Abstract
The significance of colour signals in species with strong sexual dimorphism and/or elaborated visual ornaments is rather well-understood. Less attention has, however, been devoted to study colour signals in species with weak or no apparent sexual dimorphism. In such species, an interesting possibility arises as both sexes can bear the same colour ornament(s) (i.e. sexes are mutually ornamented), but their signalling value might differ in males and females. We aimed to explore this possibility by investigating the phenotypic correlates of the black bib, a melanin-based plumage ornament, in the Eurasian tree sparrow (Passer montanus). More specifically, we studied the sex-dependent relationships between bib size and three aspects of individuals’ phenotype: body condition (i.e. size-corrected body mass), physiology (i.e. cellular innate immunity/inflammation status, expressed through total leucocyte counts, and chronic physiological stress, expressed through the ratio of heterophils to lymphocytes) and individual personality (i.e. activity in a novel environment). We found that bib size was not associated with body condition and cellular innate immunity/inflammation status, but was positively related to physiological stress levels independent of sex. Furthermore, bib size was negatively associated with activity in males but positively in females. Our findings bring important correlative evidence that mutual ornamental traits may have sex-specific signalling value.Significance statementThe signalling role of elaborated colourful ornaments, that are usually possessed only by males in sexually dimorphic species, is well-established. The function of various colour traits which are borne by both sexes (i.e. mutual ornaments), however, is less obvious. Do they have a signalling value in both sexes? If yes, do they signal the same information in males and females? Or, most intriguingly, can they convey different information in the two sexes? To test these alternatives, we studied the signalling value of the black bib, a melanin-based mutual plumage ornament, in Eurasian tree sparrows (Passer montanus). Apart from being correlated with chronic stress in both sexes, bib size was positively related to activity in females but negatively in males. Our results suggest that the information content of the same colour trait in males and females can be different in mutually ornamented species.
Highlights
Signals are elaborate and costly traits that can function via many modalities, including acoustic, visual or behavioural ones, which combines both acoustic and visual elements
Activity and scaled mass index (SMI) were related in a sex-dependent manner (Table 1, Fig. 2), post hoc tests revealing that males with better body condition (i.e. SMI) were less active (β = − 0.262, standard error (SE) = 0.107, df = 182, t = − 2.443, P = 0.016), whereas in females SMI was not related to activity
Bib size was associated with activity in a sex-dependent way (Table 2), results of post hoc tests indicating that in males, activity was marginally negatively related to bib size (β = − 0.268, SE = 0.137, df = 147, t = − 1.957, P = 0.052), but in females the two were positively associated (β = 0.183, SE = 0.090, df = 146, t = 2.032, P = 0.044; Fig. 4)
Summary
Signals are elaborate and costly traits that can function via many modalities, including acoustic (e.g. bird song; Murphy et al 2008; Nemeth et al 2012; Baldo et al 2015), visual (e.g. horns, tusks, various colourful traits; Geist 1966; West and Packer 2002; Tibbetts and Dale 2004; Loyau et al 2005; Girard and Endler 2014; Graham et al 2020) or behavioural ones, which combines both acoustic and visual elements (e.g. nuptial dance; McDonald 1989; Lukianchuk and Doucet 2014). As a consequence of this common asymmetry between the sexes, there has been a strong bias, ever since Darwin (Darwin 1871), in research interest and effort towards studying species with strong sexual dimorphism, and in particular the role of elaborated ornaments in males in the context of sexual selection (e.g. Pérez i de Lanuza et al 2013; Seddon et al 2013; Dale et al 2015) This phenomenon is well-illustrated by the strongly maleskewed sex ratio found in museum collections from all over the world for both birds and mammals (Cooper et al 2019). Studies from the last decades (e.g. Amundsen and Forsgren 2001; Jawor et al 2004; Heinsohn et al 2005; Komdeur et al 2005; Weiss 2006, Weiss et al 2009; Griggio et al 2010; Baldauf et al 2011; but see Hill 1993), provide considerable evidence that female colouration, similar to that of males, can be adaptive and evolutionary persistent in contexts of sexual (e.g. mate choice, mate competition), social (e.g. competition over nonbreeding resources) or natural selection (e.g. camouflage against predators) (Amundsen 2000; Clutton-Brock 2007; Rosvall 2011; Tobias et al 2012)
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