In the early 1970's, biologist and natural historian Amotz Zahavi struggled to understand a problem that had puzzled researchers since Darwin: Why do animals often produce costly and extravagent displays or physical ornaments? Why do peacocks have such spectacular plumage? Why do baby birds beg so loudly? Why do gazelles jump up and down when they see a lion?
To answer this question, Zahavi proposed that these extravagences are signals to other individuals. For example, a peacock's tail may be a signal used by prospective mates in order to estimate the individual's overall condition and/or genetic quality:
"An individual with a well developed sexually selected character [such as a peacock's flashy tail] is an individual which has survived a test. A female which could discriminate between a male possessing a sexually selected character, from one without it, can discriminate between a male which has passed a test and one which has not been tested. Females which selected males with the most developed characters can be sure that they have selected from among the best genotypes of the male population. "
Amotz Zahavi (1975)
Zahavi named his hypothesis "the handicap principle," and suggested that there is something about costly behaviors or physical features that make for inherently reliable signals. (Additional detail).
But what is it about these costly traits - handicaps, as Zahavi called them - that makes them believable? In the following sections, we will start to answer this question by exploring some of the messages that animals send using costly signals.