Lazy sons, self-sufficient daughters: are sons more demanding?

• Galapagos sea lion sons are larger than daughters from birth and beyond weaning.
• Juveniles start to forage by diving before they become nutritionally independent.
• Sons contributed less to their energy needs through independent foraging.
• Offspring self-contribution to their energy requirements may affect their demand.

Parent–offspring conflict exists because the optima of parental care differ between parent and offspring. In many polygynous species, males are larger than females and may require additional allocation of maternal resources. Differential allocation to sons and daughters requires either spontaneously increased supply of maternal resources in the more expensive sex or that sex to induce additional supply by its greater demand. In the Galapagos sea lion, Zalophus wollebaeki, males are larger than females from birth to weaning. However, offspring begin to forage by diving before weaning. Therefore, we investigated whether sons demand more supply of maternal resources than daughters or contribute more by independent foraging. Our results show that during the first 2 years of life, sons were much less likely than daughters to forage independently, despite very similar diving abilities. During this time, females apparently sustained sons by providing milk whereas daughters contributed substantially to their energetic needs by independent foraging. The observed difference in reliance on self-feeding between male and female offspring is consistent with the idea that males demand more, but could also be due to benefits to the mother when supplying more milk in sons. Sex differences in offspring contribution to their own maintenance during the transition to independence offer a new perspective on mother–offspring conflict worth further investigation.