Abstract

Elaborate, condition-dependent ornaments function as sexual signals in many species,
allowing individuals to gain higher dominance status or reproductive success. Ornament
expression during peak-expression, i.e. the breeding season, is well-documented.
However, dominance is often gained through year-round interactions and so signalling
may be important prior to this. Only top quality males have the resources available to cope
with prolonged investment into ornament growth and maintenance, so males of differing
quality may have different strategies of ornament growth-scheduling. I investigated this in
the lekking black grouse (Lyrurus tetrix), a species with multiple sexual signals, including
red, integumentary eye combs. Eye comb size is dynamic and grows towards the
breeding season. I tested whether condition- and dominance-related variables altered
growth 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 at
all 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 had
higher humoral immunity at the beginning of winter-flocking, which was reversed during
the breeding season, and a near-significant positive association with parasite burden.
Comb size is linked with testosterone, suggesting these males were experiencing
immunological trade-offs, increasing condition-dependence during breeding. In addition,
adults with larger eye combs also had the longest lyres, implying that only top quality
males can simultaneously express multiple traits fully. The two signals may therefore
function as multiple messages of quality, further enforcing honesty. Honesty in yearling
sexual signalling was maintained by a lower level of immunological trade-offs and
covariance with tail length. Therefore, only top quality males can afford the trade-offs
associated with expressing larger eye combs, and maintain expression of multiple
elaborate ornaments.
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Additionally to the size component of ornaments, we also investigated colouration.
Colouration is often measured using summary metrics, however colour is a product of the
full spectrum so this approach is limiting. Comb redness arises from carotenoidpigmented,
red dermal spikes overlaying a white skin-based background. Comb
enlargement is likely to ‘spread’ these spikes, affecting colouration. I used the whole
colour spectrum to unravel the variation in colour attributable to condition-based variables.
Males with larger combs were less red, but with more background reflectance. Heavier
adults with a higher humoral immunity also showed more background reflectance. Hence,
comb size is important in colouration, with condition-dependence being manifested in
background reflectance. However, the composite nature of carotenoid-based colouration
means variation to both components is likely to be important.
Overall, taking a multi-component viewpoint when studying ornament expression
may be key to understanding condition-dependence and scheduling of sexual signaldevelopment.
Multiple traits and within-trait components, e.g. colour and size, are
interlinked over prolonged periods in reflecting quality, and a full range of temporal and
spectral data should be used to unravel this.