Sharing partners pays dividends for male acorn woodpeckers

Acorn woodpeckers have a very unusual and complex social system that involves polygamy and cooperative breeding. Although some individuals form stable pairs for reproduction, the majority team up in small groups and defend a territory that contains an acorn storage granary.

A group of acorn woodpeckers typically consists of two or three males that are brothers, along with two females that are sisters but are genetically unrelated to the males. The male siblings all mate with the females and the eggs of both females are laid in the same nest hole. 

All members of the group help to raise young, search for food, and defend the granary. This unusual behavior has been the subject of much research, as scientists have tried to work out its evolutionary origins. 

It would appear that competing with one’s male siblings for the same mates is less successful, in terms of chicks raised, than being a member of a monogamous pair. However, new research published today in the Proceedings of the Royal Society B finds that this is not the case.

The long-term study, led by Sahas Barve, aPeter Buck Fellow at the Smithsonian National Museum of Natural History, followed 499 woodpeckers over the entire duration of their lifetimes.The project lasted more than 40 years and was conducted in the 2,500-acre Hastings Natural History Reservation in the Carmel Valley along California’s central coast. 

The researchers recorded reproductive output, in terms of number of chicks raised, over each woodpecker’s entire life. They also monitored the group composition, social standing and territory quality for each bird, and took samples for DNA analysis in order to link parents and their offspring.

The study revealed that males co-breeding in duos and trios produced 1.5 times more chicks than single males in monogamous pairs. In addition, co-breeding males had a longer reproductive life, spending two to three additional years as breeders, which may explain their greater lifetime reproductive success.

In the past, researchers have explained the woodpeckers’ polygamous cooperative breeding system by saying that even if a male lost out to his brother (with whom he shares 50 percent of his genes), helping to raise his brother’s offspring successfully would still ensure that some of the loser male’s genes would pass to the next generation. 

Another explanation focused on the fact that woodpeckers need teamwork in order to defend a territory. The territory is defined by the presence of a granary tree – a dead or living tree in which the woodpeckers chisel holes just big enough to hide an acorn. 

This store can sometimes be plugged with tens of thousands of acorns that ensure food during harsh winters. Having to share breeding opportunities with your brothers may be a trade off for accessing increased numbers in order to defend a territory.

“We thought acorn woodpeckers lost out on fitness by breeding cooperatively, but we show that breeding in these larger cooperative groups is actually better than breeding in pairs,” explained Barve. “This is something that hasn’t been shown before because it’s so hard to get strong enough long-term data to really study it. In that sense, our findings also highlight the value of long-term research in animal behavior.”

By contrast, the lifetime breeding success of female woodpecker duos and single-breeders was not significantly different. Duos did not appear to benefit from the co-breeding system. In fact, females that bred as trios produced 2.5 fewer chicks over a lifetime. 

“Acorn woodpeckers have some of the most complicated social systems of any organism,” said Barve. “And these findings help us understand how this social system might have evolved, while opening up the possibility that cooperative breeding behaviors may be more beneficial than previously thought in other species as well. It could even help explain why sociality evolves so commonly throughout the tree of life.”

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By Alison Bosman, Earth.com Staff Writer