Evol Ecol Res 16: 153-164 (2014)     Full PDF if your library subscribes.

Seasonal adjustment
of sex ratio and offspring masculinity by female deer mice
is inconsistent with the local resource competition hypothesis


Frances E.C. Stewart, Ronald J. Brooks and Andrew G. McAdam

Department of Integrative Biology, University of Guelph, Guelph, Ontario, Canada


Correspondence: F.E.C. Stewart, School of Environmental Studies, University of Victoria, Victoria, British Columbia V8P 5C2, Canada.



Background: Natural selection is expected to favour the dispersing sex when one sex has a greater tendency to disperse and resources are limited, i.e. during periods of high population density. This is termed the local resource competition hypothesis. Among deer mice, Peromyscus maniculatus, males are more likely to disperse than females. The masculinity of male deer mice can also affect the probability of their dispersal. ‘Masculinity’ is a continuous variable compounded from a suite of traits associated with being male (i.e. hormones, behaviours, morphology, life history).

Hypothesis: Female deer mice will produce male-biased sex ratios and more masculinized offspring when population density (and thus local resource competition) is high.

Methods: We investigated cohort sex ratio and population density in P. maniculatus during the breeding season of 16 consecutive years. Population density was the surrogate for local resource competition and was measured once every two weeks. We measured masculinity as ano-genital distance (AGD), which is a common measurement of masculinity in mammals and a correlate of testosterone levels, in 6-day-old wild P. maniculatus offspring during the breeding season (May through August) of 2011 and investigated whether masculinity was correlated with population density in a manner consistent with local resource competition.

Results: Sex ratio did not correlate as predicted with local resource competition; females produced more female-biased litters when density was high. However, maternal adjustments to masculinity did correlate with litter sex ratio, suggesting that sex ratio and masculinity might covary in response to some selection pressure other than local population density.

Keywords: ano-genital distance, sex ratio manipulation, masculinity, Peromyscus, population density, Rodentia.


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