SEX-BIASED MORTALITY IN WOODRATS OCCURS IN THE ABSENCE OF PARENTAL INTERVENTION

Male-biased mortality in young animals is often viewed as adaptive dis crimination against male offspring by parents unable to raise reproduc tively competitive sons. Unequivocal evidence of the presence or absen ce of parental discrimination against males is lacking, however, and t he adaptive interpretation of male-biased mortality is confounded by a n alternative explanation that it reflects differential energetic requ irements between the sexes (due to sexual selection for large size in; mature males) independent of parental manipulation. To determine wheth er maternal discrimination against offspring explains postnatal mortal ity in a sexually dimorphic rodent, we examined patterns of growth and mortality in offspring of food-restricted and food-enriched lactating bushy-tailed woodrats, Neotoma cinerea. We also monitored mothers and their litters daily throughout lactation for evidence of maternal dis crimination against offspring. Offspring of food-restricted mothers sh owed depressed growth, and mortality of offspring born to both food-re stricted and food-enriched mothers was male-biased, but in the absence of maternal discrimination. Offspring that died were no less likely t o be attached to their mother's teats in the 10 days prior to death th an were offspring that successfully weaned. Similarly, offspring of fo od-restricted mothers were attached as often as were offspring of food -enriched mothers. In a series of behavioural arena trials in the firs t 10 days after birth, restricted mothers were no less attentive towar d their sons than they were to their daughters, nor did mothers treat their offspring that did not survive to weaning differently from those that survived. Our findings provide empirical evidence that postnatal , sex-biased mortality in offspring is not necessarily due to parental intervention, and they call into question the adaptive interpretation s of previous examples of sex-biased offspring mortality. (C) 1998 The Association for the Study of Animal Behaviour.