Does the tie fit the female? Melanin-based colouration, aggressive personality and reproductive investment in female great tits (Parus major)

Correlated evolution of distinct signals associated with increased social selection in female white-shouldered fairywrens

Conspicuous female signals have recently received substantial scientific attention, but it remains unclear if their evolution is the result of selection acting on females independently of males or if mutual selection facilitates female change. Species that express female, but not male, phenotypic variation among populations represents a useful opportunity to address this knowledge gap. White-shouldered fairywrens (Malurus alboscapulatus) are tropical songbirds with a well-resolved phylogeny where female, but not male, coloration varies allopatrically across subspecies. We explored how four distinct signaling modalities, each putatively associated with increased social selection, are expressed in two populations that vary in competitive pressure on females. Females in a derived subspecies (M. a. moretoni) have evolved more ornamented plumage and have shorter tails (a signal of social dominance) relative to an ancestral subspecies (M. a. lorentzi) with drab females. In response to simulated territorial intrusions broadcasting female song, both sexes of M. a. moretoni are more aggressive and more coordinated with their mates in both movement and vocalizations. Finally, M. a. moretoni songs are more complex than M. a. lorentzi, but song complexity does not vary between sexes in either population. These results suggest that correlated phenotypic shifts in coloration and tail morphology in females as well as song complexity and aggression in both sexes may have occurred in response to changes in the intensity of social selection pressures. This highlights increased competitive pressures in both sexes can facilitate the evolution of complex multimodal signals.

Are plumage colour and song redundant ornaments in great tits (Parus major)? It depends on the colour

According to current theory, the development of different conspicuous ornaments may provide multiple, redundant or unreliable information. However, multiple visual and acoustic traits have not often been tested at the same time. In this study, we analyse whether black tie size, yellow breast colour indexes (chroma and hue) and song parameters (repertoire size, strophe length, maximum and minimum frequency) are inter-correlated in male great tits (Parus major). We found that males with larger black ties sang greater repertoires and had higher song frequencies, providing redundant information. However, song parameters did not correlate with hue and chroma of the yellow breast coloration, supporting the idea that they convey different units of information. Results therefore stress that since different plumaged patches can transmit very different units of information, bird song will correlate to plumage coloration traits depending on the patch and the colour considered, an aspect rarely taken into account in previous studies relating song to plumage colour traits.

Sex-specific signalling of individual personality by a mutual plumage ornament in a passerine

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 statement

The 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.