Long-term effects of sex-specific sibling interaction on the development of blue-footed boobies

Abstract

Abstract

The sex of an animal’s siblings can potentially have long-term effects on its development if it competes or cooperates with them in a brood or litter. For the first test of lifelong developmental effects of infant siblings’ sex in a wild animal, we monitored broods of two blue-footed booby (Sula nebouxii) chicks that recruited intact into their natal breeding population. Female chicks grow to be 27% heavier than male chicks and broodmates compete aggressively for food during 10 weeks. We took hatch order into account because first-hatched chicks aggressively dominate their siblings and sometimes kill them, and second-hatched chicks experience chronic stress. We confirmed only one of 20 predictions for long-term effects of sibling sex: females with elder brothers had lower odds of hatching their eggs than females with elder sisters. However, sex of their elder sibling made no difference to a female’s or male’s age of first reproduction, annual survival, fledging success, age of last sighting, or accumulated breeding success. Based on 760 recruits from 18 birth cohorts, these findings suggest that, although a sibling’s sex could, for all we know, affect an individual’s survival in the nestling or juvenile stages, long-term effects on survival and reproductive performance in the adult stage are mostly absent or inconsequential in two-chick broods that recruit intact. This resilience is likely due to evolved developmental buffering of adult survival and reproductive performance, but impacts of a sibling’s sex on development and survival during the nestling and juvenile periods remain to be tested.

Significance statement

The development of many animal infants is influenced by interacting with siblings, and in many birds the growth and behavior of female and male chicks differ during the development of a brood. It is therefore likely that female and male siblings shape a chick’s development differently, but no test of long-term developmental effects of siblings’ sex has been made in any bird. In a species where female chicks are bigger than male chicks, sibs compete aggressively for months, and sibling dominance relationships are universal, we tested for developmental impacts of the sibling’s sex in broods of two chicks that both recruited into the breeding population. We monitored 760 chicks from 18 cohorts over their lifetimes, up to age 23 years. Long-term effects of the sibling’s sex on the survival, reproduction, and fitness of females and males were absent or inconsequential.