How to vocally identify kin in a crowd: The penguin model

Abstract

Summary On penguins, individual recognition is observed between mates and between parents and chick(s). During the past five years, their particular strategies of coding—decoding have been tested by playing back modified display calls to six species, in Australia (little penguin, Eudyptula minor), in Antarctica (Adelie penguin, Pygoscelis adeliae; emperor penguin, Aptenodytes forsteri), and in subantarctic islands (king penguin, Aptenodytes patagonicus; macaroni penguin, Eudyptes chrysolophus; gentoo penguin, Pygoscelis papua). All species use only vocal cues to identify their partner, but in territorial species the nest is used as a meeting point. In large species, such as the king and the emperor penguins, which do not have a nest, the brooder carries the egg or the small chick on the feet, while the mate, and then the chick, has to be located in the noisy colony without any topographical cue. According to theory, to extract a signal from background calls, animals analyze either frequency bands or modulations (amplitude and frequency modulations) of the partner's call. The first coding-decoding system, used by nesting penguins, is easy to produce but costly in terms of analysis time. The second one, used by nonnesting penguins, is a vocal signature which is fast to analyze but costly to produce. This acoustic signal is particularly efficient as a means to locate immediately the partner on the move in a noisy crowd. Briefly, frequency analysis is enough to solve the relatively easy problem of individual recognition in nesting birds, while the complex temporal analysis of modulations of the two nonnesting penguins is an adaptation to extreme acoustic and breeding conditions. The macaroni penguin, which we have begun to test, seems to use both a frequency code similar to that of the other nesting species and a temporal code close to the one of a nonnesting penguin species, but much simpler.