Also, the special 3/4 relatedness applies only in colonies with a single, once-mated queen. Multiple queens and queen replacement reduce relatedness, as does multiple mating. Accurate estimates of relatedness among colony members are now available for hundreds of species, typically based on inherited variation in DNA microsatellite loci (Gadagkar, 1990b; Crozier and Pamilo, 1996; Peters et al., 1999). Although relatedness among female colony members in many species is near the full-sister value of 3/4, in many other species it is lower.
Finally, other possible explanations, some of them noted by Hamilton himself (Hamilton, 1964b), have been proposed for the facts the haplodiploid hypothesis seems to explain. Specifically, the high incidence of sociality in the Hymenoptera and the all-female workforce may relate to preadaptations involving parental care (Alexander et al., 1991). The solitary Hymenoptera have an unusually high level of parental care, meaning that adaptations for nest-building, prey capture, brood care, sanitation, and defense are already in place. It must be much easier to evolve alloparental care in groups that already have parental care. And because it is females that provide the parental care in solitary Hymenoptera, with special adaptations such as the sting, it is not surprising that females provide the care in social Hymenoptera (Alexander et al., 1991).
Kin selection has been so closely identified with the haplodiploid hypothesis that concerns with the latter have caused some to question kin selection in general. But of course Hamilton’s rule does not require that relatedness to beneficiaries must be higher than relatedness to one’s own offspring. If rb = rc, or even if rb < rc, Hamilton’s rule can still favor altruism if the benefit is sufficiently greater than the cost (b > c) (West-Eberhard, 1975). The question then concerns how it is possible to rear more young by aiding the beneficiary than by reproducing independently. Synergies from division of labor between helpers and reproducers are easy to see after sociality has evolved, but this kind of specialization seems unlikely to be present at the beginning of sociality.
Two kinds of factors seem especially likely to provide the necessary advantage to helping: “fortress defense” and “life insurance” (Queller and Strassmann, 1998). Fortress defenders live in protected, expandable sites that generally include food (Andersson, 1984; Alexander et al., 1991; Crespi, 1994), such as the wood galleries of termites and the plant galls of social aphids and thrips. An offspring can gain by remaining at the natal site, even if she has to rear less-related collateral relatives, because she avoids risking death by migrating to a new site (see Fig. 8.2a). Because