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Behavioral Ecology Advance Access originally published online on June 30, 2004
Behavioral Ecology 2004 15(6):997-1002; doi:10.1093/beheco/arh105
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Behavioral Ecology vol. 15 no. 6 © International Society for Behavioral Ecology 2004; all rights reserved

Sex ratio variation in the cooperatively breeding alpine marmot Marmota marmota

Dominique Allainé

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.

In cooperatively breeding vertebrate species, a clear theoretical prediction about the direction of sex ratio adjustment can be made: mothers should bias the sex ratio of their offspring towards the helping sex when helpers are absent. A consistent trend in the direction predicted by theory exists in cooperative birds, but theory is still poorly tested in cooperative mammals. Here, multivariate analyses are applied to a long-term data set to test this prediction in two ways in the alpine marmot: (1) across females in a population and (2) in individual females across multiple years. It was shown that in the alpine marmot offspring sex ratio was biased towards the helping sex (males) when helpers were absent, whereas helped mothers produced unbiased sex ratio. Unhelped mothers did not adjust the litter size but produced more sons and fewer daughters than helped mothers. These results support the theoretical prediction and explain well the male bias observed among juvenile alpine marmots at the population level. The occurrence of possible sex ratio manipulations in cooperatively breeding vertebrates is also discussed.

Key words: cooperative breeders, generalized linear models, Marmota marmota, sex ratio variation.


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