Gape size, its morphological basis, and the validity of gape indices in western diamond‐backed rattlesnakes (crotalus atrox)

Abstract

Maximum gape is important to the ecology and evolution of many vertebrates, particularly gape-limited predators, because it can restrict the sizes and shapes of prey that can be eaten. Although many cranial elements probably contribute to gape, it is typically estimated from jaw length or jaw width, or occasionally from a combination of these two measures. We measured maximum gape directly for 18 individuals of the western diamond-backed rattlesnake, Crotalus atrox. We measured each individual's body length, several external cranial dimensions, several cranial osteological dimensions from cleaned skeletons, and we calculated gape index values from two published gape indices (GI). Cranial bone lengths and gape circumference showed negative allometry with snout-vent length (SVL), indicating that small individuals have relatively larger heads and gapes than their larger conspecifics. We then used Akaike's Information Criterion to determine which external and osteological measurements were the best predictors of gape. Body size (SVL) was the best predictor of maximum gape overall; however, when SVL was excluded from the analysis, quadrate (QL) and mandible lengths (MdLs) were the best predictors of maximum gape using both external and osteological measurements. Quadrate length probably contributes directly to gape; however, the importance of MdL to gape is less clear and may be due largely to its allometric relationships with head length and SVL. The two published GI did not prove to be better indicators of actual gape than the jaw and QLs in this study, and the gape values they produced differed significantly from our empirically determined gapes. For these reasons, we urge caution with the use and interpretation of computed GI in future studies. The extensive variation in quadrate and mandible morphology among lineages suggest that these bones are more important to variation in gape among species and lineages than within a single species.

Allometry of cranial morphology, gape size and ingestion performance in the banded watersnake (Nerodia fasciata) feeding on two types of prey

“…Specifically, adult N. fasciata increase gape size relatively quickly with increasing body size. This is a different relationship from that observed for the rattlesnake Crotalus atrox, in which there is no conspicuous change in slope between juveniles and adults (Hampton and Moon, 2013). The relatively steady rate of gape growth may be because C. atrox primarily consumes mammals regardless of size.…”

“…For instance, Aubret and colleagues (Aubret et al, 2004) found a disparity in the relative jaw length of two populations of tiger snakes, in which the population consuming larger prey had longer jaws. Hampton and Moon (Hampton and Moon, 2013) found that the slope of gape against SVL for a rattlesnake (Crotus atrox) was lower (0.57) and the intercept higher (0.43) than that for N. fasciata (slope=0.72, intercept=-0.12). This suggests that individuals of C. atrox have greater gape sizes for their body sizes, and that relative gape size does not increase as rapidly during growth in individuals of C. atrox compared with N. fasciata.…”

“…Recently, a study empirically measured gape circumference in rattlesnakes and determined which skull elements were the primary contributors to maximum gape (Hampton and Moon, 2013). Hampton and Moon (Hampton and Moon, 2013) found that mandible length and quadrate length were the strongest contributors to maximum gape when corrected for body size. However, as the authors noted, skull morphology is very diverse among snakes and the primary contributors to gape may vary among species.…”

“…With regard to the size or shape of prey that can be consumed, maximum gape should be considered a two-dimensional measurement, such as circumference. Miller and Mushinsky (Miller and Mushinsky, 1990) and King (King, 2002) developed separate equations that incorporate linear dimensions to estimate gape; however, they may not accurately estimate gape or the scaling of gape with body size (Hampton and Moon, 2013). Furthermore, morphological diversity among linear dimensions does not necessarily equate to functional diversity among individuals because of the emergent properties of complex structures Wainwright et al, 2005).…”

“…For many snake species, juveniles exhibit relatively large heads compared with larger conspecifics (Vincent et al, 2006;Hampton, 2011a;Hampton and Moon, 2013). At least two studies have tested whether negative allometry of head size lowers feeding effort (time and mechanical effort) in snakes, yet ingestion performance has not been shown to vary with body size within a species (Vincent et al, 2006;Hampton, 2011a), suggesting that alternative phenomena are driving negative allometry of head size.…”