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According to kin-selection theory, social insect workers should selfishly indulge their own evolutionary interests1,2,3. But evidence for nepotistic kin discrimination has so far been weak or negative, and may even be precluded by informational constraints and prohibitive costs4. We therefore investigated the ability of F. fusca workers to discriminate between close and more-distant relatives in colonies that are headed by multiple queens (polygyny). As the workers of this species feed the queens and raise the brood, they may be in a position to manipulate brood composition if they can interpret genetic identity.

The kinship among queens in this species is 0.40 ± 0.14, and that of workers to the queens is 0.17 ± 0.12 (ref. 5). If workers are more related to one of the colony queens, theory predicts that they should favour her offspring over those of the others1,6; however, if they are equally related to all of the queens, they should have no incentive to favour the offspring of any particular one1,6.

We studied ten two-queen laboratory colonies5 of F. fusca to compare within a single cohort the reproductive apportionment at the egg and pupal stages. To determine reproductive apportionment and estimate the relatedness between workers and queens7, we genotyped at six microsatellite loci5 an average of 43 eggs, 85 pupae, both queens, and eight workers per colony. Although under laboratory conditions only the worker brood is reared, a queen's genetic representation in this brood accurately reflects her reproductive output (our unpublished results).

We found no positive association between the relatedness of workers and queens and the absolute reproductive apportionment among eggs (Fig. 1a). However, the reproductive share of the queen who was more closely related to the workers increased during brood rearing, and this change was directly proportional to the size of the relatedness difference between the workers and each of the two queens (Fig. 1b). This indicates that workers can discriminate their own kin and selectively favour a brood of closer kin. Although a difference in the viability of eggs8 or the presence of diploid males9 may cause a pattern of selective disappearance of eggs, neither explains the observed association between worker–queen relatedness and changes in reproductive apportionment.

Figure 1: Nepotism in the ant Formica fusca.
figure 1

a, Absolute reproductive share during brood development among a queen's eggs as a function of her relatedness to the colony workers (correlation coefficient, rP = −0.35, n = 10, P = 0.32). b, Increase in a queen's reproductive share as a function of her relatedness to workers. Correlation between the two variables was highly significant (rP = 0.67, n = 10, P = 0.01; one-sided test). To exclude any other factor relating to queens' fecundity5, the change in reproductive apportionment was quantified by first regressing the reproductive apportionment of each queen among the matured brood on her apportionment among the eggs, and then using the positive residuals. The relative kin value of a queen (relatedness index) is calculated as rwq+ − rwq−, where rwq+ is the relatedness of workers to the queen that increased her reproductive share, and rwq− is the relatedness of workers to the queen whose share decreased.

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We conclude that ant workers can apparently discriminate kin accurately and that they capitalize on this ability, thereby enhancing their genetic contribution to future generations1, even in the presence of several queens. Evidence of nepotism in insects apart from the honeybee (Apis mellifera)10 has not previously been found4. This selection for the selfish pursuit of fitness is likely to be a universal feature of societies, but could be modulated by constraints imposed by a lack of accurate information or a reduction in productivity4. Power affiliations may be complex in societies in which different parties pursue their individual genetic interests.