food is available at the home site, little feeding care is needed, and the first worker specialists are generally soldiers specialized for defense.
By contrast, life insurers cooperate to ensure that dependent young survive (Queller, 1989, 1994, 1996; Strassmann and Queller, 1989; Gadagkar, 1990a). They can live in a variety of sites, safe or unsafe, but generally have helpless young that need food and protection. The problem is that an adult must undertake dangerous foraging for her young, but if she dies during one of these trips, her still-dependent young also die. But when a daughter stays at the natal nest to help, and either the mother or the daughter dies, the survivor can take over feeding and protecting the young, giving rise to the synergistic advantage (Fig. 8.2b). Wasps, bees, and ants appear to fit this mold. The crucial assumption that dead individual’s investments can be saved by its surviving colony mates has been experimentally confirmed in a stenogastrine wasp (Field et al., 2000). Other advantages besides fortress defense and life insurance are also possible, and much work remains to be done on assessing their relative importance.
Whatever the fitness advantages of altruism might be, they are selectively irrelevant unless they go to relatives. Hamilton’s kin selection theory (Hamilton, 1964a,b) still provides the framework for understanding altruism, even if the altruism is not driven by extra-high relatedness. As noted above, the fact that social insect colonies consist of families, and that they exclude outsiders, shows that relatedness matters. But other studies have tested more specific predictions about the importance of relatedness.
A recent comparative study of wasps and bees (Wenseleers and Ratnieks, 2006) showed how workers modulate their altruism and selfishness according to relatedness in queenless colonies. Colonies with queens removed were used because (as we will see below) worker selfishness can be repressed in colonies with the queen present, either by the queen herself or by other workers. With the queen gone, some workers develop ovaries and lay unfertilized eggs that will develop into males. If all workers ceased working and took up laying eggs, the colony’s production of males would presumably fall, because a certain number of workers are needed to feed the larvae. In fact, the queenless colonies never had more than 40% reproductive workers; at least 60% remained as helpers. Most interesting was the finding that fewer workers laid eggs from species in which relatedness among workers was high. In other words, more workers stayed in the altruistic helping mode when relatedness was high. Relatedness explained 62% of the variance in percentage of helpers (Wenseleers and Ratnieks, 2006). Variation in relatedness also predicts variation in helping behavior in birds and mammals (Griffin and West, 2003).