Sex-specific environmental sensitivity on the postnatal growth of a sexually size-dimorphic seabird

Abstract

The environment experienced during development is a key factor determining intraspecific variation in postnatal growth. In sexually size-dimorphic species, the larger sex typically grows at a higher absolute rate and consequently is more sensitive or vulnerable to restrictive environments. In addition, this sensitivity can be intrinsic when it is caused by physiological disadvantages of the larger sex, or extrinsic when it results from environments generated by social interactions among siblings. Here, we evaluated intrinsic and extrinsic drivers of sex-specific sensitivity in the postnatal growth of the Imperial Shag Leucocarbo atriceps, a sexually dimorphic seabird that typically produces one- or two-fledgling broods. Our objectives were to evaluate sex-specific effects of: (1) good and poor years on chicks fledging as singletons, (2) different rearing environments produced by the combinations of brood size and hatching order, and (3) sibling sex and brood sex composition in two-fledgling broods. Singletons exhibited suboptimal growth in poor years, with males and females equally affected. At an extrinsic level, males were more sensitive than females, as the reduction in fledging mass between best and worst social environments was twice as high in males as in females. In addition, the presence of a younger sibling in the nest had sex-specific consequences for the older chick. Fledging mass of older female chicks was unaffected by the presence of a younger sibling, whereas males reached the highest fledging mass when raised as singletons. The sex of the sibling and the brood sex composition did not affect chick growth in two-chick broods. Overall, our results suggest that females grow at their maximum rate even in moderately favourable social environments (as senior chicks in two-chick broods), whereas males require the most favourable environment (raised as singletons) to reach the highest growth.