Abstract
Body size is a key sexually selected trait in many animal species. If size imposes a physical limit on the production of loud low-frequency sounds, then low-pitched vocalisations could act as reliable signals of body size. However, the central prediction of this hypothesis – that the pitch of vocalisations decreases with size among competing individuals – has limited support in songbirds. One reason could be that only the lowest-frequency components of vocalisations are constrained, and this may go unnoticed when vocal ranges are large. Additionally, the constraint may only be apparent in contexts when individuals are indeed advertising their size. Here we explicitly consider signal diversity and performance limits to demonstrate that body size limits song frequency in an advertising context in a songbird. We show that in purple-crowned fairy-wrens, Malurus coronatus coronatus, larger males sing lower-pitched low-frequency advertising songs. The lower frequency bound of all advertising song types also has a significant negative relationship with body size. However, the average frequency of all their advertising songs is unrelated to body size. This comparison of different approaches to the analysis demonstrates how a negative relationship between body size and song frequency can be obscured by failing to consider signal design and the concept of performance limits. Since these considerations will be important in any complex communication system, our results imply that body size constraints on low-frequency vocalisations could be more widespread than is currently recognised.
Highlights
Signal honesty is fundamental for the evolutionary maintenance of communication
The critical prediction for the hypothesis that size enforces signal honesty is not that this relationship exists between species, but rather that there is a negative relationship between vocalisation frequency and size among competing individuals
We demonstrate three approaches to testing for a relationship between body size and trill frequency using linear regression. (i) Signal design approach: using lowfrequency trill types only (Trills 1, 2, and 3, see Results), we computed an average frequency for each male, and tested tarsus length as a predictor of frequency. (ii) Lower bound approach: using the full sample of songs, we tested for a negative slope in the lower limit of the relationship between frequency and body size, following the standard approach for assessing performance limits [23,36]
Summary
Signal honesty is fundamental for the evolutionary maintenance of communication. Many sexually selected signals are reliable because of physical or physiological constraints on signal production (‘index signals’) [1]. Body size is a key sexually selected trait [2], and the limit that size imposes on producing loud lowfrequency sounds is a textbook example of an honesty-enforcing constraint [3]. The negative relationship between body size and call frequency has strong theoretical support [4,5], as well as empirical support from comparative analyses across animal taxa [4,5] and across species within taxa such as birds [6,7] and primates [8] This comparative empirical support has been demonstrated despite considerable differences between taxa and species in selection pressures, metabolic rates, and soundproducing mechanisms (for example, external in insects, internal in birds and mammals). The critical prediction for the hypothesis that size enforces signal honesty is not that this relationship exists between species (or even within a species between populations or the sexes), but rather that there is a negative relationship between vocalisation frequency and size among competing individuals
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