P. Sunnucks et Ac. Taylor, SEX OF POUCH YOUNG RELATED TO MATERNAL WEIGHT IN MACROPUS-EUGENII ANDM-PARMA (MARSUPIALIA, MACROPODIDAE), Australian journal of zoology, 45(6), 1997, pp. 573-578
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.