Integrative and Comparative Biology Integrative and Comparative Biology, volume 60, number 1, pp. 215–231 doi:10.1093/icb/icaa033 Society for Integrative and Comparative Biology

SYMPOSIUM

Reaction Forces and Rib Function During Locomotion in John G. Capano 1

Department of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912, USA Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 From the symposium “Long Limbless Locomotors: The Mechanics and Biology of Elongate, Limbless Vertebrate Locomotion” presented at the annual meeting of the Society for Integrative and Comparative Biology January 3–7, 2020 at Austin, Texas.

1 E-mail: [email protected]

Synopsis Locomotion in most tetrapods involves coordinated efforts between appendicular and axial musculoskeletal systems, where interactions between the limbs and the ground generate vertical (GV), horizontal (GH), and mediolateral (GML) ground-reaction forces that are transmitted to the axial system. Snakes have a complete absence of external limbs and represent a fundamental shift from this perspective. The axial musculoskeletal system of snakes is their primary structure to exert, transmit, and resist all motive and reaction forces for propulsion. Their lack of limbs makes them particularly dependent on the mechanical interactions between their bodies and the environment to generate the net GH they need for forward locomotion. As organisms that locomote on their bellies, the forces that enable the various modes of locomotion involve two important structures: the integument and the ribs. Snakes use the integument to contact the substrate and produce a friction-reservoir that exceeds their muscle-induced propulsive forces through modulation of scale stiffness and orientation, enabling propulsion through variable environments. XROMM work and previous studies suggest that the serially repeated ribs of snakes change their cross-sectional body shape, deform to environmental irregularities, provide synergistic stabilization for other muscles, and differentially exert and transmit forces to control propulsion. The costovertebral joints of snakes have a biarticular morphology, relative to the unicapitate costovertebral joints of other squamates, that appears derived and not homologous with the ancestral bicapitate ribs of Amniota. Evidence suggests that the biarticular joints of snakes may function to buttress locomotor forces, similar to other amniotes, and provide a passive mechanism for resisting reaction forces during snake locomotion. Future com- parisons with other limbless lizard taxa are necessary to tease apart the mechanics and mechanisms that produced the locomotor versatility observed within Serpentes.

Introduction specialized morphologies for a particular habitat, Limbless locomotion in snakes is one of the most such as marine whales, arboreal monkeys, desert versatile locomotor mechanisms in tetrapods. camels, fossorial moles, or flying bats, the conserved Within the confines of an elongate and limbless body form of snakes is equally adept in almost all body plan, snakes have managed to radiate to every environments and even capable of rapid transition continent besides Antarctica and invade almost every between multiple locomotor modes (Gans 1974; possible habitat. There are marine snakes that cruise Jayne 1986). While our understanding of the mech- the open ocean (Lillywhite 2014), arboreal snakes anisms and mechanics that govern terrestrial limbed that cantilever through complex canopies (Astley locomotion abound, the mechanisms and properties and Jayne 2009), desert dwelling snakes that ascend of limbless snake locomotion are less well slippery sand dunes (Marvi et al. 2014), fossorial understood. snakes that burrow through the earth (Greene Propulsion results from the combination of the 1997), and even flying snakes that glide through force exerted from the animal into the environment the sky (Socha 2011). In comparison to mammalian and the equivalent “reaction force” exerted from the equivalents, many of which have evolve exaggerated, environment to the animal (Biewener and Patek

Advance Access publication May 12, 2020 ß The Author(s) 2020. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: [email protected]. 216 J. G. Capano Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020

Fig. 1 Reaction forces during in limbed tetrapods and limbless snakes. Propulsion is a result of the motive force exerted from the animal being resisted by an equivalent reaction force from the environment. This three-dimensional reaction force

(G) can be described as the force vector components relative to the animal: the vertical reaction force (GV), the horizontal reaction force (GH), and the mediolateral reaction force (GML). (A) Lateral view of a bipedal bird. In typical limbed terrestrial locomotion, when the limb presses against the ground, the ground inherently pushes back and generates requisite reaction forces for motion. (B) Dorsal view of a limbless snake. In limbless locomotion in snakes, the axial musculoskeletal system functions similarly to the appendicular musculoskeletal system of limbed tetrapods and is the structure used to exert, transmit, and resist reaction forces. The direction and style of snake locomotion used is determined by mechanical properties of the environment and the direction in which the snake can exert force, as the environment does not necessarily provide enough reaction force in the required orientation for each locomotor mode.

