Social monogamy evolves in association with biparental care. However, males who tend to pursue multiple mating are expected to place less value on established partnerships, whereas females do the opposite to ensure males' participation in rearing offspring. Accordingly, selection is expected to favor paired females to behave aggressively towards females that approach their social mates. For species with long-term partnerships, female-female aggression commonly observed during the breeding period should also occur in the non-breeding period. We look at whether these expectations are held up by conducting a field experiment in Tibetan ground tits Pseudopodoces humilis, a cooperatively breeding bird that has lifetime monogamy and forms new pair bonds in winter. Paired but not unpaired females were more aggressive towards the same-sex than towards opposite-sex intruders experimentally introduced into the winter group territory, an indication that paired females may be protecting partnerships. Both paired and unpaired males, on the other hand, displayed aggression almost equally against intruders of both sexes, indicating a likely function in territory defense. Our work provides the first evidence for the female's role in maintaining long-term monogamy during the non-breeding period in birds, and contributes to the recent progress regarding intrasexual competition between females as a component of sexual selection.

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