Paul Sunnucks and Andrea C. Taylor

Abstract

Competing theories of sex allocation in mammals may best be reconciled in thelight of data from diverse species. The tammar wallaby(Macropus eugenii) is potentially a particularlyinteresting study animal because females wean only one young per year, andexhibit extreme synchronicity in the annual onset of breeding. By contrast,reproduction in the closely related parma wallaby(M. parma) is almost asynchronous. These two Australianspecies are found sympatrically only on Kawau Island, New Zealand, where theywere introduced in about 1870. We sampled wallabies on Kawau Island in Aprilof 1996, when both species were breeding. Although the sex ratios in bothspecies were not significantly different from unity, offspring ofM. eugenii were very significantly more likely to bemale with increasing maternal weight (logistic regressionχ² = 16.8,P <0.0001), and the fewerM. parma data showed anon-significant trend in the same direction (χ²= 1.9,P= 0.16). These data, at least forM. eugenii, are consistent with theTrivers–Willard hypothesis, and warrant further investigation in wildand captive populations under different measured or manipulated ecologicalconditions. We suggest an approach utilising the characteristics ofM. eugenii which might help determine whether the sexbias is determined close to conception, or is effected later in thereproductive cycle by differential survival of the sexes.