Maternal rank and local resource competition do not predict birth sex ratios in wild baboons

Joan B. Silk, Elisabeth Willoughby, Gillian R. Brown

Research output: Contribution to journalArticlepeer-review

31 Scopus citations


We test two models of adaptive adjustment of birth sex ratios that are expected to apply to Cercopithecine primate species. It has been predicted that when maternal investment differentially influences the reproductive success of male and female offspring, females in good condition will bias investment in favour of the sex that gains the greatest fitness returns from additional investment. This hypothesis was subsequently amended to take into account the effects of local resource competition on maternal investment strategies of primate females. This body of theory has been applied to primates with contradictory results, prompting some to question the conclusion that primate females facultatively adjust birth sex ratios in an adaptive manner. Here, we present a meta-analysis of the relationship between maternal rank, birth sex ratios and local resource competition in 36 groups of wild savannah baboons, Papio cynocephalus. The results do not support predictions derived from either model of facultative sex ratio adjustment, and we conclude that there is currently no evidence that baboon birth sex ratios are adjusted in an adaptive manner.

Original languageEnglish (US)
Pages (from-to)859-864
Number of pages6
JournalProceedings of the Royal Society B: Biological Sciences
Issue number1565
StatePublished - Apr 22 2005
Externally publishedYes


  • Baboons
  • Local resource competition
  • Sex ratio

ASJC Scopus subject areas

  • Biochemistry, Genetics and Molecular Biology(all)
  • Immunology and Microbiology(all)
  • Environmental Science(all)
  • Agricultural and Biological Sciences(all)


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