How to Design Experiments in Animal Behaviour

Abstract

Charles Darwin proposed a separate theory of sexual selection, as distinct from his theory of natural selection, to account for adaptations that confer success in finding a mate, which may sometimes be quite the opposite of what is best for survival. Darwin’s proposal that females have a sense of beauty and choose mating partners that appear beautiful to them was met with much scepticism. But today we have a rather detailed understanding of what animals find beautiful and why. In this article, I will describe a few very simple experiments performed by Michael J. Ryan, in collaboration with A. Stanley Rand, herpetologist extraordinaire and Merlin D Tuttle of the Bat Conservation International fame, that laid the foundation for our current understanding of the meaning and evolution of beauty. Studying the túngara frog on Barro Colorado Island, a research station of the Smithsonian Tropical Research Institute in Panama, they showed that (1) male túngara frogs can produce both simple calls, consisting of just a whine, or complex calls in which one or more chucks are added to the whine, (2) female túngara frogs have a decided preference to mate with males giving complex calls, (3) males are nevertheless reluctant to add chucks to their calls and generally do so only when they hear other males calling, and (4) the local predatory fringe-lipped bat also has a decided preference to eat males giving complex calls. Male tungara frogs thus face a trade-off between sex and survival. These experiments not only answered the question of why males don’t do their best when it comes to singing, but they also set the stage for many more sophisticated investigations that have led to an understanding of how and why natural selection has favoured this particular sexual aesthetic in the frogs and this particular culinary aesthetic in the bats.