Quantitative axial myology in two constricting snakes: Lampropeltis holbrooki and Pantherophis 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.

Keywords: Lampropeltis; Pantherophis; anatomy; cross-sectional area; muscle; snake.

Figure 1

Simplified schematic right lateral view of several epaxial muscles in Pantherophis obsoletus (top) and Lampropeltis holbrooki (bottom). Skeletal structure is gray with numbers representing the anterior (1) and posterior (25 and 27) attachment sites for the interlinked epaxial muscles. Colored areas represent contractile tissue and white areas represent tendons. See Table 2 for muscle abbreviations.

Figure 2

Right lateral view of three stretched‐out iliocostalis muscle segments and their interconnecting slips in Pantherophis obsoletus (top) and a schematic illustration of the same muscle linkages (bottom). Each muscle is pulled ventrally so that the smaller interlinkages can be seen.

Figure 3

Right lateral view of the axial musculature in a ratsnake, Pantherophis obsoletus. The large superficial epaxial muscles [semispinalis–spinalis complex, longissimus dorsi, and iliocostalis (IL)] have largely been removed. Portions of the IL have been cut and reflected laterally to show the underlying individual levator costa (LC) muscles.

Figure 4

Right lateral view of the superficial hypaxial muscles in (A) a ratsnake, Pantherophis obsoletus, and (B) a kingsnake, Lampropeltis holbrooki. This view is accomplished by making a mid‐dorsal incision through the skin and reflecting it laterally, severing the costocutaneous muscle insertions on the skin and the fascia between the muscle and skin. See Table 2 for muscle abbreviations.

Figure 5

Covariate (SVL) adjusted means of log₁₀ physiological cross‐sectional area (PCSA) for individual muscles (A) and total column PCSA for each muscle (B) at ca. 50% snout–vent length. Red triangles denote kingsnakes (Lampropeltis holbrooki), open diamonds denote ratsnakes (Pantherophis obsoletus). See Table 2 for muscle abbreviations.

Figure 6

Sum of the log₁₀ physiological cross‐sectional areas (PCSA) for the four largest epaxial muscles (semispinalis–spinalis complex, longissimus dorsi, and iliocostalis) at ca. 50% snout–vent length (SVL) regressed against log₁₀ total SVL. Red triangles denote kingsnakes (Lampropeltis holbrooki), open diamonds denote ratsnakes (Pantherophis obsoletus), and the black line represents the reduced major axis regression model (see Results).

Figure 7

Sum of the log₁₀ physiological cross‐sectional areas (PCSA) from this study regressed against log₁₀ anatomical cross‐sectional areas (ACSA) for the four largest epaxial muscles (semispinalis–spinalis complex, longissimus dorsi, and iliocostalis) at ca. 50% SVL from Penning & Moon (2017). Red triangles denote kingsnakes (Lampropeltis holbrooki), open diamonds denote ratsnakes (Pantherophis obsoletus), and the black line represents the reduced major axis regression model (see Results).