SEX OF POUCH YOUNG RELATED TO MATERNAL WEIGHT IN MACROPUS-EUGENII ANDM-PARMA (MARSUPIALIA, MACROPODIDAE)

Competing theories of sex allocation in mammals may best be reconciled in the light of data from diverse species. The tammar wallaby (Macrop us eugenii) is potentially a particularly interesting study animal bec ause females wean only one young per year, and exhibit extreme synchro nicity in the annual onset of breeding. By contrast, reproduction in t he closely related parma wallaby (M. parma) is almost asynchronous. Th ese two Australian species are found sympatrically only on Kawau Islan d, New Zealand, where they were introduced in about 1870. We sampled w allabies on Kawau Island in April of 1996, when both species were bree ding. Although the sex ratios in both species were not significantly d ifferent from unity, offspring of M. eugenii were very significantly m ore likely to be male with increasing maternal weight (logistic regres sion chi(2) = 16.8, P < 0.0001), and the fewer M. parma data showed a non-significant trend in the same direction (chi(2) = 1.9, P = 0.16). These data, at least for M. eugenii, are consistent with the Trivers-W illard hypothesis, and warrant further investigation in wild and capti ve populations under different measured or manipulated ecological cond itions. We suggest an approach utilising the characteristics of M. eug enii which might help determine whether the sex bias is determined clo se to conception, or is effected later in the reproductive cycle by di fferential survival of the sexes.