Bilateral coordination and the motor basis of female preference for sexual signals in canary song

Abstract

SUMMARYThe preference of female songbirds for particular traits in the songs of courting males has received considerable attention, but the relationship of preferred traits to male quality is poorly understood. Female domestic canaries (Serinus canaria, Linnaeus) preferentially solicit copulation with males that sing special high repetition rate, wide-band, multi-note syllables, called ʻsexyʼ or A-syllables. Syllables are separated by minibreaths but each note is produced by pulsatile expiration, allowing high repetition rates and long duration phrases. The wide bandwidth is achieved by including two notes produced sequentially on opposite sides of the syrinx, in which the left and right sides are specialized for low or high frequencies, respectively. The emphasis of low frequencies is facilitated by a positive relationship between syllable repetition rate and the bandwidth of the fundamental frequency of notes sung by the left syrinx, such that bandwidth increases with increasing syllable repetition rate. The temporal offset between notes prevents cheating by unilaterally singing a note on the left side with a low fundamental frequency and prominent higher harmonics. The syringeal and respiratory motor patterns by which sexy syllables are produced support the hypothesis that these syllables provide a sensitive vocal-auditory indicator of a maleʼs performance limit for the rapid, precisely coordinated interhemispheric switching, which is essential for many sensory and motor processes involving specialized contributions from each cerebral hemisphere.

A fine-scale, broadly applicable index of vocal performance: frequency excursion

“…Performance challenges arise as birds coordinate syrinx modulations with intricately patterned respiratory movements, and as they track changing source frequencies via precise reconfigurations of the vocal tract (Westneat et al 1993;Hoese pg. 4 of 45 et al 2000;Podos et al 2004b;Riede et al 2006;Suthers et al 2012). Studies of handreared songbirds, in which males are trained with challenging song models, have provided direct experimental evidence that aspects of song structure are indeed limited by vocal performance capacities (Podos 1996;Podos et al 2004a;Zollinger & Suthers 2004; see also Lahti et al 2011).…”

Responses to song playback vary with the vocal performance of both signal senders and receivers

“…Trills are difficult to produce, because the vocal apparatus needs to be actively reconfigured during frequency modulations, and to be 'reset' during brief intervals between syllables [15]. Evidence for performance constraints has come from several independent lines, including observations of trade-offs between two acoustic parameters: trill rate and frequency bandwidth [16][17][18]. Thus, for receivers of both sexes, trill performance may provide a reliable indicator of male quality.…”

How does testosterone act to regulate a multifaceted adaptive response? Lessons from studies of the avian song system

“…As summarised in Figure , testosterone is acting at multiple levels in the brain including nuclei in the telencephalon and the diencephalon to control different aspects of this complex learned social signal. Testosterone also acts in the syrinx to modulate aspects of the song specifically linked to mate choice (the special syllables preferred by females) that appear to be shaped by sexual selection . It is difficult to ascertain whether a neural system is really shaped by sexual selection; specifically linking variation in neural organisation to differential fitness involved in the competition for mates has rarely been established in a definitive manner .…”