Reduced mortality selects for family cohesion in a social species.

Delayed dispersal is the key to family formation in most kin-societies. Previous explanations for the evolution of families have focused on dispersal constraints. Recently, an alternative explanation was proposed, emphasizing the benefits gained through philopatry. Empirical data have confirmed that parents provide their philopatric offspring with preferential treatment through enhanced access to food and predator protection. Yet it remains unclear to what extent such benefits translate into fitness benefits such as reduced mortality, which ultimately can select for the evolution of families. Here, we demonstrate that philopatric Siberian jay (Perisoreus infaustus) offspring have an odds ratio of being killed by predators 62% lower than offspring that dispersed promptly after independence to join groups of unrelated individuals (20.6% versus 33.3% winter mortality). Predation was the sole cause of mortality, killing 20 out of 73 juveniles fitted with radio tags. The higher survival rate among philopatric offspring was associated with parents providing nepotistic predator protection that was withheld from unrelated group members. Natal philopatry usually involves the suppression of personal reproduction. However, a lower mortality of philopatric offspring can overcome this cost and may thus select for the formation of families and set the scene for cooperative kin-societies.