2018). In terrestrial animals with limbs, these forces components that are likely intrinsic to the locomotor are the force of the feet against the ground and the versatility observed in these organisms: the ribs and reaction force of the ground back to the feet. These the integument. These two structures are of crucial three-dimensional ground reaction forces can be de- importance to how snakes experience locomotor re- scribed with their force vector components: vertical action forces and their functional roles vary depend- (GV), horizontal (GH), and mediolateral (GML) ing on the specific locomotor mode the snake uses. (Fig. 1). Typically, the vertical reaction force (GV) results from a combination of the organism’s mass, the normal force of gravity, and any vertical propul- Snake locomotion: A continuum of propulsive kinematics and mechanics sion components; the horizontal reaction force (GH) is attributed to the craniocaudal propulsive force mi- Terrestrial snake locomotion is a continuum of nus any frictional interactions; and the mediolateral movement patterns often categorized into discrete reaction force (GML) is attributed to sideways forces, locomotor modes. Classically, the four predominant which can be associated with maneuverability modes are considered to be lateral undulation, rec- (Fig. 1). As a limbed terrestrial animal moves tilinear crawling, , and concertina loco- through the environment, these reaction forces are motion and most species are capable of some variant first experienced via individual limbs and then trans- of all four (Gans 1974; Jayne 1986). However, most mitted into, and experienced by, the axial musculo- snakes can simultaneously employ two different skeletal system (Carrier 1991; Ritter 1995; Schilling modes in disparate body regions or combine the 2011). mechanics from multiple modes, blurring the lines In snakes, the axial system is all that remains and between these discrete categories (Gray 1946; Greene this musculoskeletal framework becomes the primary 1997; Marvi and Hu 2012; Gart et al. 2019; Jayne structure with which snakes exert, transmit, and ex- 2020). Moreover, while these four locomotor pat- perience all locomotor forces (Jayne 1988a, 1988b; terns share similar characteristics of force exertion Ritter 1995; Schilling 2011). As a snake exerts force and kinematics, there can be large differences in into the environment, locomotor forces are gener- how a snake executes each particular mode, and ated through epaxial activity and then transmitted even variation within that mode. into the substrate through the body wall in contact In a broad context, we can decouple snake loco- with the environment (Cundall 1987; Capano et al. motion into two distinct patterns of body contact. 2019a, 2019b). As such, the body wall of snakes is When snakes move, each body region can either be their sole locomotor structure and contains two key (1) in static contact or (2) non-static contact with Reaction forces and rib functions in snakes 217 the environment (Gans 1974; Cundall 1987). In against objects in the environment that provide ad- static contact modes, force is exerted with regions equate GH. Hence, these push points are essential for of the body that do not move relative to the sur- resisting the caudally directed forces required for for- rounding environment, enabling other body regions ward motion. to move toward or away from the anchored zone. If the environment does not provide push points Rectilinear crawling, concertina locomotion, and against which the snake can exert force in the nec- sidewinding all employ these mechanics, where single essary direction, the snake cannot move and or multiple body regions provide a static base becomes more heavily reliant on friction-induced re- through which locomotor forces are exerted and action forces. While limbed locomotion also depends transmitted (Gans 1974; Marvi and Hu 2012; on friction between the foot and the ground for GH Marvi et al. 2013, 2014; Newman and Jayne 2018). and GML, friction is generally not in short supply Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 In non-static contact modes, the entirety of the due to the large GV in limbed locomotion, where snake is in constant motion and body segments friction against a surface is proportional to the nor- that generate propulsive force move relative to spa- mal force. Therefore, a fundamental difference be- tially constant contact zones, sliding past them. tween legged and limbless terrestrial locomotion is Lateral undulation is the primary example of this, that it imparts particular importance on friction to as the traveling waves of muscle contraction contin- generate the requisite reaction forces for propulsion. uously propel the whole animal forward, while force This is evident from the consistently low Froude is exerted at environmentally stationary push points numbers generated during snake locomotion, a mea- (Jayne 1988b; Moon and Gans 1998; Lillywhite surement of the relative importance of inertial to 2014). With most species of snakes capable of such frictional forces, which indicate that frictional and an extensive repertoire of locomotor modes, it is gravitational forces are an order of magnitude larger important to understand why an individual may se- than inertial forces in snake locomotion (Hu et al. lect one mode over another. 2009). Hence, relative to legged locomotion, where In typical terrestrial locomotion in limbed tetra- the single plane ground provides enough reaction pods, the used is related to energy efficiency, force for motion, snakes rely more heavily on locomotor speed, and movement of the center of three-dimensional force exertion, particularly the mass (Cavagna et al. 1976; Hildebrand 1989; Farley frictional interactions that produce the GH and and Ko 1997; Farley and Ferris 1998; Biewener and GML they struggle to achieve in the absence of limbs. Patek 2018). A limbed tetrapod may use a different gait, for example, run instead of walk, to change the ratios of kinetic to gravitational potential energies The importance of snakeskin and capitalize on inertia to increase energetic effi- As limbless organisms that locomote on their bellies, ciency (Biewener and Patek 2018). In contrast, the the integument of snakes is integral to the frictional specific locomotor style a snake uses in any given interactions these organisms are reliant upon to gen- situation is primarily determined by mechanical erate much of their locomotor reaction forces. Each properties of the environment and the direction in locomotor mode of snakes uses the integument to which the snake can exert propulsive forces (Gans contact the substrate and produce a friction-reserve 1962; Jayne 1986). In legged locomotion, when the that exceeds their muscle-induced propulsive forces, limb presses against the ground, the ground inher- otherwise they experience slippage rather than force ently pushes