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Behavioral Ecology Advance Access published online on June 30, 2004
Behavioral Ecology, doi:10.1093/beheco/arh105
© 2004 by International Society for Behavioral Ecology
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Received November 14, 2003
Revised February 4, 2004
Accepted February 27, 2004

Sex ratio variation in the cooperatively breeding alpine marmot Marmota marmota

Dominique Allainé 1*

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

* To whom correspondence should be addressed. E-mail: allaine{at}biomserv.univ-lyon1.fr.


  Abstract

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.

Keywords: cooperative breeders; generalized linear models; Marmota marmota; sex ratio variation.
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