Variation in Sex Ratios of Banner-Tailed Kangaroo Rats in Relation to Population Density

Abstract

Over a 5-year period, sex ratios of juvenile Dipodomys spectabilis varied significantly at one study site, whereas there was a recurrent and significant male bias at a second site. Population density was variable at the first site, but at the second site it was consistently at high levels. The proportion of males was correlated positively with population density. Demographic and dispersal data revealed no variation in competition for local resources or in differential mortality that ultimately could account for the variation in sex ratio. However, male weights relative to female weights at the end of parental care were lower where sex ratios were male-biased, suggesting a correspondence between male-biased sex ratios and relatively cheaper males. The adjustment to male-biased sex ratios apparently occurred in utero or during lactation. A hypothesis that might explain the variation in sex ratio is that it is an adaptive response either to variation in the relative costs of producing sons and daughters or to variation in the life-history expectations of sons and daughters that occurs with variable population density. Another hypothesis is that competition among siblings results in higher mortality in one sex, and this effect is more pronounced at higher population density.