back and the animal is able to generate transmission. As such, the amount of force a snake GV to counter gravity, GH to move forward, and can exert is limited by the maximum amount of GML to maneuver. This is not the case in snakes. friction they can produce. Snakes also appear capable Although the mass of the snake does generate GV, of modulating the amount of friction they generate this single dimension does not necessarily provide and often produce a static-friction reservoir or re- enough reaction force for the direction the snake is serve (Gans 1970) that exceeds the minimum attempting to travel, as forward motion often amount necessary for propulsion. The ventral scutes, requires GH and GML. For example, in generalized the enlarged belly scales snakes most often locomote lateral undulation, the sweeping bends of a snake upon, are of particular importance because of the exert force laterally and caudally. The multiple op- direction-dependent frictional anisotropy these scales posite lateral forces cancel each other out and the exhibit; measurements of their coefficients of friction vector sum of the forces propels the snake forward. in the forward (lf), backward (lb), and transverse This requires more than the ground below, however, (lt) directions indicate that the ventral scutes most as the snake must be able to exert force caudally easily slide forward, strongly resist sliding backward, 218 J. G. Capano and most strongly resist sliding sideways or trans- rib kinematics in any animal, let alone snakes. The versely (Hazel et al. 1999; Berthe et al. 2009; Hu advent of XROMM, X-ray Reconstruction of Moving et al. 2009; Abdel-Aal et al. 2010). In contrast, the Morphology, (Brainerd et al. 2010; Gatesy et al. lateral and dorsal scales of snakes exhibit significantly 2010) and the expanded use of biplanar fluoroscopy less frictional anisotropy, although these regions have in the field of comparative biomechanics, have since significantly higher absolute frictional coefficients provided the necessary techniques to begin teasing than the ventral scales (Berthe et al. 2009; Abdel- apart how ribs operate, including their contribution Aal et al. 2010; Marvi et al. 2016). The magnitude to snake locomotion (Brainerd et al. 2016; of the anisotropic frictional properties of the ventral Brocklehurst et al. 2017, 2019; Cieri et al. 2018; scales, related to the micro- and nanostructure prop- Capano et al. 2019a, 2019b). erties of the scales (Hazel et al. 1999; Abdel-Aal et al. Although the exact role of the ribs during snake 2010), also appears to be under partial active control. locomotion remains elusive, it is abundantly clear Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 Relative to unconscious snakes, conscious snakes ex- that snakes use their ribs for a wide array of behav- hibit nearly 30% higher sliding friction coefficients iors. Rib rotations contribute to cross-sectional body and almost double their static friction coefficients shape changes that help generate lift and enable glid- (Hu et al. 2009; Marvi and Hu 2012). These results, ing between trees (Socha 2011), mediolateral com- combined with quantitative observations of scale ac- pression to increase swimming performance tuation during locomotion (Marvi et al. 2016) and (Pattishall and Cundall 2008), production of an intrinsic cutaneous muscle activation associated with edge to dig into the sand for crypsis (Young and skin stiffening for locomotor force transmission Morain 2003), dorsoventrally flattening to increase (Newman and Jayne 2018), demonstrate that snakes surface area during basking (Greene 1997), threaten- actively control their ventral scutes to modulate the ing body inflation during hissing (Lillywhite 2014), reaction forces they experience and therefore, the defensive signaling during hooding (Greene 1997; locomotor forces they can transmit. The mechanics Young and Kardong 2010; Lillywhite 2014), and of the various locomotor modes each use the integ- the volume changes associated with an ancestral ument differently to exert motive force and likely function of ribs for all amniotes, lung ventilation synergize with and involve another often- (Rosenberg 1973; Brainerd and Owerkowicz 2006). overlooked component of snakes: their ribs. Moreover, as squamates, snakes’ ribs are highly mo- bile and have the capacity to rotate about three po- tential axes of rotation, colloquially described as (1) Biological roles of ribs in snakes “bucket handle” rotation about a dorsoventral axis In the absence of limbs, the hundreds of repeating (Fig. 2A and D)(Supplementary Video S1), (2) pairs of ribs in snakes are their primary skeletal “caliper” rotation about a craniocaudal axis structures to interact with the environment, although (Fig. 2B and E)(Supplementary Video S2), and (3) their functional role during locomotion is unclear. “pump handle” rotation about a mediolateral axis, a While electromyography experiments have demon- rotation that can also be thought of as long-axis strated that the epaxial muscles produce much of rotation about the neck of the rib (Fig. 2C and F) the propulsive forces during most serpentine loco- (Supplementary Video S3) (Jordanoglou 1970; motor modes (Jayne 1988a; Moon and Gans 1998; Osmond 1985). Within the variety of behaviors for Newman and Jayne 2018), our understanding of the which snakes use rib motions, the axes each rib locomotor contributions of the over fifteen serially rotates about appear to differ depending on the de- repeated hypaxial muscles, most of which attach to sired output; for example, the axes a rib rotates the ribs, is nowhere near as complete (Mosauer 1935; about during hooding, a probable combination of Gasc 1981; Gasc et al. 1989; Moon and Gans 1998; caliper and pump handle rotations (Young and Newman and Jayne 2018; Penning 2018; Martins Kardong 2010), are likely quite different from those et al. 2019). As organisms that locomote on their for the lateral compression for swimming, presumed bellies, however, the locomotor forces a snake exerts to be primarily bucket and pump handle rotations must inherently be transmitted through the ribs, to (Pattishall and Cundall 2008). Additionally, snakes the integument, and into the substrate (Cundall may exhibit further rotational permissiveness because 1987; Capano et al. 2019a, 2019b). This intrinsic of their complete lack of a sternum and absence of costal involvement during snake locomotion was first sternal ribs; an individual snake’s hundreds of pairs noted over three decades ago but has received little of ribs are all unipartite (composed of a single costal attention since (Cundall 1987), likely due in part to element homologous with the vertebral ribs of other the difficulty of quantifying and comparing in vivo amniotes; Hoffstetter and Gasc 1969). This unipartite Reaction forces and rib functions in snakes 219 Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020

