BEHAVIOURAL ECOLOGY Henri Weimerskirch Æ Matthieu Le Corre Yan Ropert-Coudert Æ Akiko Kato Æ Francis Marsac Sex-specific foraging behaviour in a seabird with reversed sexual dimorphism: the red-footed booby Received: 8 April 2005 / Accepted: 21 July 2005 / Published online: 30 September 2005 Ó Springer-Verlag 2005 Abstract Most hypotheses attempting to explain the evolution of reversed sexual dimorphism (RSD) assume that size-related differences in foraging ability are of prime importance, but the studies on sex-specific differ- ences in foraging behaviour remain scarce. We compare the foraging behaviour of males and females in a seabird species with a RSD by using several miniaturised activity and telemetry loggers. In red-footed boobies males are 5% smaller and 15% lighter than females, but have a longer tail than females. Both sexes spend similar time on the nest while incubating or brooding. When foraging at sea, males and females spend similar time foraging in oceanic waters, forage in similar areas, spend similar proportion of their foraging trip in flight, and feed on similar prey—flying fishes and flying squids—of similar size. However, compared to males, females range farther during incubation (85 km vs. 50 km), and furthermore feed mostly at the extremity of their foraging trip, whereas males actively forage throughout the trip. Males are much more active than females, landing and diving more often. During the study period, males lost mass, whereas females showed no significant changes. These results indicate that males and females of the red-footed boobies differ in several aspects in their foraging behaviour. Although some differences found in the study may be the direct result of the larger size of females, that is, the slightly higher speeds and deeper depths attained by females, others indicate clearly different foraging strategies between the sexes. The smaller size and longer tail of males confer them a higher agility, and could allow them to occupy a foraging niche different from that of females. The higher foraging effort of males related to its different foraging strategy is probably at the origin of the rapid mass loss of males during the breeding period. These results suggest that foraging differences are probably the reason for the differential breeding investment observed in boobies, and are likely to be involved in the evolution and maintenance of RSD. Keywords Telemetry Æ Diving behaviour Æ Flying fish Æ Foraging effort Æ Sula sula Introduction In birds, mammals and reptiles, males are generally larger than females (Andersson 1994). The greater body size of males is most commonly attributed to sexual selection acting on males, that is, as the outcome of evolutionary increases in body size of males as a result of competition for access to females (Andersson 1994). Reverse sexual dimorphism (RSD, females are larger than males) occurs in a number of groups, and is well known in several bird families, raptors, owls, jacanas, phalaropes, skuas and jaegers, boobies and frigate birds. Whereas in some groups such as jacanas or phalaropes sex roles are reversed and RSD is also attributed to sexual selection, in the other groups where species are all monogamous there is no sex-role reversal. The origin and maintenance of RSD in these groups has been the subject of much debate, and no definitive answer has been proposed (Mueller 1990). One of the most popular hypothesis, the ‘division of labour’ hypothesis, suggests Communicated by Roland Brandl H. Weimerskirch Æ F. Marsac IRD, Centre de la Re´union, UR 109 Thetis, BP 172, 97492 Sainte Clotilde, Ile de la Re´union, France M. L. Corre Laboratoire d’Ecologie Marine, Universite´ de la Re´union, 15 Avenue Rene´ Cassin, BP 7151, 97715 Saint Denis, Ile de la Re´union, France Y. Ropert-Coudert Æ A. Kato National Institute of Polar Research, 1-9-10 Kaga, Itabashi-ku, Tokyo 173-8515, Japan Present address: H. Weimerskirch (&) CNRS Chize´, 79360 Villiers en Bois, France E-mail: henriw@cebc.cnrs.fr Fax: +33-549-096526 Oecologia (2006) 146: 681–691 DOI 10.1007/s00442-005-0226-x