Study finds polygamy helps male acorn woodpeckers thrive
The company of two and the crowd of three – unless you’re a male woodpecker vying for the biggest prize in the brood.
A new study, published in the journal Proceedings of the Royal Society B, found that during polygamous breeding, these future feathered sires produced 1.5 times more offspring and continued to breed two to three years older than their monogamous counterparts.
The results suggest that male acorn peaks gain an evolutionary advantage through polygamy, as they pass their DNA on to more offspring.
“Acorn woodpeckers have some of the most complex social systems of all organisms,” said Sahas Barve, ornithologist and Peter Buck Fellow at the Smithsonian’s National Museum of Natural History and lead author of the study. âThese results help us understand how this social system may have evolved. “
Keep it in the family
Acorn Woodpeckers are a particularly social species. They live in territorial groups of 5 to 10 adults and their offspring generally stay a few years to care for the younger generations before leaving.
These birds are famous for their fierce battles around attics or dead trees stuffed with acorns. But their love lives are just as dramatic. Several brothers nest with different pairs of sisters to form a community that reproduces and co-parent. Their behavior causes parents to compete for evolutionary ability, as measured by the success of DNA transmission.
âMore offspring is considered an evolutionary advantage because you are leaving more copies of your DNA in the gene pool,â Barve said. “Evolutionary fitness is all about genetic longevity over time. The more traits that survive, the higher the fitness.”
Biologists originally believed that polygamy would result in fewer offspring, so they turned to a concept called parentage selection to justify this polygamous behavior. Parentage selection occurs when a bird takes care of its related non-descendants instead of mating to produce its own offspring. This behavior, in theory, does not interfere with the evolutionary success of the bird, as the bird still protects some of its DNA that survives into the younger generation.
âParent selection theory suggests that since your nephew is a quarter related to you, helping to raise two nephews is the same as direct offspring, which would be half related,â said Barve. âBut because so few species do, cooperative breeding was seen as the best strategy. It was better than not leaving any copies of your DNA at all. “
Most animals practice monogamy and non-cooperative breeding. Based on this premise, biologists speculated that co-breeding and polygamy must be less progressive. Now, the new research questions that assumption.
A long-term labor of love
Proving that parentage selection, or other cooperative animal behavior, is occurring takes time. Scientists must observe and collect data over many generations before they can get a full picture of the dynamics of an entire species.
âYou can’t really test this without some super detailed long-term recordings,â Barve said. “Fortunately, that’s exactly what we had for this study.”
Barve and his colleagues analyzed more than 40 years of observations and genetic samples from 499 birds in the 2,500-acre Hastings Nature Reserve in California.
The results showed that polygamy was less important for female acorn woodpeckers, but very useful for their male counterparts. Polygamous males produced more offspring during their lifetime and reproduced for a few years longer on average than monogamous males.
Beyond the birds
By studying 40 years of data on the relationships between acorn woodpeckers, Barve and his team were able to analyze the evolution of birds’ mating behavior.
âIt’s something that has never been shown before,â Barve said. “And that highlights the value of long-term research on animal behavior.”
After discovering that co-breeding has evolutionary benefits for male acorn woodpeckers, they hope these benefits could help scientists learn more about changing social behaviors in other animals.
âThe mechanisms of natural selection are the same for all life. We can use the peaks as a study system to understand the evolution of cooperative behaviors, âsaid Barve.
Eight of nature’s wildest mating rituals
Shocking study finds electric eels hunt together
How seven of nature’s freshest species resist the cold
Landmark study shares Smithsonian bird DNA collected over three decades