Are elder siblings helpers or competitors? Antagonistic fitness effects of sibling interactions in humans.

Determining the fitness consequences of sibling interactions is pivotal for understanding the evolution of family living, but studies investigating them across lifetime are lacking. We used a large demographic dataset on preindustrial humans from Finland to study the effect of elder siblings on key life-history traits. The presence of elder siblings improved the chances of younger siblings surviving to sexual maturity, suggesting that despite a competition for parental resources, they may help rearing their younger siblings. After reaching sexual maturity however, same-sex elder siblings' presence was associated with reduced reproductive success in the focal individual, indicating the existence of competition among same-sex siblings. Overall, lifetime fitness was reduced by same-sex elder siblings' presence and increased by opposite-sex elder siblings' presence. Our study shows opposite effects of sibling interactions depending on the life-history stage, and highlights the need for using long-term fitness measures to understand the selection pressures acting on sibling interactions.

Sibling competition, dispersal and fitness outcomes in humans.

Determining how sibling interactions alter the fitness outcomes of dispersal is pivotal for the understanding of family living, but such studies are currently scarce. Using a large demographic dataset on pre-industrial humans from Finland, we studied dispersal consequences on different indicators of lifetime reproductive success according to sex-specific birth rank (a strong determinant of dispersal in our population). Contrary to the predictions of the leading hypotheses, we found no support for differential fitness benefits of dispersal for either males or females undergoing low vs. high sibling competition. Our results are inconsistent with both hypotheses that family members could have different fitness maximizing strategies depending on birth rank, and that dispersal could be mainly driven by indirect fitness benefits for philopatric family members. Our study stresses the need for studying the relative outcomes of dispersal at the family level in order to understand the evolution of family living and dispersal behaviour.

Phonological-dependent territorial responses in yellowhammers (Emberiza citrinella).

In songbirds, males sing to attract potential mate and to defend their territory. Information about the sender such as its sex, its motivational state, its strength or identity can be encoded through subtle modifications in songs. In this study, we investigated whether territorial responses of yellowhammer males may be affected by modifications of song syntax and phonology. Yellowhammer song is mainly composed of three elements among which the last one is a long and low frequency note that is supposed to be the main component used by males to assess their potential rival. We carried out field experiments and played back either fully natural songs or partly natural and partly artificial songs to test this hypothesis. We identified that phonology rather than syntax modifications altered territorial responses. Our results also suggest that the phonology of the first song element plays a critical role in driving territorial responses.