Cooperative Breeders Adjust Offspring Sex Ratios to Produce Helpful Helpers

Abstract

Whether birds and mammals adaptively adjust their offspring sex ratios in response to their environment is much debated. A source of confusion is that different studies show different patterns, with sex ratio adjustment appearing to occur in some cases but not others. The extent to which this reflects interesting biological variation due to differences in the underlying selective forces, as opposed to statistical noise, is not clear. Cooperatively breeding species offer an opportunity to address this problem because the strength of selection on sex ratio adjustment can be estimated. When helping behavior is sex dependent, parents are predicted to overproduce the helping sex when this sex is rare or absent. We show here that the extent of this behavior depends on the benefit that helpers bring to parents: there is greater sex ratio adjustment when helpers bring larger benefits. Variable selection on sex ratio adjustment may thus explain variable empirical findings.

Sex Allocation Patterns across Cooperatively Breeding Birds Do Not Support Predictions of the Repayment Hypothesis

“…Breeding females are expected to benefit from biasing their offspring's sex ratios toward the helping sex only if the help actually improves their fitness in turn (Emlen et al 1986;Griffin et al 2005). For this reason, from a data set of 32 candidate species we excluded four for which research suggests help has no positive effect on the fitness of breeding females (table A1; tables A1-A3 are available online).…”

“…Long-term data on red-cockaded woodpeckers (Picoides borealis) have been used to estimate direct fitness differentials between the sexes and incorporate them into a repayment model; however, the model still failed to predict brood sex ratios at the population level (Koenig and Walters 1999). A meta-analysis by Griffin et al (2005) suggests that individuals are more likely to adjust their offspring's sex ratio in systems where the benefits of help are high. This provides some explanation for failures to observe adjustment, but such negative results have proliferated since Griffin et al's (2005) study, perhaps because authors realize that they run counter to the prevailing view of adaptive sex allocation (Cockburn and Double 2008).…”

“…In cooperative breeders, one benefit that has attracted a great deal of attention is the fitness gain that parents receive from offspring that help them in future breeding attempts (Malcolm and Marten 1982;Koenig and Walters 1999;Griffin et al 2005). Help is usually more likely to come from one sex than the other (Komdeur 2004).…”

“…A meta-analysis by Griffin et al (2005) suggests that individuals are more likely to adjust their offspring's sex ratio in systems where the benefits of help are high. This provides some explanation for failures to observe adjustment, but such negative results have proliferated since Griffin et al's (2005) study, perhaps because authors realize that they run counter to the prevailing view of adaptive sex allocation (Cockburn and Double 2008).…”

Do males hatch first and dominate sex ratios in White Stork Ciconia ciconia chicks?

“…Donald 2007). Griffin et al (2005) argued that across species the selection on sex ratio adjustment may be variable due to differences in breeding system, sexual dimorphism, and life-histories, whereas within species such variability in selection is less straightforward. Studies investigating the repeatability of facultative sex ratio adjustment in the same species between years, and for the same individuals, are scarce, yet crucial to determine with confidence the frequency of sex ratio modification in specific taxa.…”