title: Sex-specific demography and generalization of the Trivers-Willard theory.
abstract: The Trivers-Willard theory proposes that the sex ratio of offspring should vary with maternal condition when it has sex-specific influences on offspring fitness. In particular, mothers in good condition in polygynous and dimorphic species are predicted to produce an excess of sons, whereas mothers in poor condition should do the opposite. Despite the elegance of the theory, support for it has been limited. Here we extend and generalize the Trivers-Willard theory to explain the disparity between predictions and observations of offspring sex ratio. In polygynous species, males typically have higher mortality rates, different age-specific reproductive schedules and more risk-prone life history tactics than females; however, these differences are not currently incorporated into the Trivers-Willard theory. Using two-sex models parameterized with data from free-living mammal populations with contrasting levels of sex differences in demography, we demonstrate how sex differences in life history traits over the entire lifespan can lead to a wide range of sex allocation tactics, and show that correlations between maternal condition and offspring sex ratio alone are insufficient to conclude that mothers adaptively adjust offspring sex ratio.
authors.affiliation: University of Oxford, Department of Zoology, Oxford OX1 3PS, UK.
authors.forename: Susanne
authors.initials: S
authors.lastname: Schindler
authors.affiliation: Laboratoire de Biométrie et Biologie Evolutive (UMR 5558), Université Claude Bernard Lyon 1, 43 boulevard du 11 novembre 1918, 69622 Villeurbanne Cedex, France.
authors.forename: Jean-Michel
authors.initials: JM
authors.lastname: Gaillard
authors.affiliation: Department of Computer Science, University of Surrey, Guildford GU2 7XH, UK.
authors.forename: André
authors.initials: A
authors.lastname: Grüning
authors.affiliation: Department of Biological Sciences, University of Calgary, Calgary, Alberta T2N 1N4, Canada.
authors.forename: Peter
authors.initials: P
authors.lastname: Neuhaus
authors.affiliation: School of Animal, Plant and Environmental Sciences, University of the Witwatersrand, Wits 2050, South Africa.
authors.forename: Lochran W
authors.initials: LW
authors.lastname: Traill
authors.affiliation: Department of Biology, Stanford University, Stanford, California 94305, USA.
authors.forename: Shripad
authors.initials: S
authors.lastname: Tuljapurkar
authors.affiliation: University of Oxford, Department of Zoology, Oxford OX1 3PS, UK.
authors.forename: Tim
authors.initials: T
authors.lastname: Coulson
mesh_terms: D000220:Adaptation, Biological / Q000502:physiology; D000375:Aging / Q000502:physiology; D000818:Animals; D001522:Behavior, Animal / Q000502:physiology*; D005260:Female; D008297:Male; D008954:Models, Biological; D009026:Mortality; D009035:Mothers; D015203:Reproducibility of Results; D012098:Reproduction / Q000502:physiology*; D012309:Risk-Taking; D012589:Sciuridae / Q000502:physiology; D012727:Sex Characteristics; D012744:Sex Ratio; D012756:Sheep / Q000502:physiology