In a sequence of sounds, our ears pay the most attention to the first, and our brains assume all sounds in the sequence come from the same location. This is what's known as the precedence effect, and it biases animal hearing. When male frogs "sing" to attract mates, they also broadcast their locations to eavesdropping predators, and animal behaviorists have long been interested in the balance of risk versus reward in male mating displays.
But male pug-nosed tree frogs have developed a trick to stay safe without sacrificing their mating chances: they chorus in unison. These males have a unique opportunity because — for some, currently unknown reason — female pug-nosed tree frogs aren’t subject to the precedence effect. Singing in unison can therefore overwhelm predators’ senses without confusing their prospective mates.
To study this risk versus reward, a trio of researchers compared the preferences of female pug-nosed frogs to those of two eavesdropping predators (midges and bats) and a related species (túngara frogs) whose males do not chorus. For each experiment, researchers positioned two speakers far from the target animal (for the bats, speakers were baited with fish to sweeten the deal). Male pug-nosed frog calls were piped in through the speakers asynchronously: either the left or the right speaker would play the call first, with the other following shortly after. If a target moved towards one of the speakers, the researchers interpreted that as it “preferring” that speaker over the other, evidence of the precedence effect.
Researchers found that túngara, bats, and midges all preferred the first-call speaker; therefore, predators and related frogs do experience the precedence effect. Female pug-nosed frogs, however, did not show any speaker preference. It is clear that males are taking advantage of this lack of preference. This suggests that, having removed any advantage for singing first, the male frogs would rather not tip off predators to their individual locations.
Why, then, would any male start the chorus? Well, someone has to lead — and the biological urge to reproduce is strong.