Philip A. Downing, Ashleigh S. Griffin, and Charlie K. Cornwallis (June 2020)
Helpers benefit breeder reproductive success in cooperative birds
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“The benefits of help in cooperative birds – non-existent or difficult to detect?”
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Live long and reproduce, a lot. This is a good way of passing on genes to future generations. But in a variety of species, from hairy-faced hover wasps to grey-crowned babblers (pictured), some individuals give up their opportunity to reproduce and help raise the offspring of others. Why do this? Evolutionary theory has the answer: help when you can pass on more genes this way than by reproducing yourself. For this to work, your help must boost the reproductive success of individuals with whom you share genes. Only part of this solution has empirical support. Helpers are typically related to those they help, but they don’t always boost their reproductive success.
To make sense of the inconsistent evidence, researchers from the University of Lund in Sweden and Oxford University in the UK used a statistical technique called meta-analysis. This allowed them to synthesize the results of studies on 19 bird species where helping has been studied in remarkable detail. When considered together, the result is clear. Helpers indeed boost the reproductive success of their relatives. Inconsistencies between studies arise because of the methodological techniques used to measure the reproductive boost. So, live long and reproduce lots? Sure, but it's not the only way to effectively pass on your genes.
Abstract
In birds that breed cooperatively in family groups, adult offspring often delay dispersal to assist the breeding pair in raising their young. Kin selection is thought to play an important role in the evolution of this breeding system. However, evidence supporting the underlying assumption that helpers increase the reproductive success of breeders is inconsistent. In 10/19 species where the effect of helpers on breeder reproductive success has been estimated while controlling for the effects of breeder and territory quality, no benefits of help were detected. Here we use phylogenetic meta-analysis to show that the inconsistent evidence for helper benefits across species is explained by study design. After accounting for low sample sizes and the different study designs used to control for breeder and territory quality, we found that helpers consistently enhanced the reproductive success of breeders. Therefore, the assumption that helpers increase breeder reproductive success is supported by evidence across cooperatively breeding birds.