Release from ecological constraint erases sex difference in social ornamentation

Ecological Effects on Female Bill Color Explain Plastic Sexual Dichromatism in A Mutually-Ornamented Bird

“…The explanation for the different results here and in Funghi et al 13 or Eraud et al 11 is not clear. It may be related with the time of the year in which the different studies were conducted, since our experiments were made in winter and the previous correlational study with waxbills was conducted across spring and summer 13 . Spring and summer correspond to the breeding season of waxbills in this region 53 , which may alter various aspects of their biology.…”

“…Dietary enrichment with either a carotenoid or a non-carotenoid antioxidant increased female but not male bill color saturation, and protection from winter cold increased the stability of female but not male bill color saturation. It was previously suggested that a greater sensitivity to environmental conditions in female than male ornamentation could be explained by sex differences in life-history and in reproductive physiology 13 . Namely, in species with conventional sex roles, females carry the largest physiological burden of reproduction, including egg production, and therefore may prioritize maintaining physiological condition in detriment of investing in costly ornamentation.…”

“…3. Instead, a correlational test found that changes in the bill color saturation of females, but not males, were positively related to uctuations in temperature, especially night temperatures that can pose the greatest energetic stress to small birds 13 .…”

“…The phenomenon of plastic sex differences in ornamentation, i.e., those differences that result from sex differences in phenotypic plasticity, is poorly documented or investigated in species with conventional sex roles. Some examples were nonetheless reported, such as the ornamental bill color of some birds, which can change plastically 7,8,9,10 and where changes in color appear affected by ecology or physiology differently in females and in males 11,12,13 . Plastic sex differences in ornamentation are predicted to evolve when ornamentation bene ts both sexes, but females and males differ in whether or how they bene t from adjusting ornamentation in response to external or physiological conditions 14,15 .…”

Evolution of female coloration: What have we learned from birds in general and blue tits in particular

“…They have found that female coloration is sensitive to the availability of food resources (Morales, Velando, & Torres, 2009;Siefferman & Hill, 2005a) and greater reproductive costs (Doutrelant, et al, 2012). In the common waxbill (Estrilda astrild), favorable environmental conditions (higher nighttime temperatures) were found to positively affect red bill coloration in females: both sexes had similar bill coloration when nighttime temperatures were high but only males had redder beaks under most conditions (Funghi, Trigo, Gomes, Soares, & Cardoso, 2018).…”