Sexually dimorphic swallows have higher extinction risk

Abstract

The effect of sexual selection on extinction risk remains unclear. In theory, sexual selection can lead to both increase and decrease extinction probability depending on the ecology of the study system. Thus, combining different groups might obscure patterns that can be found in groups that share similar ecological features. Using phylogenetic comparative analysis, we studied sexual plumage dimorphism in relation to the perceived risk of extinction in hirundines (subfamily: Hirundininae), in which all species are socially monogamous aerial foragers. Among the 72 species studied, five species are facing a perceived threat of extinction. Species with sexually dimorphic plumage had a higher risk of extinction than did species with sexually monomorphic plumage. Likewise, when focusing solely on tail ornamentation, species that exhibit a sexual dimorphism in tail length had a higher risk of extinction than did other species. In Hirundininae, which are affected a great deal by severe weather, sexual selection and the resultant sexual dimorphism would increase extinction risk.

Opposing population trends of fork‐tailed swallows and reddish‐coloured swallows in our changing world

“…In summary, we found ornament‐specific population trends and extinction risk, supporting theoretical predictions together with the previous study that showed higher extinction risks of species with distinctive sexually dimorphic tails (Hasegawa & Arai, 2018a). Although previous empirical studies have mostly focused on the overall effect of sexual selection using large clades while combining different kinds of presumably sexually selected traits, such over‐simplification is misleading.…”

“…As in our previous studies (e.g., Hasegawa & Arai, 2017, 2018a; Hasegawa et al., 2016; M. Hasegawa & E. Arai unpublished manuscript), information on wing length, relative fork depth, migratory habit (migratory or not), dorsal plumage coloration (greyish or glossy blackish) and reddish plumage coloration (see Hasegawa et al., 2016) was obtained from Turner and Rose (1994). Turner and Rose (1994) was published more than 25 years ago and thus was not affected by the current population trends and extinction risks.…”

“…Information on Hirundo striolata is given as H. daurica, and thus, we used the information of H. daurica . As before (Hasegawa & Arai, 2018a), using del Hoyo and Collar (2016), which follows the IUCN red list, we regarded ‘threatened species’ (i.e., critically endangered, endangered and vulnerable) as species facing a perceived threat of extinction. Based on the map and description in Turner and Rose (1994), species distributions were classified as restricted (i.e., those breeding in a ‘restricted’ area, as written in the literature with the word ‘restricted’ or a similar description: e.g., ‘restricted range’ as written for the black‐capped swallow Notiochelidon pileate ; or, when no such description was available, species with ranges similar to those described as such were also considered to have a restricted distribution) or unrestricted (i.e., those that breed across a wider area, sometimes described as being ‘widespread’ or a similar description).…”

“…Likewise, we used a Bayesian phylogenetic mixed model with a binomial distribution to analyse extinction risk. Although we used a phylogenetic logistic regression in the previous study (Hasegawa & Arai, 2018a), here we used a Bayesian framework to handle complete separation (i.e., the dependent variable is completely separated by the independent variables, which is difficult for the former analysis to handle: Gelman et al., 2008) because only five species face a perceived extinction threat. For this purpose, we used a Gelman prior for the fixed effects while standardizing each variable using ‘gelman.prior’ in the package ‘MCMCglmm’.…”

Selection in two-sex stage-structured populations: Genetics, demography, and polymorphism

“…Interestingly, using fossil records of ostracods, Martins et al (2018) recently found that species with greater sexual dimorphism exhibit an increased risk of extinction. Similarly, Hasegawa and Arai (2018) estimated perceived extinction risk in 72 species of swallow and found a higher extinction risk for swallows with sexually dimorphic plumage than for species with sexually monomorphic plumage.…”