The Oxymoron of Darwinian Cooperation in Birds

What is ‘Natural selection’?

‘Natural selection’ is the evolutionary process where organisms better adapted to their environment tend to survive and produce more offspring. This was proposed by Charles Darwin, a famous naturalist who noted that species varied by island in his voyages to areas like the Galápagos Islands. An example of a trait that varies among birds is beak size/shape, which is associated with their diet. For instance, thicker beaks are associated with seed-crushing whereas longer, sharper beaks are associated with nectar collection.

Due to its focus on individual traits, cooperative breeding does not fit well in the Darwinian model of natural selection. Cooperative breeding is an extreme form of cooperation that has evolved in numerous animals like arthropods (ex. ant colonies), fish, birds, and mammals. This form of breeding involves related individuals helping to raise offspring that are not their own while sacrificing their own reproduction. Despite cooperative breeding in birds being widespread in nature, the reason for why birds evolved this behaviour is not understood. It is known that family living, where related individuals only interact because they are in proximity to each other, can provide more opportunities for offspring to socially acquire critical life skills and potentially increase their cognition. In addition, family living helps birds deal with variable and harsher living conditions.

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Image taken from article.

In July 2016, several phylogenetic comparative analyses on 3005 terrestrial bird species were created to determine the evolutionary development and diversification of social systems. The relative rates of evolutionary changes among all possible social systems were estimated to investigate whether family living was necessary for the evolution of cooperative breeding. Then, possible conditions were examined to determine which selective forces favoured the evolution of cooperative breeding.

The hypotheses were that cooperative breeding evolved from a non-cooperative ancestor, and that the evolution of cooperative breeding involved two separate stages. These were found to be correct. The researchers determined that the evolution of cooperative breeding occurred in two stages: first was the continued parent-offspring association beyond supervision, and second was helping at the nest. It was confirmed that non-family and family-living species are associated with very different climatic conditions and reproductive strategies (whereas the conditions were similar for family living and cooperative breeding). The common traits of family-living bird species include: larger body size, relatively sedentary lifestyle, denser habitats, and higher food specialization.

These results provide insights into the geographic distribution of different social systems, where hotspots for cooperative breeding can be seen in Southern Africa, Australia, and South America. Because many of these hotspots are disproportionately affected by climate change, family-living may be a useful trait that will be retained in a rapidly warming world.

Summary written by: Emma Stevens

To read the full article, please click the following link:

Family living sets the stage for cooperative breeding and ecological resilience in birds

Michael Griesser, Szymon M. Drobniak, Shinichi Nakagawa, Carlos A. Botero

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