Supplementary materials for: Female‐female aggression in a cooperatively breeding bird during the non‐breeding period: The behavioral strategy to maintain long‐term partnerships

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.