Fig. 2 Three-dimensional rotations about each costovertebral joint. Rib motions are conventionally described relative to three ana- tomical axes; (A) and (D) “Bucket handle” rotation about a dorsoventral axis (blue arrow; see Supplementary Video S1); (B) and (E) caliper rotation about a craniocaudal axis (green arrow; see Supplementary Video S2); and (C) and (F) Pump handle rotation about a mediolateral axis (red arrow; see Supplementary Video S3), that is, long-axis rotation about the neck of the rib.

Note: (A), (B), and (C) are cranial (frontal) views of vertebrae and left rib; (D), (E), and (F) are lateral views of the same vertebrae and left rib (cranial is to the left); colored tracers indicate path of distal rib tip; transparent colored ribs indicate rib position after rotation about the corresponding axis. morphology may therefore permit an even wider distention during prey ingestion or force exertion range of motion due to the absence of potentially during constriction (McDonald 1959; Rosenberg constraining ventral connections. 1973; Wallach 1998). In these scenarios, the ribs are presumably unable to rotate because they are Regional control of rib motion in snakes either displaced by the bolus or pressed against the ribcage of the prey. Recent XROMM experiments of The serially repeated ribs of snakes are present al- lung ventilation in Boa constrictor have confirmed most the entire length of all snake species and rib this capacity to spatially relocate (Capano et al. motions do not appear to be restricted to specific 2019a, 2019b). In these experiments, rib kinematics regions. Rib rotations can be spatially localized, were measured (1) during normal ventilation at rest such as during the hooding display of cobras, or and (2) while an inflated blood pressure cuff was present throughout the length of the animal, such applied to this same region, in an attempt to pre- as the rib motions during lateral compression for clude rib motion and simulate constriction. The swimming or horizontal compression during gliding findings were that each individual tested ceased rib and basking (Greene 1997; Pattishall and Cundall rotations in regions where the cuff was applied and 2008; Young and Kardong 2010; Socha 2011). shifted the location of ventilation posteriorly, a clear Furthermore, snake locomotion often involves simul- demonstration of active control of rib rotations taneous activity of disparate body segments, with the throughout the body (Capano et al. 2019a, 2019b). function of each region, and likely their underlying ribs, fluctuating throughout a locomotor cycle (Cundall 1987; Greene 1997; Gart et al. 2019). Reaction-force perspective on terrestrial As in all squamates, a critical function of rib rota- locomotion in snakes tions in snakes is for lung ventilation. Within the For locomotion, the extensive mobility and indepen- elongate bodies of snakes are similarly elongate dent control of rib motions in snakes suggest the ribs lungs, where the left lung is reduced or entirely ves- may have different mechanical functions depending tigial and the right lung extends 30–80% of snout- on the locomotor style used and the reaction forces vent length (Wallach 1998; Lillywhite 2014). experienced. Each of the four predominant modes of Numerous anecdotal observations have suggested snake locomotion provides examples of the multiple that lung ventilation can shift throughout the lung, mechanisms used to generate reaction forces and typically in response to external stimuli, such as plausible functional roles of the ribs that combine 220 J. G. Capano to produce the versatility and variation observed in ribs, and the ribs are highly mobile, why do they not the continuum of snake locomotor styles. rotate or fold under these contractile forces? Moreover, the CCS and CCI are functional antago- nists that both attach to the ribs with no overlapping Rectilinear crawling or counterbalancing activation (Newman and Jayne Of all the forms of snake locomotion, rectilinear 2018). As such, the lack of motion suggests a new crawling is the only one that does not use axial potential hypothesis: that each rib is actively immo- bending to generate motion. Instead, this mode pro- bilized by other hypaxial musculature to enable an- duces unidirectional propulsion through movement terior or posterior costocutaneous musculature to of the external skin relative to the internal skeleton pull against the rib, depending on the locomotor (Lissmann 1950; Newman and Jayne 2018; Jayne phase. 2020). The ventral scutes interact with the substrate Isometric contractions of numerous costal muscles Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 to simultaneously produce numerous, at times up- could provide the necessary stabilization for costocu- wards of 100 (Newman and Jayne 2018), static con- taneous contraction, and the architecture of these tact zones along the body and generate GV and GH muscles suggests tractable hypotheses to explain the via weight of the animal and friction of the scales observed rib immobility during rectilinear crawling. (Marvi et al. 2013). Regions of skin between these For example, costal muscles with posterior fiber ori- zones are then simultaneously lifted and pulled for- entation, relative to the vertebrae, such as the levator ward toward the static areas in alternating waves to costae, tuberculocostalis, or intercostalis quadrangu- produce propulsive and recovery phases. Two laris, may hold the ribs steady while the CCS pulls muscles known to effect this form of locomotion caudal scales cranially toward static contact zones; are the costocutaneous superior (CCS) and costocu- while anteriorly directed costal muscles such as the taneous inferior (CCI) (Lissmann 1950; Gasc 1981; iliocostalis, obliquus internus ventralis, obliquus Newman and Jayne 2018). From each rib, a CCS internus dorsalis, or transversus dorsalis stabilize courses posteriorly from approximately one-third the ribs as the CCI pulls the entire skeleton toward the length down the rib from the costovertebral joint static scales. This new active stabilization hypothesis to connect to multiple caudal lateral and ventral could be thoroughly tested through a combination of scales. The CCI runs anteriorly from each ventral XROMM of the ribs, along with electromyography of rib tip for approximately six intercostal spaces to the CCS, CCI, and potential stabilizing muscles men- attach to more cranial lateral and ventral scales tioned above. A lack of rib motion, combined with (Gasc 1981; Newman and Jayne 2018). As the animal activity of the CCS and CCI, would suggest the ribs progresses, the CCI shortens, pulling the skeleton are actively immobilized, while activity of other cos- forward toward anchored zones, while the CCS tal muscles may provide evidence for how this im- shortens and pulls posterior scutes anteriorly, to gen- mobilization occurs. Nevertheless, the absence of rib erate a new anchored zone (Lillywhite 2014; motion is likely an active mechanism in this behav- Newman and Jayne 2018). The result is that the ior and presents an interesting line of inquiry to skeleton moves forward at a nearly constant rate better understand the ribs’ role in this specialized while regions of skin either slide faster than the en- form of vertebrate locomotion. tirety of the animal or remain stationary to transmit locomotor forces. Over two centuries ago it was proposed that rec- Concertina tilinear crawling involved literal “rib-,” with Concertina locomotion is generally described as the snake crutching and crawling on its ribs similar “accordion style” movement, where the snake folds to the legs of a caterpillar (Home 1812; Sibson 1846; itself like the pleats of an accordion for ratcheted Sood 1946). Numerous experimental methods in- progression (Jayne 1988a; Marvi and Hu 2012; cluding X-ray fluoroscopy, electromyography, and Xiao and Chen 2013). In this mode, at least one marker-based XROMM have since overturned this portion of the body is statically anchored within hypothesis and shown that the ribs do not move the environment, which enables anterior or posterior relative to the vertebrae during rectilinear progres- regions to move toward or away from the anchored sion (Supplementary Video S4; Lissmann 1950; zone. Generally speaking, the reaction forces in this Newman and Jayne 2018). However, this presents mode are produced by the weight of the animal an interesting conundrum: if musculature is short- (GV), the frictional coefficient and activation of the ening to either physically pull the entire skeleton scales (GH), and any muscle-induced laterally di- forward by the ribs or retract the skin toward the rected forces (GML)(Marvi and Hu 2012; Reaction forces and rib functions in snakes 221

