Conflict of interest between queens and workers in Hymenoptera

Conflict of interest between queens and workers in Hymenoptera is a conflict over who controls the colony’s reproductive investment and, especially, the balance of female and male reproductives. Its significance in the book’s gene-centered argument is that the same inherited gene can have different optimal outcomes depending on whether it acts through a queen or a worker: the queen’s reproductive interests favor equal investment, while workers can benefit by producing more sisters. The conflict is therefore not simply a contest between a reproductive individual and non-reproductive servants; it is a struggle between different routes by which genes can reach later generations.

Relatedness and the worker’s advantage

In the account, the key genetic circumstance is that full sisters in Hymenoptera have a relatedness of three-quarters. They share the same father, and the two sperm cells that conceived them are identical in every gene. A particular gene inherited by one sister from her father is therefore certain to be present in her full sister; if it came from her mother, there is a one-half chance that her sister also received it. Averaging these possibilities gives the three-quarter relatedness. This exceeds a female’s relatedness to her own offspring, which is one-half.

That difference creates a possible advantage in making sisters indirectly rather than reproducing directly. A worker that helps her mother produce reproductive sisters may, from the gene’s-eye perspective used in the book, propagate more copies of her genes than she would by producing offspring herself. The account connects this logic to the evolution of worker sterility: it proposes that a female can gain through the reproductive output of close relatives even when she does not reproduce. It also notes the scale of the evolutionary pattern: true sociality with worker sterility is said to have evolved at least eleven times independently in Hymenoptera, compared with once elsewhere in the animal kingdom, in termites.

The competing sex-ratio optima

The worker advantage is conditional, because workers benefit from sisters more than brothers. A particular gene in a worker has only a one-quarter chance of being shared with a brother. If the queen produces male and female reproductives in equal proportions, the workers’ indirect route through sisters is less advantageous than it could be. The workers’ predicted interest is therefore to shift reproductive investment toward females.

Drawing on Fisher’s calculations of optimal sex ratios, Trivers and Hare adapted the calculation to the unusual relatedness structure of Hymenoptera. Their account distinguishes investment from simple head counts: the mother’s stable optimum is equal investment in male and female reproductives, while the sister’s optimum is three-to-one investment in females. Thus a female that breeds directly is predicted to invest equally in sons and daughters, whereas a worker that does not breed is predicted to favor a three-to-one ratio of reproductive sisters to brothers.

The difference does not mean that queens and workers possess different genes. The same gene can have different interests according to the body in which it finds itself and the influence available to it. In a queen’s body, the gene can benefit from equal investment in male and female offspring. In a worker’s body, it can benefit from influencing the queen to produce more females. The conflict is between the effects that the gene can achieve through different positions in the colony, not between two genetically separate classes.

Ant colonies as a test of control

The book presents this disagreement as a testable conflict over practical power in the nursery. If workers can control the reproductive output, the sex ratio should approach their three-to-one female-biased optimum. If the queen can impose her own interests and workers function chiefly as her attendants, the expected result is closer to equal investment. Trivers and Hare tested the prediction across twenty ant species, estimating investment rather than merely counting reproductives because males and females can differ greatly in size. They weighed the reproductive forms and found a close fit to the predicted three-to-one female-to-male investment ratio.

The result supports the interpretation that workers prevail in the ant species examined. Their practical position as guardians of the nurseries gives them more influence over what happens there than the queen’s reproductive body alone can exercise. In the book’s formulation, genes acting through workers can outmaneuver genes acting through queens. The outcome makes the colony’s sex ratio evidence not just of relatedness arithmetic, but of which class can translate its genetic interest into control of reproduction.

Slave-making ants and a reversal

Slave-making ants supply a contrasting case because the identity of the nursery’s workers changes the balance of power. In the species described, specialized workers raid nests of other ants and carry off unhatched young. Those captured ants mature in the captors’ nest and perform its ordinary work, including caring for the brood, although they are unrelated to the queen and reproductives they serve. The book notes that selection may favor adaptations against enslavement in the raided species, but that such defenses have not eliminated the practice.

For the sex-ratio conflict, the decisive consequence is that the queen’s own workers no longer hold practical control of the nursery. The slave workers behave according to rules that would ordinarily favor the production of their own close relatives, but the captor queen’s reproductives are not their kin. Because the slave workers’ genes do not pass through those reproductives, selection does not favor their evolving the ability to counter the queen’s adjustments to reproductive output. The workers may therefore be unable to detect or defeat a queen’s means of shifting the ratio.

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 the expected one-to-one result in the two slave-making species they examined. In this exceptional arrangement, the queen is better able to secure the balance that benefits her own reproductive interests. The contrast with the twenty ordinary ant species makes the proposed mechanism especially clear: the predicted ratio changes with the kinship and practical power of those tending the brood.

Limits and significance of the model

The account presents these patterns as a model rather than a claim that every social insect fits neatly into two opposed strategies. It cautions that real biological arrangements can complicate the comparison. Honeybees, for instance, appear to show a surplus of investment in males, which at first seems inconsistent with both the workers’ and the queen’s predicted interests. Hamilton’s proposed answer is that a departing queen takes a large swarm of workers with her to establish a new colony. Since producing those workers is part of the cost of establishing reproductive females, their investment should be counted alongside the queens rather than ignored in the sex-ratio calculation.

The relation also belongs to the book’s broader distinction between kinship as a biological estimate and kinship as a practical or social category. In humans, names, formal marriages, written records, and cultural customs make relations recognizable in ways not available to the simple behavioral rules proposed for wild animals. Human systems of family obligation can reflect subjective and cultural ideas of kinship as well as genetic relationships. The ant model, by contrast, explains how a genetic asymmetry can generate competing reproductive interests without requiring the participants to understand those interests. Its central lesson is that apparently unified colony life can contain conflict: who benefits depends on relatedness, reproductive role, and the practical power to shape the next generation.