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.

Cranio‐facial remodeling in domestic dogs is associated with changes in larynx position

“…Much of the variation in cranio‐facial size and shape among individuals was, predictably, attributable to differences in overall size (Plotsky et al. ). At the same time, there was additional variation in cranio‐facial size and shape that could not be attributed simply to such differences, and this variation was significantly associated with differences in laryngeal position, as hypothesized.…”

“…Plotsky et al. ). Principal components analysis (PCA) was then used to identify orthogonal dimensions of cranio‐facial variation for subsequent tests of the extent to which such variation might be associated with variation in larynx position.…”

“…The sample involved radiographic images of adult (> 2 years) males (n = 55) and females (n = 51). Details of imaging procedures and morphological measurements have been published previously (Plotsky et al 2013). For this study, we focused on 36 cranio-facial dimensions collectively characterizing variation in the size and shape of the skull and face (Supporting Information Table S1; Chase et al 2002).…”

Auditory Communication in Domestic Dogs

“…The grey wolf is the most sexually dimorphic canid species; males have around 18% greater body mass than females (Hillis & Mallory, 1996). Within dog breeds, those of comparable size to wolves show similar levels of body mass dimorphism, which was found to relate to a difference of 5%-7% in vocal tract length (VTL) between adult males and females for Portuguese water dogs (Plotsky et al, 2013). However, the size difference between sexes decreases in smaller breeds (Frynta et al, 2012), and no sex-related differences in vocal anatomy or in F0 have been identified across breeds when controlling for body weight (Riede & Fitch, 1999;Taylor et al, 2008).…”

The howl of Rancho La Brea: Comparative anatomy of modern and fossil canid hyoid bones

“…It has also been shown to correlate with overall skull shape in domestic dogs, with brachycephalic individuals having more descended laryngeal structures (including hyoids) than dogs with longer, narrower heads (Plotsky, Rendall, Chase, & Riede, 2016). While studies have sought to characterize the vocal repertoire of canids (e.g., Schassburger, 1993) and explored the relationship between body size and vocal tract length (Plotsky, Rendall, Riede, & Chase, 2013), no attempt has been made to relate the morphology of the hyoid apparatus to the vocal capabilities of specific species. Here, we present morphological data for several large extinct and extant canid taxa: the dire wolf ( Canis dirus ), the gray wolf ( Canis lupus ), the red wolf ( Canis rufus ), and both modern and fossil coyotes ( Canis latrans ).…”