It is dusk in a Central American rainforest. The sun drops below the hidden horizon and there is a brief silence as the calls of many birds and diurnal (active during daylight) insects fade away. In the nearby creek, a male toad makes his first, tentative call of the evening, rather like the trilling of a mobile phone. He is soon joined by tens of others of his kind and the contrasting calls of several treefrogs from the leaves overhead.
To a human listener, the multitude of different sounds to be heard under the rainforest canopy represents a bewildering variety of squeaks, chirrups and whistles. How does any animal make sense of the diverse bombardment of sounds in this complex environment? The human analogue might be a busy office where the e-mail has crashed: the boss is shouting for his coffee; the office manager is dictating a memo about desk allocation and Brian from Personnel is chatting up the secretary again.
The rainforest is little different: everybody is shouting because of resources (often food, space and sex). The toad in the creek’s call says many things about him to others of his species. It tells them that he is an adult (and older, larger toads often have deeper voices!); it tells them that he has been successful in his foraging for insects and has accumulated enough energy to expend some of it on calling and it tells them that he is macho enough to defend a good spot in the creek where a female toad might be persuaded to deposit her eggs. Other males should keep away and females should take a look at what he has to offer!
Each animal species which uses sound in this way has evolved its own unique way of communicating its resource status (what it has and what it needs). As well as the advertisement call (here I am, this is what I’ve got), described above, our toad uses a recognisably different aggression call as a direct challenge to an approaching rival male – a very definite “keep away, this is my bit of creek!”.
The females of toads (and other amphibians) rarely call but may give a short squeak if grabbed by an overenthusiastic male. This release call says either “I am not ready to mate” or “I haven’t decided you’re the right one yet”! Female amphibians have been shown to choose potential mates on characteristics of their calls (pitch and length of call, for example). Males with longer (or perhaps deeper, depending on the species) calls tend to father tadpoles which grow faster and into larger youngsters. They therefore have a shorter larval period (time spent as a tadpole) where they are vulnerable to predators, and a greater variety of invertebrate prey available to them when they metamorphose (become terrestrial). The female’s choice of mate, based on the characteristics of his call, may be directly translated into the survival of her offspring!