Condition-dependence, colouration and growth of red eye combs in black grouse Lyrurus tetrix
Creators: name: Harris, Sarah
Publisher: name: University of Lincoln
Year: 2023
Subjects: D300 - Animal science

description: Elaborate, condition-dependent ornaments function as sexual signals in many species,allowing individuals to gain higher dominance status or reproductive success. Ornamentexpression during peak-expression, i.e. the breeding season, is well-documented.However, dominance is often gained through year-round interactions and so signallingmay be important prior to this. Only top quality males have the resources available to copewith prolonged investment into ornament growth and maintenance, so males of differingquality may have different strategies of ornament growth-scheduling. I investigated this inthe lekking black grouse (Lyrurus tetrix), a species with multiple sexual signals, includingred, integumentary eye combs. Eye comb size is dynamic and grows towards thebreeding season. I tested whether condition- and dominance-related variables alteredgrowth scheduling in the lead-up to (winter flocking) and during the breeding season.Heavier adult males, i.e. those with more resources, grew eye combs larger and faster.Dominant adults, in this case those with higher lek attendance, had larger eye combs atall times. This suggests growth-scheduling is condition-dependent but not dominancerelated.I then investigated the interdependence of eye comb size with condition further,alongside its covariance with other morphological traits. Adults with larger eye combs hadhigher humoral immunity at the beginning of winter-flocking, which was reversed duringthe breeding season, and a near-significant positive association with parasite burden.Comb size is linked with testosterone, suggesting these males were experiencingimmunological trade-offs, increasing condition-dependence during breeding. In addition,adults with larger eye combs also had the longest lyres, implying that only top qualitymales can simultaneously express multiple traits fully. The two signals may thereforefunction as multiple messages of quality, further enforcing honesty. Honesty in yearlingsexual signalling was maintained by a lower level of immunological trade-offs andcovariance with tail length. Therefore, only top quality males can afford the trade-offsassociated with expressing larger eye combs, and maintain expression of multipleelaborate ornaments.7Additionally to the size component of ornaments, we also investigated colouration.Colouration is often measured using summary metrics, however colour is a product of thefull spectrum so this approach is limiting. Comb redness arises from carotenoidpigmented,red dermal spikes overlaying a white skin-based background. Combenlargement is likely to ‘spread’ these spikes, affecting colouration. I used the wholecolour spectrum to unravel the variation in colour attributable to condition-based variables.Males with larger combs were less red, but with more background reflectance. Heavieradults with a higher humoral immunity also showed more background reflectance. Hence,comb size is important in colouration, with condition-dependence being manifested inbackground reflectance. However, the composite nature of carotenoid-based colourationmeans variation to both components is likely to be important.Overall, taking a multi-component viewpoint when studying ornament expressionmay be key to understanding condition-dependence and scheduling of sexual signaldevelopment.Multiple traits and within-trait components, e.g. colour and size, areinterlinked over prolonged periods in reflecting quality, and a full range of temporal andspectral data should be used to unravel this.