Hymenoptera

Hymenoptera—ants, bees, and wasps—are distinguished in this account by a sex-determination system that changes how relatedness and reproductive interests work. Unfertilized eggs develop into males, which have only one set of chromosomes; fertilized eggs develop into females, which have two. The resulting three-quarter relatedness between full sisters helps explain how worker sterility could evolve, but it also sets up a contest over whether a colony’s reproductive investment serves the queen or the workers.

Sex Determination and Relatedness

In a typical hymenopteran nest, one mature queen stores sperm from a single mating flight and uses it to fertilize some eggs over the course of her life, which may last ten years or longer. Unfertilized eggs become males, with a single set of chromosomes inherited from their mother. Females have a father and the usual double set of chromosomes. Whether a female develops as a worker or a queen depends not on different genes but on her rearing, particularly the food she receives; the account also notes specialized worker castes such as soldiers.

This arrangement makes the usual shorthand for relatedness harder to apply without careful attention to whose genes are being considered. A son receives all his genes from his mother, but he carries only half the usual number of genes, so from the queen’s point of view a son and a daughter are each related to her by one-half. The apparent contradiction disappears when the calculation returns to the actual inheritance process rather than treating a relatedness index as absolute.

Full sisters share the same father, and the sperm that produced them are identical in every gene. A gene inherited from their mother has a fifty-percent chance of being shared by a sister; a gene inherited from their father is shared with certainty. Their average relatedness is therefore three-quarters, not the one-half typical of full siblings in ordinary sexual reproduction. The distinction is central to the argument: a female hymenopteran can be more closely related to her full sisters than to her own offspring.

Worker Sterility and the Colony’s Reproductive Economy

That relatedness creates a possible genetic advantage for a worker that does not reproduce but helps its mother produce sisters. A gene promoting the vicarious production of sisters can be copied through those sisters more rapidly than a gene promoting direct reproduction by the worker. The account links this logic to the evolution of worker sterility and notes that true sociality involving sterile workers evolved at least eleven times independently within Hymenoptera, compared with one other occurrence in the animal kingdom, among termites.

The queen and workers nevertheless have different predicted interests. A queen is related by one-half to offspring of either sex and is predicted to invest equally in male and female reproductives. For a worker, a brother shares only one-quarter of a particular gene, while a sister may share three-quarters. On that basis, workers are predicted to favor a three-to-one ratio of investment in female rather than male reproductives. If a worker breeds on her own, by contrast, the predicted investment in her sons and daughters is equal. The same genes can thus have different optimal strategies according to whether they act through a queen’s body or a worker’s.

The conflict is framed as a difference in practical as well as genetic leverage. Workers tend the brood and can influence what happens in the nursery, while the queen supplies the eggs. The account describes the possibility that workers might be able to detect concealed male eggs, setting up an evolutionary contest in which queens change the signals and workers counter them. This is not a conflict between separate kinds of female: an embryo female has the genes associated with both queen and worker development, with rearing determining which caste she becomes.

Ant Evidence and the Queen–Worker Conflict

To test the predicted investment ratio, Trivers and Hare counted the winged male and female reproductives that emerge in periodic mating flights. Because the sexes may differ substantially in size, they weighed them rather than simply counting individuals: the relevant prediction concerns investment, not numerical proportions. Across twenty ant species, their estimates showed a close fit to the predicted three-to-one female-to-male ratio. In the species studied, the workers’ predicted interest appeared to prevail over the queen’s equal-investment interest.

The result is attributed to the workers’ control over the nursery: genes acting through worker bodies could outmanoeuvre genes acting through queen bodies. The argument therefore treats the colony not as a unit with a single reproductive interest, but as a setting where different relatedness positions create competing strategies. A colony’s sex ratio becomes evidence about which reproductive pathway has greater influence.

Slave-making ants offer a contrasting test. Their workers are poor at ordinary nest work or do none of it; they specialize in raids on colonies of other species, carrying off unhatched young. The captured ants hatch in the new nest and perform the ordinary tasks for which their nervous programs equip them, including cleaning, foraging, and caring for brood. Because these workers are not kin to the slave-making queen or to the reproductives produced by her colony, their genes cannot be propagated through those reproductives.

That separation changes the evolutionary contest. A worker gene for detecting a queen’s deceptive signals would not benefit by influencing the slave-makers’ reproductives, so the captured workers cannot evolve the same counter-strategy on behalf of the queen’s own offspring. The queen can therefore alter the signals without selecting for worker genes that break the code. The prediction is that slave-making species should approach equal investment in male and female reproductives rather than the workers’ three-to-one female bias. Trivers and Hare found that result in the two slave-making species they examined, a case in which the queen’s interests could prevail.

Scope and Limits of the Account

The argument distinguishes a general prediction from the complications of particular species. Honey bees appear at first to be a serious exception: their colonies invest more in males than in queens, which does not seem to fit either the workers’ or the mother queen’s predicted interests. Hamilton’s proposed solution is to include the large swarm of attendant workers that accompanies a departing queen and helps establish a new colony. Since those workers are lost to the parent hive, their production is part of the cost of reproduction; counting them as investment in reproductive females can change the apparent balance against males.

The account thus uses Hymenoptera to show both the explanatory power and the conditional nature of gene-centered reasoning. Relatedness does not dictate a single colony-wide purpose: it generates different predicted interests for queens and workers, while caste, reproductive pathways, and control of the brood affect which interests shape the outcome. The ant ratios and the contrasting slave-making result are presented as empirical tests of those predictions, while the honey-bee case demonstrates why reproductive investment must be measured broadly rather than inferred from the numbers of reproductives alone.