Lillywhite 2014). These general principles govern fold under these forces? How do they transmit force? most concertina mechanics, but variations in body Undesired rib rotation would represent a substantial contact, mechanisms of friction generation, and rib energy dissipation and although concertina is not function produce a spectrum of concertina styles. particularly efficient, the metabolic demand is attrib- Although rarely observed, concertina may be used uted to the transitions of acceleration and decelera- on flat surfaces. In the absence of lateral contact tion associated with this mode (Marvi and Hu 2012). zones such as tunnel walls or push points, snakes Moreover, observations of snakes using concertina using concertina mechanics on flat surfaces must do not suggest high degrees of body deformation generate locomotor reaction forces solely through that would be associated with rib folding. Potential their mass and frictional interactions of their ventral explanations might be that (1) the costal muscles scales (Gray and Lissmann 1950; Hirose 1993; Hazel stabilize the ribs to immobilize them for force trans- et al. 1999; Lillywhite 2014). This form is therefore mission, similar to rectilinear crawling or (2) the ribs Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 inherently reliant on the direction of force applica- actively rotate laterally and therefore contribute to tion and will generate the highest static friction per force exertion during contact. Stabilization against surface area of substrate contact when the ventral GML or mediolateral exertion would likely involve scutes are oriented sideways or backward, due to lateral rib rotations and muscles such as the levator the relatively high friction coefficients (lt and lb) costae, tuberculocostalis, or intercostalis quadrangu- in these directions. This suggests that the S-shape laris are possible candidates with mechanical orien- often observed during flat concertina may increase tations to provide such function. frictional forces, as it orients more scales sideways Additionally, the anchor-slide mechanics of con- rather than backward to generate higher GH and in- certina locomotion also enable snakes to climb rela- crease the reserve of friction-induced reaction force tively steep and cylindrical surfaces such as those required for locomotor force exertion. Evidence of found in arboreal habitats. In the absence of discrete snakes modulating scale orientation, contact time, push points, snakes are able to generate static contact and stiffness (Marvi and Hu 2012; Newman and zones by coiling with their ventrum against the Jayne 2018) to increase friction and force transmis- branch-like object to periodically grip a surface sion also suggests that snakes may actively regulate through medial force exertion (Astley and Jayne reaction force production, although robust measure- 2007, 2009). These kinematics are remarkably similar ments of reaction forces and correlation with scale to those of constriction, although constriction coils orientation during snake locomotion are lacking. are applied with ventral and lateral body walls, de- Typically, however, snakes employ concertina lo- pendent upon species and number of loops (Mehta comotion in narrow tunnels and burrows (Gray and 2005; Mehta and Burghardt 2008; Moon et al. 2019). Lissmann 1950; Jayne 1986, 1988a). In these circum- As the slope of the incline increases, the animal’s stances, the lack of cranially oriented contact zones mass contributes less to the production of normal prevents the animal from experiencing sufficient GH forces and contact becomes more reliant on muscu- that enables caudal propulsive force exertion because lar effort and friction, the latter aided by the lt and the snake is unable to create the cranially directed lb of the scales. A similar question remains, how- force vectors needed for lateral undulation. Forward ever: how are reaction forces resisted by the snake propulsion is, therefore, a stepwise process of an- and what role may the ribs play? Snake ribs are ca- choring and sliding, where the animal bends the pable of dorsal caliper rotations, as observed in body to contact the straight-sided walls and produce hooding displays, basking behaviors, and large prey a static contact zone. Transverse forces are exerted ingestion (Greene 1997; Lillywhite 2014), but uncon- across the channel width, predominantly with the trolled ventral displacement during concertina loco- robust epaxial muscles (Jayne 1988a), and these motion would dissipate locomotor energy and place forces are often in excess of the force required to undue pressure on viscera. It is possible that the ribs prevent sliding out of contact. Snakes have been maintain their posture and position similarly as pro- shown to push against sidewalls with forces up to posed in tunnel-based concertina: through active ro- nine times their own body weight, a safety factor of tation or stabilization to exert or transmit force 400% (Marvi and Hu 2012). This inherently involves during climbing. Muscles of interest may include force transmission through the lateral body wall and, those active during exhalation, including the trans- by proxy, the ribs, that are in contact with the tun- versus dorsalis, obliquus internus dorsalis, transver- nel. Considering the rotational mobility of the ribs sus ventralis, and obliquus internus ventralis, as and substantial GML experienced, this presents an- these produce medially directed rotations and would other interesting dilemma: How do the ribs not provide favorable orientations to avoid the ribs from 222 J. G. Capano splaying dorsally (Rosenberg 1973; Gasc 1981). although measurements of the frictional anisotropy Nevertheless, a more concentrated focus on the re- in sidewinders and their influence on this locomotor action forces generated during concertina locomo- mode are underexplored. In instances where in- tion, and the implicit function of the ribs and creased reaction forces are required, however, such associated muscles, would provide an opportunity as inclined sand dunes, sidewinders do not increase to more deeply understand the mechanics of this sand penetration depth but rather increase the rela- serpentine mode. tive proportion of their body in contact with the substrate, in effect increasing GH, relative to the di- rection of travel (Marvi et al. 2014). It is also possi- Sidewinding ble that these snakes actuate their scales to increase Sidewinding, a mesmerizing testament to the incred- frictional coefficients, using active rib motions to ible neuromuscular control of snake locomotion, is a increase body width and surface area contact, or Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 specialized mode particularly effective for traversing contract musculature attached to their ribs to granular and slippery substrates (Gans 1970). It is sharpen the lateral keel of their scales to dig into most commonly and effectively executed by stout, the sand (Jayne et al. 2015; Marvi et al. 2016). desert dwelling viper species adapted to navigate Similar to other bending forms of locomotion, the steep sand dunes, and aquatic snake species that reg- motive forces of sidewinding are generated with ularly negotiate exposed mud flats (Jayne 1986; epaxial muscles and transmitted into the substrate Marvi et al. 2014). When a snake side winds, indi- with the lateral side of the body and highly mobile vidual body segments maintain static contact while ribs. Effective force transmission therefore likely other regions simultaneously bend and lift; in effect, involves active stabilization of these skeletal ele- the static segments “roll over,” are lifted out of con- ments, although another potential is that individual tact, and are then placed down at the next static ribs rotate to exert propulsive force or press into the contact zone, producing distinct, discontinuous trough of the trackway, similar to the rib motions tracks. The track orientation and the snake’s body, used for digging in other desert-dwelling vipers however, are oblique relative to the snake’s direction (Young and Morain 2003). of travel, a peculiar pattern that appears crucial to this mode’s stability on granular substrates. The propulsive force a sidewinding snake can ex- Lateral undulation ert is constrained by the maximum static-friction Terrestrial lateral undulation is the fastest, most reservoir generated through body contact. As a snake ubiquitous form of snake locomotion and is the ascends a sand dune, the oblique orientation of the characteristic slithering associated with snakes body directs any slippage laterally, such that sliding (Jayne 1986; Greene 1997; Lillywhite 2014). Unlike frictional forces are converted into static friction. A other forms of snake locomotion, lateral undulation sliding sidewinder merely digs into the sand more, involves non-static contact with the environment, with slippage producing an accumulation of material such that body segments that exert propulsive force posterior to the long axis of the body and giving are in constant motion relative to spatially fixed con- them a deeper trough to push against. This com- tact zones, sliding past them. The snake bends from bined with active twisting and torsion of the body side-to-side to exert force caudally and laterally that directs the posterior edge of their scales into the (Gans 1962; Jayne 1988b), and when these arcing sand, increases and maintains the potential reaction bends exert force at multiple locations, the lateral forces that enable their ascent (Gans 1970). The forces cancel each other out and the vector sum of three-dimensional nature of sidewinding is of equal the forces propels the snake forward. importance, as lifting behavior reduces sliding con- Slithering, however, is more than mere side-to- tact, which reduces frictional drag and lessens the side bending, and the presence or absence of discrete yielding of sand that will diminish available reaction push points changes the fundamental mechanisms forces (Marvi et al. 2014; Astley et al. 2015). used for motive force production. In the presence A sidewinder employs numerous mechanisms to of push points, such as rocks, branches, or the increase reaction force generation, many of which “pegs” often used in controlled experiments (Jayne may involve participation of the ribs. With the 1988a; Lillywhite 2014), snakes press their bodies body oriented obliquely relative to the direction of against each push point and exhibit differential travel, force is applied by regions with sideways- body curvature, where the vertebral column bends oriented scales with high frictional coefficients (lt). widely around the peg and the body wall asymmet- This may increase absolute reaction forces generated, rically deforms on either side of the peg (Moon and Reaction forces and rib functions in snakes 223

