Quantitative axial myology in two constricting snakes:Lampropeltis holbrookiandPantherophis obsoletus

Abstract

A snake's body represents an extreme degree of elongation with immense muscle complexity. Snakes have approximately 25 different muscles on each side of the body at each vertebra. These muscles serially repeat, overlap, interconnect, and rarely insert parallel to the vertebral column. The angled muscles mean that simple measurements of anatomical cross-sectional area (ACSA, perpendicular to the long-axis of the body) serve only as proxies for the primary determinant of muscle force, physiological cross-sectional area (PCSA, area perpendicular to the muscle fibers). Here, I describe and quantify the musculature of two intraguild constrictors: kingsnakes (Lampropeltis holbrooki) and ratsnakes (Pantherophis obsoletus) whose predation performance varies considerably. Kingsnakes can produce significantly higher constriction pressures compared with ratsnakes of similar size. In both snakes, I provide qualitative descriptions, detail previously undescribed complexity, identify a new lateral muscle, and provide some of the first quantitative measures of individual muscle and whole-body PCSA. Furthermore, I compare measurements of ACSA with measurements of PCSA. There was no significant difference in PCSA of muscles between kingsnakes and ratsnakes. There is, however, a strong relationship between ACSA and PCSA measurements. I could not identify a significant difference in musculature between kingsnakes and ratsnakes that explains their different levels of constriction performance. Unmeasured components of muscle function, such as endurance and force production, might account for differences in performance between two species with similar muscle structure.

The scaling of terrestrial striking performance in western ratsnakes (Pantherophis obsoletus)

“…Herrel et al () measured the CSA of five epaxial muscles (spinalis–semispinalis complex [a 2‐muscle complex], multifidus, longissimus dorsi, and iliocostalis) at approximately 16.5% SVL. However, as axial musculature varies along the body in snakes (Jayne & Riley, ; Nicodemo, ; Penning, ; Pregill, ), we measured muscle CSA at five locations (20%, 40%, 60%, 80%, and 100% SVL; Penning & Moon, ) to assess any differences in the scaling of muscle CSA along the body. For these measurements, we took digital photographs of body cross‐sections using Canon EOS Rebel T5i and Olympus Stylus Tough TG‐630 cameras with scale markers in the field of view, and measured muscle CSA using ImageJ software (http://imagej.nih.gov/ij/).…”

Moving beyond the surface: Comparative head and neck myology of threadsnakes (Epictinae, Leptotyphlopidae, Serpentes), with comments on the ‘scolecophidian’ muscular system

“…Despite the great amount of cephalic myology studies available for snakes (e.g., [11–17]), much of the knowledge remain obscure for several key taxa, with new muscles still being identifyied and described [18]. Some studies on the cephalic myology of snakes provide a series of relevant data regarding their biology (e.g., functional morphology), as well as several systematic issues [19].…”

“…Besides the classical descriptions of the trunk myology by Mosauer [24] and Gasc [25], very few additional studies are available, which are mostly focused on Alethinophidia (e.g., [26–31]). Descriptive and/or comparative studies of the craniovertebral myology in snakes have received even less attention in the past (e.g., [18,32–36]).…”

The axial anatomy of monitor lizards (Varanidae)

“…) and sustained exercise (Farmer & Hicks, ). Although some descriptions of the axial musculature of snakes exist (Mosauer, ; Pregill, ; Penning, ), comparisons between varanids and snakes are problematic due to the extreme specializations for limblessness in snakes (Gasc, ).…”

“…Furthermore, varanids are active foragers with high metabolic rates (Clemente et al 2009) that use buccal pumping to overcome ventilatory-locomotor constraints (Wang et al 1997;Owerkowicz et al 1999), whereas green iguanas are herbivores that are constrained from simultaneous breathing and walking (Wang et al 1997) and sustained exercise (Farmer & Hicks, 2000). Although some descriptions of the axial musculature of snakes exist (Mosauer, 1935;Pregill, 1977;Penning, 2018), comparisons between varanids and snakes are problematic due to the extreme specializations for limblessness in snakes (Gasc, 1981).…”