The higher ornamentation of males is usually explained by sexual selection: males have to compete with other males for access to mates and females are picky about who they mate with. However, there are also many highly ornamented females. Scientists from the Max Planck Institute for Ornithology and colleagues quantified plumage colour of almost 6000 species of passerine birds and found that selection acts on the ornamentation of both sexes, even in opposite direction. Strong sexual selection on males led to an increase in their plumage colouration, but also to a more pronounced reduction in female ornamentation. The researchers found colourful females predominantly in larger species, in species living in the tropics and in cooperatively breeding species.
Sperm is small and cheap and eggs are large and expensive – this fact of life is the reason why females invest more in their offspring than males, and why males have to compete and females can be choosy. Ornamental traits such as plumage colour may signal a male’s competitiveness or attractiveness to the opposite sex, and therefore ornamented males will enjoy higher reproductive success. Sexual selection will thus lead to increased male ornamentation.
But how then can it be explained that females of many species are also highly ornamented? A widely held view is that female ornamentation is a side-effect of selection on males. Under strong sexual selection, males should become more ornamented, and because females inherit the same genes, they will become more ornamented as well. However, often both sexes compete for resources such as food and territories, and plumage ornamentation could be advantageous in this competition for females as well. Birds are ideal to study this: they show extraordinary variation in plumage colouration in both sexes, with males being more colourful than females, with both sexes looking alike or with females being the more colourful sex.
To answer the question of which selective forces act on plumage colouration in males and females, a team of researchers from the Max Planck Institute for Ornithology in Seewiesen together with colleagues in New Zealand, Canada, and Australia quantified plumage colour of males and females in almost 6000 species of passerine birds, listed in the “Handbook of the birds of the World”. The scientists found the colour elaboration of males being highly correlated with that of females, suggesting that there are limitations to independent evolution of plumage ornamentation in each sex. However, contrary to the expectation, strong sexual selection on males – which led to increased colouration – had an antagonistic and stronger effect on females. “Strong sexual selection leads to larger differences in ornamentation between the sexes, but the most obvious is not that males become more colourful, but that females becoming duller” summarizes Mihai Valcu.
The scientists also found that larger species and species that live in the tropics are more colourful, and this is true in both sexes. Being large reduces predation risk and hence being colourful may be less “costly” in those species. In the tropics, resource competition is typically higher, and therefore it may be more important to signal quality via increased ornamentation.
Overall, interspecific variation in plumage colour can be better explained in females than in males: females are more colourful in monogamous species and in cooperatively breeding species, where the competition among females over mating opportunities is higher. “Our study shows that plumage ornamentation of females is not simply a by-product of the ornamentation in males”, says Bart Kempenaers, director in Seewiesen. In fact, it seems that females are highly ornamented when they also benefit from signalling their quality or competitiveness, either because it plays a role in mate choice, or via competition among females.
Prof. Dr. Bart Kempenaers