Vocal recognition and social cognition in Acorn Woodpeckers
A leading hypothesis for the evolution of intelligence posits that complex cognitive abilities developed in order to help animals navigate complex social interactions. The Acorn Woodpecker (Melanerpes formicivorus) is a cooperatively breeding polygynandrous bird with an unusually complex social system, suggesting that their social cognitive abilities may be similarly complex. Acorn Woodpeckers in California live on year-round territories in family groups that consist of one or more related females breeding with one or more related males (opposite sex breeders are unrelated to each other). Adult offspring also remain in their natal territory for years as non-breeding “helpers” and assist their parents in raising subsequent generations of offspring. This atypical social structure occurs because suitable territories for breeding are limited; therefore, when all the breeders of one sex die, creating a breeding vacancy, individuals from surrounding territories fight in dramatic “power struggles” for the opportunity to fill this vacancy. Such power struggles often start within a few hours of a vacancy opening up, and the competitors typically form coalitions with same-sex relatives, including relatives living on other territories, to improve their chances of winning. Thus, keeping track of the membership of other groups and recognizing kin are crucial for reproductive success in Acorn Woodpeckers. My doctoral research investigated how Acorn Woodpeckers recognize the calls of both kin and non-kin and keep track of the relationships among members of other groups. My field site and second home during my Ph.D. was Hastings Natural History Reserve in central coastal California, where the Acorn Woodpecker population has been continuously studied for over 50 years and nearly all of the individuals are color-banded for individual identification.
I investigated vocal recognition in Acorn Woodpeckers by playing back calls I had recorded from known individuals. The woodpeckers responded more quickly to playbacks of territorial neighbors with whom they had never shared a group than to playbacks of their own current group members or former group members who had dispersed to a nearby territory. However, they responded equally quickly to playbacks of dead or missing former group members and playbacks of non-group members. This suggests that Acorn Woodpeckers continue to recognize their former group members post-dispersal as long as the former group members are still living nearby, but either forget former group members who have died or left the area or classify them differently from living group members.
Acorn Woodpeckers often produce vocalizations known as waka calls in an overlapping chorus with one or more members of their social group; however, they rarely if ever perform this display with individuals with whom they have no affiliative relationship.
Waka call (single caller)
To determine if Acorn Woodpeckers recognize associations between members of other social groups, I simulated waka choruses by artificially overlapping waka calls recorded from two different birds and played them back to breeding female woodpeckers from a different social group. Subjects responded more quickly to simulated choruses in which the two callers did not belong to the same group as each other than to simulated choruses in which the two callers were from the same group. This provided the first experimental evidence that free-ranging animals could keep track of the social affiliations among individuals from social groups other than their own. In a subsequent experiment, female Acorn Woodpeckers responded more quickly to simulated choruses in which the two callers had never lived together than to simulated choruses in which the callers used to live together but dispersed into separate groups before the subject hatched. This suggests that not only do Acorn Woodpeckers vocally recognize their own former group members who have dispersed into other groups, but at least the females can also recognize such cross-group relationships among others, likely by observing the birds in question visit one another post-dispersal.
Most studies of social cognition have focused on animals' knowledge about the members of their own social group, and it is often implicitly assumed that the challenges of navigating within-group social interactions are the strongest selection pressure on the evolution of social cognition. However, the results of my Ph.D. research highlight the importance of considering the full social environment in which animals exist, including social interactions and relationships that take place across group boundaries.