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Behavioral Ecology Advance Access originally published online on January 6, 2007
Behavioral Ecology 2007 18(2):427-432; doi:10.1093/beheco/arl099
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© The Author 2007. Published by Oxford University Press on behalf of the International Society for Behavioral Ecology. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org

Litter size and fetal sex ratio adjustment in a highly polytocous species: the wild boar

Sabrina Servantya,b, Jean-Michel Gaillardb, Dominique Allainéb, Serge Brandta and Eric Baubeta

a Office National de la Chasse et de la Faune Sauvage, Centre National d'Etudes et de Recherches Appliquées sur les Cervidés-Sangliers, 85bis avenue de Wagram, 75017 Paris, France b Laboratoire de Biométrie et Biologie Evolutive (UMR 5558), CNRS, Université Lyon 1, 43 boulevard 11 novembre 1918, 69622 Villeurbanne Cedex, France

Address correspondence to J-M. Gaillard. E-mail: gaillard{at}biomserv.univ-lyon1.fr.


  Abstract

For species in which reproductive success is more variable in one sex than the other, the Trivers and Willard model (TWM) predicts that females are able to adjust their offspring sex ratio. High-quality mothers should provide greater investment to one sex than the other. Previous tests of the TWM have been inconsistent, and whether the TWM applies to species with several offspring per litter is unclear due to possible trade-offs between size, number, and sex of the offspring. Williams' model (WM) accounts for confounding effects of these trade-offs on sex ratio variation. Lastly, the "extrinsic modification hypothesis" predicts changes in offspring sex ratio in relation to climatic conditions and population density. Using wild boar as a model, we tested 1) whether the WM fitted observed sex ratio variation and 2) whether sex ratio variations were related to maternal attributes (test of the TWM) and/or to resource availability (test of the extrinsic modification hypothesis). Females adjusted their litter size rather than their litter composition, so that the WM was not supported. Likewise, changes in resource availability did not influence the fetal sex ratio, so that the extrinsic modification hypothesis was not supported. The fetal sex ratio was negatively related to increasing litter size, providing some support for the TWM. Sex ratio was male biased for litter sizes up to 6 and then became female biased in larger litters. Our results provide the first case study showing marked changes in sex ratio in relation to litter size in a large mammal.

Key words: litter size, primary sex ratio, sex ratio variation, Sus scrofa scrofa, Trivers and Willard model, Williams model.

Received 28 March 2006; revised 2 December 2006; accepted 5 December 2006.


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