Gans 1998). This produces higher curvature on the are either (1) stabilized to prevent folding and enable anterior side of the peg relative to the posterior side. force transmission, as this mode is driven at least in As the snake bends, force is then differentially ap- part by the large epaxial muscles (Jayne 1988a), or plied to the region of higher curvature, causing the (2) rotated laterally via hypaxial musculature to con- contact zone to be displaced from the area of higher tribute to force exertion (Moon and Gans 1998). curvature, anterior to the peg, to the area of lower In contrast, when no discrete push points are curvature, posterior to the peg (Moon and Gans available, the mechanics of lateral undulation are en- 1998). In effect, this results in forward translation tirely reliant on frictional interactions. A snake on a of the whole animal with a locomotor mechanism flat surface can still laterally undulate by using the analogous to a modified cam follower (Gasc et al. asperities of the surface to function as microscopic 1989; Moon and Gans 1998). push points. These irregularities enable the anisot- Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 Although curvature is essential to push-point lat- ropy of the ventral scales to generate sufficient GH eral undulation, it is not entirely clear how snakes and GML to prevent slippage and enable force trans- produce these body shape adjustments. mission (Hu et al. 2009; Hu and Shelley 2012). As Electromyography experiments in Python regius sug- the snake moves, the lf of the scales in contact with gest anterior curvature about the peg is associated the ground produces frictional resistance that must with activity of the supracostalis ventralis and dor- be overcome with additional muscular effort. When salis, two superficial hypaxial muscles that course snakes slither at high speeds using this method, they between two ribs numerous interspaces apart, and often raise sections of the body off of the substrate. that this musculature produces bulging to increase This has two important consequences: (1) it reduces curvature (Gasc et al. 1989). In this scenario, how- frictional resistance due to lf and (2) redistributes ever, contraction of the supracostalis to produce weight to areas still in substrate contact (Hu et al. bulging would require active stabilization of the cor- 2009). This phenomenon of “sinus-lifting” has been responding ribs to prevent interspace collapse and observed and noted in the literature since the late instead enable body wall deformation. It is also pos- 1800s, although the mechanical underpinnings are sible that the supracostalis or other musculature may only now beginning to be understood (Stradling actively displace ribs laterally to produce asymmetri- 1882; Hirose 1993). cal curvature. The large levator costae, a hypaxial Sinus-lifting, therefore, appears to be a mechanism muscle with substantial physiological cross-sectional that allows snakes to slither faster without slipping. area (PCSA) and already hypothesized to participate Body lifting occurs in regions that exhibit the highest in snake axial bending (Moon and Gans 1998; degree of curvature and appears to be a mechanism to Penning 2018), is mechanically oriented to move modulate frictional forces (Hu et al. 2009). The three- ribs as it does during lung ventilation and defensive dimensionality of this lifting behavior means that the hooding (Rosenberg 1973; Young and Kardong normal force exerted by the snake is a function of 2010) and may contribute to body wall deformations mass distribution and torque, not necessarily curva- at contact sites. Additional muscles such as the ture (Hirose 1993). Furthermore, the normal force tuberculocostalis or intercostalis quadrangularis exerted is concentrated in regions with the lowest cur- have similar architecture to produce such deforma- vature, the same regions where tangential force is low- tion through rib motion, and future studies of mus- est, indicating that this is where slippage is most likely cle function in this mode would provide useful to occur (Hirose 1993; Hu et al. 2009; Hu and Shelley information into these complex mechanics. 2012). In order to modulate those normal forces, Unlike most other serpentine modes, the mechan- however, theoretical modeling suggests that body lift- ics of push-point lateral undulation do not necessar- ing non-uniformly distributes the animal’s weight to ily require friction to function. Experimental load less curved regions and changes the directionality manipulations with a snake slithering in a nearly of frictional forces (Hirose 1993; Hu et al. 2009; Hu frictionless system composed of a smooth lubricated and Shelley 2012). This increases the normal force, surface with fixed, rotatable vertical pegs, found that effectively increasing the magnitude of friction against the snake moved faster than through a standard peg- which the snake can push and enabling the snake to board (Gans 1970). These observations and the lf of exert more motive force. While, admittedly, the non- scales demonstrate that sliding friction detracts from uniform weight distribution model did not account propulsion, static friction is not required to produce for non-uniform body contact or craniocaudal varia- motion, and the pegs merely enable the requisite GH tion in cross-sectional width and mass, researchers and GML to enable force exertion. At the point where werestillunabletoaccountforthehighrangeof the body exerts force into the peg, the mobile ribs speeds observed in their slithering trials, discrepancies 224 J. G. Capano potentially related to the intricacies of snake skin, the provide multiple functions depending on how the modulation of weight distribution, and the forces as- snake moves. As described above, locomotor propul- sociated with body lifting (Hu et al. 2009). sion in snakes often includes mediolateral reaction In the sinus-lifting configuration, the snake dis- forces, activity of musculature with one or multiple tributes weight to and exerts force in body regions costal attachment sites, differential weight distribu- oriented obliquely relative to their direction of travel. tion along the body, and active modulation of fric- Theoretical models of lateral undulation and mass tion (Gans 1962, 1974; Jayne 1988a; Moon and Gans distribution (Hu et al. 2009; Hu and Shelley 2012) 1998; Hu and Shelley 2012; Marvi et al. 2016; demonstrated that lb had little effect on locomotor Newman and Jayne 2018). In each instance, the speed, suggesting that snakes rarely exerted force di- ribs and their associated musculature likely provide rected backward relative to their continually re- different roles that, given the modularity in snake orienting body axis. As the snake bends from side design and body control, plausibly fluctuate between Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 to side, however, it is evident that the scales contin- functions throughout a locomotor cycle or in re- uously reorient with respect to one another and the sponse to environmental variables. Ribs may (1) ex- direction of travel, potentially to optimize the use of ert forces, through muscle-induced active rotation or the scales’ high lt. It has also been observed that (2) transmit forces, through muscular stabilization to snakes actuate individual scales to modify their fric- prevent rib cage collapse and energy dissipation dur- tional coefficients (Marvi et al. 2016), contract cuta- ing substrate contact. They also (3) provide an at- neous musculature to stiffen the scales for force tachment site for locomotor musculature and may transmission (Newman and Jayne 2018), and modify be actively held still as antagonists for other muscles body width and surface area contact, often through to pull toward or against, particularly cutaneous rib motions (Greene 1997; Pattishall and Cundall musculature likely involved in friction modulation 2008; Socha 2011; Lillywhite 2014). Therefore, ven- (Marvi et al. 2016). Additionally, rib motions may tral scale contact and friction generation is not con- (4) change body shape to contribute to the genera- stant, and snakes may preferentially distribute weight tion or maintenance of frictional reserves. Therefore, to specific regions, potentially to those with the most ribs may actively rotate or be stabilized to contribute advantageous scale or body axis configuration, in an to overall force production and optimization and attempt to optimize force output. Furthermore, lift- present a wealth of opportunities for tractable hy- ing may also enhance force optimization, as hypoth- potheses about the function of these structures dur- esized in other studies, through increased torsion, ing locomotor behaviors (Cundall 1987; Gasc et al. which may increase the normal force more than an- 1989; Moon and Gans 1998; Penning 2018). All of ticipated by models and enable higher overall force these proposed functions involve active muscular exertion and locomotor speed (Hirose 1993; Moon control, whereas the intrinsic morphology of the and Gans 1998). Nevertheless, although it is clear rib heads and costovertebral joints of snakes suggest that snakes modulate the directionality of frictional a passive mechanism of equal importance. forces, it is not clear what reaction forces are mod- ified and through what mechanisms this occurs. Continued study of lateral undulation is necessary Functional morphology of to understand the extent to which slithering snakes costovertebral joints in snakes distribute weight non-uniformly, the interactions of An important consideration when thinking about scale actuation and stiffness on overall speed, and skeletal kinematics are the joints about which the how snakes modulate these variables in relation to elements move, and costovertebral joints are no ex- the frictional coefficients of their own scales and the ception. The ancestral costovertebral joint morphol- substrate. The use of small pressure sensors, X-ray ogy for amniotes is two-headed ribs that have a videography, and electromyography would prove tuberculum and capitulum on each rib that articu- useful in teasing apart the intricacies of flat lateral late with a diapophysis and parapophysis on each undulation and likely provide a wealth of inspiration vertebra, respectively. This bicapitate condition is for the next generation of biomimetic applications. retained in almost all extant amniotes except squa- mates, that are notably different in having unicapi- tate ribs which articulate with each vertebrae at a Biomechanics of ribs during locomotion single articular surface, the synapophysis (Romer in snakes 1956; Hoffstetter and Gasc 1969). Snakes, however, In terms of locomotion, the extensive mobility of have double-headed ribs that articulate with the ver- costovertebral joints in snakes means the ribs may tebrae at two distinct articular surfaces (Fig. 3A–C). Reaction forces and rib functions in snakes 225

