The effects of life history and sexual selection on male and female plumage colouration

• Published in: Nature (London) Vol. 527; no. 7578; pp. 367 - 370
• Main Authors: Dale, James, Dey, Cody J., Delhey, Kaspar, Kempenaers, Bart, Valcu, Mihai
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 19.11.2015; Nature Publishing Group
• Subjects: 631/158/2450; 631/158/856; 631/158/857; 631/181/2469; 631/181/2470; Animal behavior; Animal coloration; Animal reproduction; Animals; Biological Evolution; Bird migration; Birds; Body Size; Color; Evolution; Evolutionary biology; Feathers; Feathers - anatomy & histology; Feathers - physiology; Female; Females; Humanities and Social Sciences; letter; Life history; Male; Males; Mating Preference, Animal - physiology; Models, Biological; multidisciplinary; Natural history; Passeriformes - anatomy & histology; Passeriformes - physiology; Perching birds; Phylogeny; Physiological aspects; Pigmentation - physiology; Science; Sex Characteristics; Sexual selection (Natural selection); Studies; Tropical Climate; Tropical environments; Zoological research
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature15509

By quantifying the colouration of all approximately 6,000 species of passerine birds, certain life-history traits such as large body size and tropical distribution are found to increase ornamentation in both male and female birds, whereas cooperative breeding increases it in females only, and sexual selection diminishes it in females more than it increases it in males. Colour clash in male versus female birds Sexual selection theory can potentially explain the fact that male birds are usually more colourful and ornamented than females, but it does not explain why this is not always the case and why the difference between the sexes varies between species. James Dale et al . develop a method to quantify the colouration of all of the approximately 6,000 species of passerine birds to test alternative explanations. They find that patterns in the two sexes are correlated to a considerable extent, but that certain life history traits such as large body size and tropical distribution increase ornamentation in both sexes, whereas cooperative breeding increases ornamentation in females only, and sexual selection diminishes ornamentation in females more than it increases it in males. Classical sexual selection theory 1 , 2 , 3 , 4 provides a well-supported conceptual framework for understanding the evolution and signalling function of male ornaments. It predicts that males obtain greater fitness benefits than females through multiple mating because sperm are cheaper to produce than eggs. Sexual selection should therefore lead to the evolution of male-biased secondary sexual characters. However, females of many species are also highly ornamented 5 , 6 , 7 . The view that this is due to a correlated genetic response to selection on males 1 , 8 was widely accepted as an explanation for female ornamentation for over 100 years 5 and current theoretical 9 , 10 and empirical 11 , 12 , 13 evidence suggests that genetic constraints can limit sex-specific trait evolution. Alternatively, female ornamentation can be the outcome of direct selection for signalling needs 7 , 14 . Since few studies have explored interspecific patterns of both male and female elaboration, our understanding of the evolution of animal ornamentation remains incomplete, especially over broad taxonomic scales. Here we use a new method to quantify plumage colour of all ~6,000 species of passerine birds to determine the main evolutionary drivers of ornamental colouration in both sexes. We found that conspecific male and female colour elaboration are strongly correlated, suggesting that evolutionary changes in one sex are constrained by changes in the other sex. Both sexes are more ornamented in larger species and in species living in tropical environments. Ornamentation in females (but not males) is increased in cooperative breeders—species in which female–female competition for reproductive opportunities and other resources related to breeding may be high 6 . Finally, strong sexual selection on males has antagonistic effects, causing an increase in male colouration but a considerably more pronounced reduction in female ornamentation. Our results indicate that although there may be genetic constraints to sexually independent colour evolution, both female and male ornamentation are strongly and often differentially related to morphological, social and life-history variables.