Selective Abortion of Entire Litters in the Coypu: Adaptive Control of Offspring Production in Relation to Quality and Sex

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Abstract
There are strong theoretical reasons for expecting the evolution of an adaptive control of offspring production in relation to their quality and sex, but little empirical evidence that it exists (Williams 1979; Clutton-Brock and Albon 1982). Data from a 12-yr study of coypu reproductive biology suggest that adaptive control may exist in this species and that one mechanism is the selective abortion of entire litters. Young females, in better than average physical condition and expected to litter in the summer, abort small litters of predominantly female embryos at around wk 13-14 of the 19-wk gestation period. Large litters and small, predominantly male litters are retained, and parturition follows normally. Females conceive soon after selective abortion, and the new litter is significantly larger than that aborted: about 5.82 (.+-. 2.07 SD) as opposed to about 4.17 (.+-. 1.74). Neonate size is positively correlated with female condition (and body size) and inversely related to litter size. The differences in body size at birth are carried through to differences in adult size. These data are consistent with the hypothesis that when females are in above-average condition, it would pay them to invest preferentially in offspring of the sex whose chances of future reproductive success will benefit most (Trivers and Willard 1973). Males would be expected to benefit most in the coypu and other species that have polygynous mating systems. Females that abort small, predominantly female litters (with large embryos) abandon offspring that would continue to be costly during the last 6 wk of pregnancy and during the 8-wk lactation period but that would pay off, in fitness terms, little better than small females; instead, they invest their large fat reserves in the production of a larger litter.