Exploring the functional meaning of head shape disparity in aquatic snakes

Abstract

19Phenotypic diversity, or disparity, can be explained by simple genetic drift or, if functional 20 constraints are strong, by selection for ecologically relevant phenotypes. We here studied 21 phenotypic disparity in head shape in aquatic snakes. We investigated whether conflicting 22 selective pressures related to different functions have driven shape diversity and explore 23 whether similar phenotypes may give rise to the same functional output (i.e. many-to-one 24 mapping of form to function). We focused on the head shape of aquatically foraging snakes as 25 they fulfil several fitness-relevant functions and show a large amount of morphological 26 variability. We used 3D surface scanning and 3D geometric-morphometrics to compare the 27 head shape of 62 species in a phylogenetic context. We first tested whether diet specialization 28 and size are drivers of head shape diversification. Next, we tested for many-to-one mapping by 29 comparing the hydrodynamic efficiency of head shapes characteristic of the main axis of 30 variation in the dataset. We 3D printed these shapes and measured the forces at play during a 31 frontal strike. Our results show that diet and size explain only a small amount of shape variation. 32Shapes did not functionally converge as more specialized aquatic species evolved a more 33 efficient head shape than others. The shape disparity observed could thus reflect a process of 34 niche specialization under a stabilizing selective regime. 35

Morphological integration and modularity in the hyperkinetic feeding system of aquatic‐foraging snakes

“…Yet, aquatic‐foraging snakes show a large amount of morphological variability along with an exceptional ecological diversity in terms of diet, behavior, and habitat use (Segall et al. 2020), which may be related to the disparate morphology of their feeding bones (Klaczko et al. 2016).…”

“…The feeding sequence is highly constrained under water, from prey detection, to the hydrodynamic constraints generated by an accelerated strike (Segall 2019; Segall et al. 2020), to the subjugation and manipulation of slippery (e.g., fish, tadpoles), hard (e.g., crustaceans), and elongated preys (e.g., eels), to the lack of constriction in most species, to swallowing a (sometimes living) neutrally buoyant prey item (Moon et al. 2019).…”

Specialized morphology, not relatively large head size, facilitates competition between a small‐bodied specialist and large‐bodied generalist competitors

“…Although prey size is a major factor in influencing head shape within most snakes (Arnold, 1993;Cundall & Greene, 2000;Jayne, Voris & Ng, 2018;Segall et al, 2020), the link between prey size and head size possibly only accounts for challenges relating to ingestion (Vincent et al, 2006). It is important to also consider other selective pressures that may have been involved in the evolution of head shape for different snake species.…”

“…It is important to also consider other selective pressures that may have been involved in the evolution of head shape for different snake species. Factors such as habitat use, prey capture and anti-predator defensive behaviour can play prominent roles in the evolution of the feeding apparatus and head sizes of many snake species (Hibbits & Fitzgerald, 2005;Fabre et al, 2016;Segall et al, 2020). For example, the defensive display of Dasypeltis involves flattening and triangulation of their heads which they achieve by manipulating their quadrates (Young, Lalor & Solomon, 1999), a feature strongly associated with gape size (King, 2002;Moon et al, 2019).…”

Morphological Integration and Modularity in the Hyperkinetic Feeding System of Aquatic-foraging Snakes

“…The head shape of aquatic foraging snakes has functionally converged in response to the physical constraints related to prey capture under water (Fabre et al 2016;Segall et al 2016;). Yet, aquatic-foraging snakes show a large amount of morphological variability along with an exceptional ecological diversity in terms of diet, behavior, and habitat-use (Segall et al 2020), which may be related to the disparate morphology of their feeding bones (Klaczko et al 2016). The feeding sequence is highly constrained under water, from prey detection, to the hydrodynamic constraints generated by an accelerated strike Segall et al 2020), to the subjugation and manipulation of slippery (e.g.…”