Parental preference for investment risk incites family strife

Abstract

Older stronger siblings in many birds and mammals enjoy decisive and often lethal advantages in sibling competitions by virtue of being born or hatching first. Asynchronous hatching in birds divides the brood into core (advantaged) and marginal (disadvantaged) elements generating an asymmetric sibling rivalry with often fatal outcomes. Given that these asymmetries are under parental control, the obvious question is why parents seem to incite family conflict. Here I show in a model system (an altricial songbird) that family strife in the form of fatal sibling rivalry arises as a consequence of parental investor's appetite for risk: in accord with an axiom of investment theory, obtaining greater potential rewards required taking greater risks. I analyze the risk and reward of investment in different family structures using a tool borrowed from modern portfolio theory, and show that the most successful parents gambled with brood size, placing bets on incipient families that often proved too large. Failed bets triggered a fatal sibling rivalry for insufficient resources. Parents driven to maximize reproductive success incited family strife among their offspring.