How to assess musculature and performance in a constricting snake? A case study in the Colombian rainbow boa ( Epicrates cenchria maurus )

Abstract

The ability of a living organism to perform specific actions such as prey capture or predator avoidance is a critical feature that should affect individual fitness. Snakes have an elongate morphology that lacks the regional variation associated with girdles found in limbed vertebrates. In this context, this group offers interesting opportunities to study specific forms and functions. In this study, we examined musculature in a medium-sized boid snake, the Colombian rainbow boa Epicrates cenchria maurus, by combining two methods: magnetic resonance imaging (MRI) and calliper measurements after axial palpation. The MRI demonstrated an important development of epaxial musculature consistent with the predatory mode of constriction. We showed that MRI and palpation data were strongly correlated, indicating that musculature is an easily measurable phenotypic feature in the studied species. The calliper method was then applied in combination with assessments of physical performance (constriction and defence capabilities) in a group of snakes after a prolonged fast and again after a re-feeding period. A clear relationship was detected between dorsal musculature and both maximal traction and constriction forces. Re-feeding significantly increased dorsal musculature and traction capabilities. While musculature assessments are classically difficult to achieve among tetrapods, this study suggests that snakes are interesting models for examining variation in musculature and performance in natural and experimental conditions.

The king of snakes: performance and morphology of intraguild predators (Lampropeltis) and their prey (Pantherophis)

“…1) in each section of the body (following Jayne and Riley, 2007;Herrel et al, 2011) using ImageJ software (NIH; https:// imagej.nih.gov/ij/) (following Herrel et al, 2011). We chose this method of measuring muscle CSA to follow previous methods (Lourdais et al, 2005;Jayne and Riley, 2007;Herrel et al, 2011) and because a simple measure of external body width (a dimension of length) would represent only half of the possible variation in muscle cross-sectional area (length 2 ); furthermore, previous work has shown that although linear measures are significantly related to muscle crosssectional areas, they miss a considerable portion of the variation in muscle cross-sectional area (Lourdais et al, 2005, found that R 2 =0.73 for this relationship).…”

“…Once the scale was attached, we manually straightened the snake to its maximum length and anchored the spring scale with tape to the flat surface. We then gently agitated the anchored snake to elicit a pulling motion for 5 min (Lourdais et al, 2005, methodology summarized in fig. 2).…”

“…Potential ways of escaping from a constriction coil include pulling out of the coil, counter-constricting to make the aggressor release its coil, and clawing or biting to gain release. Therefore, pulling force is a potentially important measure of performance in snakes because it indicates a snake's ability to escape from the grip of a predator (Lourdais et al, 2005). Geometric scaling offers testable a priori expectations as to how force production scales with body size (Pennycuick, 1992).…”

“…Because muscle force is proportional to muscle cross-sectional area (MacIntosh et al, 2006), we expect pulling force to scale with snake body mass 0.66 . Both predation (constriction pressure) and escape ( pulling force) performance are affected by the cross-sectional area (CSA) of muscle (Moon and Candy, 1997;Lourdais et al, 2005), and in principle can use up to about half of a snake's axial musculature (e.g. all the muscles on the concave parts of a constriction coil or on the concave parts of multiple axial bends used in pulling movements).…”

“…Given that the relationship between constriction pressure and the surface area of contact has not yet been quantified for snakes of any size, we do not have enough information to generate testable hypotheses about the predicted scaling of constriction pressure with body size. Although constriction pressure and pulling force are distinct variables that are typically related to different behaviors, they are appropriate indicators of predation and escape performance in snakes (Moon, 2000;Lourdais et al, 2005;Moon and Mehta, 2007;Boback et al, 2015;Penning et al, 2015;Penning and Dartez, 2016). Therefore, they can be compared across species and sizes to understand the factors that affect the outcome of this predator-prey interaction.…”