A new long-term study finds that male acorn woodpeckers breeding polygamously in duos or trios of males actually fathered more offspring than males breeding alone with a single female, contrary to conventional thinking among biologists that monogamous males necessarily produce more offspring than those in polygamous groups. For females, polygamy is less of a slam dunk but co-breeding duos left behind the same number of offspring as the birds that coupled up, while female trios left behind fewer offspring than either group.
The study, published today in the journal Proceedings of the Royal Society B, suggests that even complex cooperative breeding strategies may offer direct evolutionary benefits over an animal’s lifetime, and perhaps offers clues into how social behavior first evolved in humans and other animals.
The love life of the typical acorn woodpecker is a complex, polygamous affair. A common arrangement for a male of this species, which inhabits oak savannas from Oregon to Colombia, might be ruling over a patch of woodland alongside two of his brothers and a pair of sisters from another family that the brothers all mate and raise chicks with. Some of the group’s offspring may even hang around their childhood territory for years, not to breed with their parents or aunts and uncles, but to help raise the next generation before striking out to become breeders in other groups.
Researchers had long assumed that the woodpeckers opting for this rare form of polygamy, in which co-breeding siblings are forced to compete to mate, were making an evolutionary compromise. To researchers, this sibling rivalry seemed likely to result in individual woodpeckers leaving behind fewer offspring each year than if they had opted for a more traditional coupled pairing with a guaranteed opportunity to mate. To balance out this short-term loss of evolutionary fitness, researchers hypothesized that the woodpeckers’ polygamy must confer some indirect or long-term advantages, but quantifying those benefits in wild populations before this study had proven extremely challenging.
«For the longest time we have thought polygamous breeding was a compromise, and breeding as couples was considered the gold standard for leaving behind the highest number of chicks,» Barve said. «But you can’t really test that without super-detailed, long-term data. Fortunately, that’s exactly what we had for this study.»
The data underpinning this research spans more than 40 years and tracks 499 individual birds over their entire lifetimes at the 2,500-acre Hastings Natural History Reservation in the Carmel Valley along California’s central coast, where a rotating cast of some 150 scientists and interns has been observing acorn woodpeckers since 1968. Researchers working at the Hastings Reserve recorded each bird’s reproductive output from their first attempt to their last along with information including territory quality, group composition, social standing and genetic data linking parents to their offspring for birds hatched between 1984 and 2006.
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