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Sexual selection drives elaboration in animal displays used for competition and courtship, but this process is opposed by morphological constraints on signal design. How do interactions between selection and constraint shape display evolution? One possibility is that sexual selection continues exaggeration under constraint by operating differentially on each signal component in complex, modular displays. This is seldom studied on a phylogenetic scale, but we address the issue herein by studying macroevolutionary patterning of woodpecker drum displays. These territorial displays are produced when an individual rapidly hits its bill on a hard surface, and drums vary across species in the number of beats included (length) and the rate of drumbeat production (speed). We report that species body size limits drum speed, but not drum length. As a result of this biomechanical constraint, there is less standing variation in speed than length. We also uncover a positive relationship between sexual size dimorphism and the unconstrained trait (length), but with no effect on speed. This suggests that when morphology limits the exaggeration of one component, sexual selection instead exaggerates the unconstrained trait. Modular displays therefore provide the basis for selection to find novel routes to phenotypic elaboration after previous ones are closed.

Abstract1. Many species perform social displays that incorporate complex body movements.However, the reason why such exaggerated behavioural signals evolve in the first place is unclear.2. Recent work posits that physical displays arise in part because they showcase an animal's motor skill-that is, the ability to produce challenging motor acts with great coordination, precision and/or speed. Support for this idea is largely correlational, with few studies attempting to manipulate metrics of motor skill to assess their effect on physical display efficacy. Here, we address this issue in the downy woodpecker (Dryobates pubescens).Individuals of this species compete for territories through the performance of drums, which are complex displays produced by rapidly hammering the bill against a resonate surface at rates of 16 hits/s. This display is a whole-body endeavour, and its production relies on the ability to swiftly oscillate the head forward and backward at fraction-of-a-second periods.4. Using a series of playback studies, we expose resident birds to experimentally engineered drums that reflect putative fine-scale differences in the motor command of the head and neck. We show that resident individuals respond more aggressively to drums characterized by a cadence with a 9-ms faster beat speed. These residents even modulate their own drum speed to resemble this high-performance stimulus, although they often fail to reach it. Residents also appear to appraise drum acceleration by listening to the time intervals between successive beats in a single drum, while tracking how these time intervals change as the signal progresses. 5. Our data support a connection between motor skill and the effectiveness of a physical display produced through elaborate body movement. We, therefore, suspect that motor skill is adaptive and evolves in response to selection by competition for effective drum displays. K E Y W O R D Sdowny woodpecker, evolution, gestural signal, motor skill, territorial behaviourThis is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

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Physical gestures are prominent features of many species' multimodal displays, yet how evolution incorporates body and leg movements into animal signaling repertoires is unclear. Androgenic hormones modulate the production of reproductive signals and sexual motor skills in many vertebrates; therefore, one possibility is that selection for physical signals drives the evolution of androgenic sensitivity in select neuromotor pathways. We examined this issue in the Bornean rock frog (Staurois parvus, family: Ranidae). Males court females and compete with rivals by performing both vocalizations and hind limb gestural signals, called "foot flags." Foot flagging is a derived display that emerged in the ranids after vocal signaling. Here, we show that administration of testosterone (T) increases foot flagging behavior under seminatural conditions. Moreover, using quantitative PCR, we also find that adult male S. parvus maintain a unique androgenic phenotype, in which androgen receptor (AR) in the hind limb musculature is expressed at levels ∼10× greater than in two other anuran species, which do not produce foot flags (Rana pipiens and Xenopus laevis). Finally, because males of all three of these species solicit mates with calls, we accordingly detect no differences in AR expression in the vocal apparatus (larynx) among taxa. The results show that foot flagging is an androgen-dependent gestural signal, and its emergence is associated with increased androgenic sensitivity within the hind limb musculature. Selection for this novel gestural signal may therefore drive the evolution of increased AR expression in key muscles that control signal production to support adaptive motor performance.androgen receptor | testosterone | courtship behavior | signal evolution | frogs