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First published online March 30, 2006
Journal of Experimental Biology 209, 1413-1420 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02138

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Kidnapping of chicks in emperor penguins: a hormonal by-product?

Frédéric Angelier^1,*, Christophe Barbraud¹, Hervé Lormée², François Prud'homme¹ and Olivier Chastel¹

¹ Centre d'Etudes Biologiques de Chizé, Centre National de la Recherche Scientifique, F-79360 Villiers en Bois, France
² Office National de la Chasse et de la Faune Sauvage, F-79360 Villiers en Bois, France

^* Author for correspondence (e-mail: angelier{at}cebc.cnrs.fr)

Accepted 31 January 2006

The function and causes of kidnapping juveniles are little understood because individuals sustain some breeding costs to rear an unrelated offspring. Here we focus on the proximal causes of this behaviour in emperor penguins (Aptenodytes forsteri), whose failed breeders often kidnap chicks. We experimentally tested the hypothesis that kidnapping behaviour was the result of high residual levels of prolactin (PRL), a hormone involved in parental behaviour. Penguins with artificially decreased PRL levels by bromocriptine administration kidnapped chicks less often than control penguins. Within the bromocriptine treated group, kidnapping behaviour was not totally suppressed and the probability of kidnapping a chick was positively correlated to PRL levels measured before treatment. During breeding, emperor penguins have to forage in remote ice-free areas. In these birds, PRL secretion is poorly influenced by chick stimuli and has probably evolved to maintain a willingness to return to the colony after a long absence at sea. Therefore, penguins that have lost their chick during a foraging trip still maintain high residual PRL levels and this, combined with colonial breeding, probably facilitates kidnapping. We suggest that kidnapping in non-cooperative systems may result from a hormonal byproduct of a reproductive adaptation to extreme conditions.

Key words: kidnapping, hormones, prolactin, non-cooperative breeding, seabird, emperor penguin, Aptenodytes forsteri

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