as an adaptation by these frogs to try to prevent rival males from hearing their song and attempting to come fight for their territory and mate, but this result seems unclear, given its disagreement with some previous research.
Other studies will surely be done in the future to try to verify this surprising result, and to investigate the complicated dynamics of frog interactions when looking for mates. But the major result of the impact of the aggressive calls on other frogs helps us better understand how frogs find and compete for their mates, and may help explain satellite males.
Reference: C.J. Leary. 2014. Close-range vocal signals elicit a stress response in male green treefrogs: resolution of an androgen-based conflict. Animal Behavior, 96 (2014), pp. 39-48
