The evolution of eusociality: Reproductive head starts of workers

Abstract

In eusocial species, many individuals forego their personal reproduction to aid the reproduction of their mother or other relatives. Kin selection can favor such behavior for any positive degree of relatedness to the individuals being helped, provided the helper gives them a sufficiently large fitness gain compared to the fitness cost of giving up its own reproduction. Yet, little attention has been given to the question of how helpers can aid relatives more than they can aid themselves, particularly in species lacking morphologically specialized castes. One answer to this question is explored here. When a potential helper is born into a colony that has immature young, its help may quickly bring those young to the age of independence. If it left to reproduce alone, it would have to bring its own offspring all the way to independence. The consequences of early mortality therefore differ: solitary reproducers that die early will fail to bring any young to independence, while helpers that die at the same age may have made substantial contributions. Published data from four polistine wasps show that high adult mortality rates and long periods of offspring dependence combine to provide a large selective advantage for worker behavior.