Radiographic analysis of vocal tract length and its relation to overall body size in two canid species

Abstract

Body size is an important determinant of resource and mate competition in many species. Competition is often mediated by conspicuous vocal displays, which may help to intimidate rivals and attract mates by providing honest cues to signaler size. Fitch proposed that vocal tract resonances (or formants) should provide particularly good, or honest, acoustic cues to signaler size because they are determined by the length of the vocal tract, which in turn, is hypothesized to scale reliably with overall body size. There is some empirical support for this hypothesis, but to date, many of the effects have been either mixed for males compared with females, weaker than expected in one or the other sex, or complicated by sampling issues. In this paper, we undertake a direct test of Fitch's hypothesis in two canid species using large samples that control for age- and sex-related variation. The samples involved radiographic images of 120 Portuguese water dogs Canis lupus familiaris and 121 Russian silver foxes Vulpes vulpes. Direct measurements were made of vocal tract length from X-ray images and compared against independent measures of body size. In adults of both species, and within both sexes, overall vocal tract length was strongly and significantly correlated with body size. Effects were strongest for the oral component of the vocal tract. By contrast, the length of the pharyngeal component was not as consistently related to body size. These outcomes are some of the clearest evidence to date in support of Fitch's hypothesis. At the same time, they highlight the potential for elements of both honest and deceptive body signaling to occur simultaneously via differential acoustic cues provided by the oral versus pharyngeal components of the vocal tract.

Keywords: animal communication; body size; vocal tract length.

Figure 1

Radiograph of head and vocal tract of Portuguese water dog illustrating vocal tract landmarks and distances between them used to evaluate overall vocal tract length and the length of constituent components of the vocal tract. (1) basihyoid; (2) cricoid; (3) bulla tympanica; (4) incisors; (A) hyoid–bulla distance; (B) cricoid–bulla distance; (C) bulla–incisor distance; (D) spinal axis.

Figure 2

Radiographs of the upper body (top) and zoomed head and vocal tract regions (bottom) of foxes illustrating vocal tract landmarks and distances between them used to evaluate overall vocal tract length and the length of constituent components of the vocal tract. Landmarks and distances are the same as those labelled in Fig. 1.

Figure 3

Radiographs of Portuguese water dogs illustrating variation in the degree of head extension or flexion quantified as the angle of deviation from the spinal axis. (a) 164°; (b) 128°; (c) 87°.

Figure 4

Relationship between body size (humerus length) and vocal tract length measurements in Portuguese water dog sample. Vocal tract length measurements have been corrected for the effects of head position.

Figure 5

Relationship between body size (humerus length) and vocal tract length measurements in fox sample. Vocal tract length measurements have been corrected for the effects of head position.