convex prominence on the vertebra (Fig. 3C; blue arrow), analogous to a diapophysis. The ventral as- pect of each costovertebral joint has (2) a slightly convex and flattened articular surface on the rib (Fig. 3A, B, and E; red arrow), analogous to a capit- ulum, which articulates with a broad, slightly con- cave articular surface on the vertebrae (Fig. 3C; red arrow), analogous to a parapophysis. This ventral portion is also obliquely oriented and medially recessed relative to the dorsal prominence (Fig. 3D and E; Sood 1946; Hoffstetter and Gasc 1969; Pattishall and Cundall 2008; Young and Kardong Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 2010). The dorsal articular concavity of the rib also Fig. 3 Biarticular costovertebral morphology in B. constrictor. exhibits a conspicuous ridge along the caudal edge of Unlike most squamates, the costovertebral joints in snakes have the joint (Fig. 3A and B; black arrow) that is sug- two distinct articular surfaces. (A) Oblique lateral view of a left rib head and (B) a cranial view of the same left rib head. The rib gested to preclude cranial translation and enable the heads of B. constrictor have a concave, dorsal articular facet (blue dorsal articulation to function similar to a ball-and- arrow), a slightly convex and flattened ventral articular facet (red socket (Hoffstetter and Gasc 1969; Pattishall and arrow), and a prominent caudal ridge (black arrow). (C) Lateral Cundall 2008). Anatomical observations also demon- view of the left side of the corresponding vertebra shows the strate that, at rest, the ribs’ ventral flattened articular convex dorsal prominence (blue arrow) that fits with the dorsal surface is flush against the similarly flattened ventral facet of the rib similar to a ball-and socket (blue arrow) and the vertebral surface and permits craniocaudal shear, broad, slightly concave ventral articular surface (red arrow) that while rotation occurs primarily about the dorsal is medially recessed and articulates with the ventral facet of the rib. (D) A cranial view and (E) caudal view of the costovertebral facet (Pattishall and Cundall 2008). These general- joint showing the oblique orientation of the costovertebral joint. ized morphological characteristics have been docu- Note the ventral facet of the rib buttressed against the ventral mented in numerous phylogenetically diverse taxa, articular surface of the vertebra; in this configuration, medial including Pythonidae (Sood 1946; Hoffstetter and rotation about a craniocaudal axis, that is, caliper rotation (green Gasc 1969), Viperidae (Hoffstetter and Gasc 1969), arrow), would be precluded by this joint articulation. Elapidae (Hoffstetter and Gasc 1969; Young and Kardong 2010), and Colubridae (Sood 1946; This biarticular morphology has been noted in the Hoffstetter and Gasc 1969; Pattishall and Cundall literature for over two centuries, and the distinct 2008), and parsimoniously suggest this trait origi- articular surfaces have been described as nated at the latest within Alethinophidia (Hsiang “capitulum,” “tuberculum,” “diapophysis,” or et al. 2015; Streicher and Wiens 2017). Contrary “parapophysis,” although there is no developmental descriptions exist for blind snakes, for example, evidence that suggests such homology (Home 1812; Typhlops spp., with some suggesting unicapitate Sibson 1846; Sood 1946; Hoffstetter and Gasc 1969; morphology, while other accounts and diagrams de- Pattishall and Cundall 2008; Young and Kardong note similar biarticular morphology to all other 2010). Rather, the synapophyses of squamates have snakes (Sood 1946; Hoffstetter and Gasc 1969). been shown to be a developmental fusion of the These conflicting descriptions and the variable mor- parapophysis and diapophysis of each vertebrae phology present throughout Serpentes suggest that a (Romer 1956; Hoffstetter and Gasc 1969). This evi- more robust morphological analysis of these traits dence, combined with snakes’ phylogenetic nesting and their phylogenetic variation would be productive within Squamata (Fig. 4)(Streicher and Wiens directions for future inquiry. Nevertheless, the ubiq- 2017), suggests the biarticular condition in snakes uity and prominence of this biarticular morphology is a convergent and derived secondary evolution of within snakes beg the question: what functional role multiple articular surfaces from the plesiomorphic does this morphology serve? unicapitate morphology for Squamata. One of the most notable characteristics of snakes The two articular surfaces on each rib and verte- is their lack of limbs, and at first glance one might brae of snakes have distinct anatomical features likely presume that this trait evolved in relation to limb- related to their function. The dorsal aspect of each lessness. An elongate, limbless, “snake-like” body costovertebral joint has (1) a concave articular sur- plan has convergently evolved within Squamata at face on the rib (Fig. 3A, B, and E; blue arrow), anal- least 25 separate times and provides a phylogeneti- ogous to a tuberculum, which articulates with a cally and ecologically diverse set of comparisons 226 J. G. Capano Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020

