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Behavioral Ecology Advance Access originally published online on June 16, 2004
Behavioral Ecology 2004 15(6):916-924; doi:10.1093/beheco/arh096
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Behavioral Ecology vol. 15 no. 6 © International Society for Behavioral Ecology 2004; all rights reserved

Is there an optimal number of helpers in Alpine marmot family groups?

Dominique Allainé and Fabienne Theuriau

UMR-CNRS 5558 Biométrie et Biologie Evolutive, Université Claude Bernard, Lyon 1, 43, Bd du 11 novembre 1918, F-69622 Villeurbanne cedex, France

Address correspondence to D. Allainé. E-mail: allaine{at}biomserv.univ-lyon1.fr.

The consequence of helping behavior on breeders fitness is still controversial. We used multivariate analyses to investigate for the effects of male and female subordinates on breeders' components of fitness in the Alpine marmot, Marmota marmota. We found that male and female subordinates, respectively, increased and decreased juvenile survival during winter. Thus, we give evidence that male subordinates should be considered as helpers, and that helpers provided breeders with immediate reproductive success gains, whereas subordinates females were costly. Helpers had no positive effects on female body condition, on persistence (future survival) of dominants, and on future reproduction (occurrence and size of a litter). Helpers thus did not provide breeders with delayed fitness benefits, and therfore, the load-lightening hypothesis was not supported. On the contrary, helpers had delayed fitness cost for dominant males and, consequently, for dominant females. Immediate benefits counterbalanced by delayed costs suggested an optimal number of helpers in the family group both from male and female perspectives. An optimality model well predicted the observed mean number of helpers in Alpine marmot family groups. Optimal numbers of helpers were slightly different for males and females, suggesting a potential conflict of interest between dominants. We finally discuss the possible mechanisms of helping that may explain the observed pattern in the Alpine marmot.

Key words: cooperative breeding, load-lightening hypothesis, Marmota marmota, mixed models, optimality.


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