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

Abstract

Classical sexual selection theory 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. The view that this is due to a correlated genetic response to selection on males was widely accepted as an explanation for female ornamentation for over 100 years and current theoretical and empirical evidence suggests that genetic constraints can limit sex-specific trait evolution. Alternatively, female ornamentation can be the outcome of direct selection for signalling needs. 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. 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.

Species variation in the degree of sex differences in brain and behaviour related to birdsong: adaptations and constraints

“…R. Soc. B 371: 20150117 are competing for mates in both sexes [11]. This comprehensive analysis of factors driving sex differences in plumage among all passerines can also be informative when thinking about the evolution of sex differences in song behaviour and the song system in the same taxa.…”

“…Studies that followed the original Nottebohm & Arnold work [1] immediately illustrated that there are diverse patterns of sex differences in relation to sex differences in behaviour ( [17], figure 2). Plumage and size variation between males and females varies from being identical to being dimorphic to such a degree that they were thought to be from different species [11]. A recent study on variation in the colour of the plumage among songbirds comprehensively sampled all 6000 living species and found fascinating diversity that the authors linked to sexual selection in many cases (such as when males were clearly brighter than females and engaged in competitive mating behaviours for females), though a surprisingly large number of species exhibited bright coloration in females as well as males, especially in the tropics where intra-sexual competition was relatively high in both males and females and where long-lived sedentary species Figure 1.…”

“…Songbirds, although named after their signature behaviour, are technically a taxonomic group. They are members of the order Passeriformes that consists of approximately 6000 out of the 10 500 extant species of birds [11]. In particular, the term songbirds is thought to correspond to species in the suborder Passeri (also known as oscines), which is sometimes referred to as the 'true' songbirds and consists of just under 50% of the 10 500 extant species of birds themselves [12].…”

Multifaceted origins of sex differences in the brain

“…Ball [51] notes that contrary to original ideas that song is ancestral to males and was subsequently derived in females, emerging views suggest that song evolved in both males and females but then diverged. Whether this divergence was owing to sexual selection, natural selection or both is subject to debate [53], but the critical point is that preadaptation (i.e. existing song nuclei) may constrain the neuroanatomical substrates in males and females, so that the nuclei remain larger in males, regardless of singing behaviour, yet other factors could evolve to subserve duetting or femalebiased song.…”

Testosterone activates sexual dimorphism including male-typical carotenoid but not melanin plumage pigmentation in a female bird

“…Although males typically are the more ornamented sex, females often produce some elements of the male ornamental plumage that function in female-female signaling or other contexts (Amundsen, 2000). Furthermore, females are more likely than males to show evolutionary gains and losses in elaborate plumage color (Burns, 1998;Irwin, 1994;Johnson et al, 2013;Omland, 1997;Price and Eaton, 2014), indicating that sexual dichromatism is not driven solely by selection acting on mechanisms in males (Dale et al, 2015). Male-typical color expression is considered to be T dependent in Charadriiformes and luteinizing hormone (LH) dependent or genetically determined in Passeriformes (Kimball, 2006;Kimball and Ligon, 1999).…”