Trevor's and Hale's ant studies

Trivers and Hare’s ant sex-ratio study tested a gene-centered account of conflict within insect colonies by comparing the queen’s reproductive interests with those of the workers who care for the brood. Its striking result was that workers appeared to prevail: among 20 ant species, reproductive investment closely matched a three-to-one female-to-male ratio, while two slave-making species showed the contrasting one-to-one pattern predicted when the nursery’s workers were unrelated to the queen’s offspring.

The conflict and its predictions

The study addressed a conflict over who could shape a colony’s investment in male and female reproductives. A queen’s genes, the book argues, are best served by an equal investment in sons and daughters. A worker’s genes can instead benefit when the queen produces more daughters than sons. In this framework, the difference between queen and worker is not a difference in their genes or an expression of conscious preference: it is a difference in the bodies through which genes can influence reproduction.

The queen’s position and the workers’ practical control of the nursery therefore imply competing outcomes. If the queen can direct reproductive investment, the expected ratio is one-to-one. If workers effectively run the colony for their own genetic benefit, the prediction is three female reproductives for every male. Trivers and Hare treated this as a testable question about which party could exert greater influence over the colony’s brood.

Measuring reproductive investment

The comparison depended on counting the right ants. The relevant reproductives are the large, winged males and females that emerge from the nest in periodic mating flights; young queens may subsequently attempt to establish colonies. But the sexes can differ substantially in size, so simply counting males and females would not measure how much a colony had invested in each.

The study therefore accounted for reproductive size by weighing the individuals, estimating investment rather than relying on head counts alone. Trivers and Hare examined 20 ant species, using this measure to test the workers’ predicted three-to-one female-to-male ratio. The result was described as a convincingly close fit to that prediction, supporting the interpretation that workers, rather than queens, were winning the conflict in the species studied.

Why workers could prevail

The proposed explanation was practical as well as genetic. Worker bodies guard and tend the nurseries, giving workers greater power over the rearing of reproductives than queen bodies possess. Genes influencing the world through workers could therefore outmanoeuvre genes influencing it through queens. The observed bias did not mean workers consciously planned the ratio; it was the predicted outcome of differences in whose bodies could affect the colony’s reproductive investment.

The account also identifies a circumstance in which that balance could change: slave-making ants. In these species, workers raid another species’ nest and carry off its young. Once the captured ants hatch in the new nest, they perform ordinary colony work, including brood care, while the slave-making workers can continue their raids. The slaves are unrelated to the queen and to the reproductives they tend, although they behave according to the programs that would normally guide work in their own colony.

The slave-making test and its result

Because slave workers are unrelated to the queen’s offspring, their genes are not represented in the reproductives emerging from the slave-making nest. The workers’ usual genetic stake in biasing investment toward sisters is therefore absent. The queen, by contrast, can benefit from an equal investment in her own sons and daughters. The book describes this as a change in who has effective power in the nursery: the slave workers do the care, but the queen can alter the outcome without a corresponding response evolving in those unrelated workers.

This reasoning led to a specific prediction. In slave-making species, investment in the two sexes should approach one-to-one rather than the workers’ three-to-one female bias. Trivers and Hare found the predicted result in the two slave-making species they examined. The evidence was narrower than the 20-species comparison, but the contrasting pattern supplied the critical test: when the workers’ genetic connection to the brood was removed, the balance shifted toward the queen’s predicted ratio.

Scope and significance

Taken together, the ordinary and slave-making ant comparisons turn a general account of reproductive conflict into a contrast between measurable outcomes. In the broader sample, the close fit to a three-to-one female-to-male investment supported the view that related workers could shape the sex ratio for their own benefit. In the two slave-making species, the movement toward one-to-one was consistent with the prediction that a queen gains influence when the workers tending her brood are unrelated to it.

The result illustrates the book’s wider argument that reproductive behavior and social organization can reflect conflicts among genetic interests, even within a colony whose members cooperate in raising young. It also depends on a specific distinction: numbers of males and females are not equivalent to investment in them, and the study’s weighing procedure was essential to the comparison. The finding is presented as support for the proposed account, not as a claim that every colony is governed by a perfectly tidy contest; the surrounding discussion explicitly cautions that real cases need not match an idealized model in every detail.