Fig. 4 Phylogenetic distribution and evolution of costovertebral joint morphology in Amniota. The plesiomorphic costovertebral condition for Amniota (0), inherited from early tetrapods, is bicapitate ribs articulating with two articular facets on the vertebrae, the diapophysis and parapophysis. Bicapitate morphology is retained in many extant amniote taxa including Mammalia, Crocodylia, and Aves. Squamata shows a derived condition (1) in which unicapitate ribs articulate with a single articular facet, the synapophysis. Serpentes (2) have re-evolved biarticular costovertebral joints as a modification of the ancestral squamate uniarticular condition. (Asterisk denotes images from limbless squamate taxa.) Images after Hoffstetter and Gasc (1969).

(Brandley et al. 2008). In no other instance, however, and often slip into and out of contact with push do we see a similar biarticular costovertebral mor- points (Moon and Gans 1998; Marvi and Hu 2012; phology arise (Fig. 4)(Hoffstetter and Gasc 1969). Gart et al. 2019). In contrast, snakes are capable of Even in the second-most specious and widespread numerous modes of locomotion that may be simul- group of limbless squamates, amphisbaenians, unica- taneously performed in disparate body segments, pitate joints are retained (Fig. 4)(Hoffstetter and each of which is capable of propulsive force exertion Gasc 1969; Gasc 1981). This suggests that biarticular in multiple dimensions with magnitudes much morphology is not necessarily correlated with a mere higher than their own body weight (Gans 1970; lack of limbs and may instead be associated with Marvi and Hu 2012). This suggests that snakes, character traits present only in snakes. clearly capable of regionalized body deformation Of the numerous derived features of snakes, there and local control, have finer neuromuscular control are two important traits that may be linked to the of each segment and overall force modulation. evolution of biarticular costovertebral morphology: In addition to complex locomotor mechanics, locomotion and constriction. Unlike snakes, most constriction is a behavior found only within limbless lizards are capable of only a few forms of Serpentes and is proposed to be a major innovation locomotion, often “simple undulation” and concer- in snake evolution that contributed to their extensive tina, and display little to no localized body deforma- radiation (Greene and Burghardt 1978; Mehta 2005). tion (Gans 1986; Gans and Gasc 1990; Gasc and In this prey subjugation mode, the snake wraps one Gans 1990). Of the two limbless lizard species exten- or more loops of its body around the prey to form a sively tested, Anguis fragilis and Ophisaurus apodus, constriction coil (Greene and Burghardt 1978; A. fragilis do exert a higher degree of regionalized Greene 1983; Mehta and Burghardt 2008). The snake control than O. apodus (Gasc and Gans 1990), but exerts pressure at two or more of these points of neither species is capable of the full continuum of contact, effectively using its own rib cage to com- limbless locomotor mechanics observed in snakes, press and pressurize the ribcage of its prey, thereby Reaction forces and rib functions in snakes 227 disrupting the prey’s cardiovascular blood flow, and locomotion (Carrier 1990, 1991; Brainerd and inducing death likely via cardiac arrest (Boback et al. Owerkowicz 2006). This negative influence of loco- 2015; Moon et al. 2019). Although this clearly motion on ventilation is thought to be reduced by involves tremendous force, this behavior does not strut-like costovertebral joints, transferring energy substantially pressurize the systemic blood pressure into the axial skeleton rather than requiring muscu- of the snake exerting the force; systemic blood pres- lar stabilization (Claessens 2015; Capano et al. 2019a, sures reach higher levels throughout hissing and prey 2019b). While the reaction forces experienced during ingestion than during constriction (Wang et al. snake locomotion are quite different than those in 2001). limbed locomotors, the biarticular costovertebral The key component that these two traits have in morphology of snakes may serve a similar functional common is the medially directed reaction force ex- role. As previously suggested, muscle-induced stabi- perienced by the ribs. In many of the various modes lization would enable the transmission of locomotor Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 of snake locomotion described, the ribs experience forces via the ribs, although considering the number substantial GML, such as when the body wall and and likely involvement of ribs to various snake associated ribs transmit force into a tunnel with con- behaviors, this stabilization would represent a non- certina (with up to 400% of the snake’s body weight) trivial use of energy. A plausible alternative is that or push points throughout lateral undulation (Gasc the biarticular joints in snakes serve as a passive but- et al. 1989; Moon and Gans 1998; Hu et al. 2009; Hu tress; as the ribs are pressed into the environment, and Shelley 2012; Marvi and Hu 2012). When a the flattened ventral facet of the rib may abuts the snake constricts, it generates pressure through activ- corresponding ventral articular surface of the verte- ity of the epaxial musculature that is transmitted brae (Fig. 3D and E; curved arrows), thereby con- through the ribs in contact with the prey, with force straining medial-directed caliper rotation and instead transmission only possible due to the GML of the transmitting force into the environment or prey. prey pushing back against the snake (Moon 2000). Functionally, a buttress mechanism may specifi- This direct transmission and application of force cally constrain medially directed caliper rotation with the ribs is remarkably similar to concertina me- while enabling rotation about other axes. Precise chanics, where snakes often exert large forces with XROMM measurements of rib rotations during cos- their lateral body wall and rib cage (Jayne 1986; tal ventilation in B. constrictor suggest rotational per- Marvi and Hu 2012). In these scenarios, if the mo- missiveness about axes other than caliper. In normal bile ribs reactively rotate under the requisite GML lung ventilation and hissing, B. constrictor rotates forces, this represents a substantial dissipation of en- their ribs primarily about bucket and pump handle ergy that would require active rib rotation and mus- axes, with only small amounts of caliper rotation cular stabilization to prevent, thereby increasing the (Fig. 5)(Capano et al. 2019a, 2019b). In the >55 metabolic demands of constriction and locomotion. individual rib rotations analyzed for that study, the It is, therefore, possible that the biarticular costover- average medial caliper rotation, including standard tebral morphology of snakes is passive mechanism to deviations, never exceeded 5 degrees (Fig. 5), and enable force transmission in the absence of muscular no individual rib ever rotated medially in excess of effort and would be similar to the function of cost- 11 degrees (Supplementary Fig. S1). Physical manip- overtebral joints in other amniotes. ulations of anesthetized and deceased specimens used In limbed amniotes, there is considerable variation in that study further corroborate a buttress mecha- in costovertebral morphology that has been sug- nism. When the ribs are postured almost orthogonal gested to functionally resist locomotor reaction to the body axis and force is mediolaterally exerted forces. Crocodilians, birds, and some lizard species on them, they are unable to rotate toward the mid- have convergently evolved dorsoventrally elongated line; when the ribs are postured more cranially or costovertebral joints, particularly near the pectoral caudally, however, mediolateral pressure exertion girdle regions of the thorax that experiences high produces pronounced medial caliper rotations. magnitudes of locomotor forces (Claessens 2015; While these observations do not conclusively dem- Brocklehurst et al. 2017; Capano et al. 2019a, onstrate that medial caliper rotations are prevented 2019b). This alignment has been hypothesized to rather than not performed, they do provide the function as a skeletal strut that dissipates locomotor framework for tractable hypotheses as to the func- forces away from the rib cage. In the absence of this tion of this perplexing biarticular costovertebral morphology, axial musculature typically used for morphology in snakes. This morphology does appear ventilation is co-opted to stabilize limbed locomotor to vary among major snake taxa and morphometric reaction forces, constraining ventilation during analyses, ex vivo range of motion studies, and 228 J. G. Capano

Fig. 5 Joint coordinate systems for lung ventilation in B. constrictor and mean rotations. (A) Joint coordinate system orientation relative Downloaded from https://academic.oup.com/icb/article-abstract/60/1/215/5836307 by guest on 27 July 2020 to a left rib of i > B. constrictor; all rotations measured used the right-hand rule, that is, positive bucket (blue) ¼ cranial rotation, positive caliper (green) ¼ dorsal rotation, positive pump handle (red) ¼ caudal rotation. (B) Mean rib rotation about each of the three anatomical axes during lung ventilation (n ¼ 56 individual rib rotations; three individuals; shaded region shows 6 1 SD); each breath was defined as exhalation followed by inhalation (maximum exhalation at 25% breathing cycle; maximum inhalation at 75% breathing cycle); all rotations were zeroed and normalized to 100% cycle duration. Note that the average caliper rotations never exceeded 5 degrees in either direction; combined with morphology in Fig. 3 this suggests that costovertebral morphology may preclude medial caliper motions. See Fig. 3 for costovertebral morphology and Supplementary Fig. S1 for non-averaged rotational traces. phylogenetic analyses would provide invaluable in- non-academic support, mentorship, invaluable con- sight into the evolution of this trait. These data, servations, and editorial help. Thanks to Erika coupled with in vivo kinematic work concentrated Tavares and the two reviewers who provided con- on rib motions throughout each locomotor mode, structive and helpful comments on this manuscript. including the rib rotations and posture during con- I also thank Jeremy “JJ” Lomax for his animal assis- certina locomotion, and also constriction, could pro- tance, Armita Manafzadeh for her constructive com- vide evidence for the function of biarticular joint and ments and figure critiques, Elska Kaczmarek for her yield interesting results as to how snakes integrate rib helpful discussions, and Hannah Weller for discus- motions into their complex movement patterns. sions and her coding help. Thanks to Stephen Gatesy While ribs are of clear importance to snake locomo- for his Maya expertise and teaching, along with tion, similar in vivo work could be extended to better David Baier for his invaluable XROMM tools. All understand the role of the integument. The measure- experimental procedures were approved by the ment of three-dimensional reaction forces and scale Brown University Institutional Animal Care and orientation in response to changes in substrate fric- Use Committee [IACUC Protocol 19-10-0004]. The tion, or species-specific frictional anisotropy, could opinions and conclusions in this material are those attempt to decouple how snakes modulate their ex- of the author and do not necessarily reflect the views ertion of motive forces and generation of requisite of the funding sources. reaction forces. On the whole, snake locomotion presents numerous perplexing mechanisms and me- chanics, and to understand these fully, we need to learn more about the mechanical roles of both the Funding integument and the ribs across the all locomotor This research was partially supported by grant modes and major groups of these versatile limbless awards to JGC from: the Society of Integrative and locomotors. Comparative Biology Grants-in-Aid of Research Program, the Sigma Xi Grants in Aid of Research Program [grants G201603152024996 and Acknowledgments G2017100190748016], an EEB Dissertation Thanks to Henry Astley for organizing this sympo- Development grant from the Drollinger Family sium and the inspirational discussions with all the Charitable Foundation, the American Society of symposium participants, particularly Bruce Jayne Ichthyologists and Herpetologist Gaige Fund and Philip Bergmann. A special thanks to Scott Award, and the Bushnell Research and Education Boback, to whom I owe my fascination with snakes, Fund. This work was also partially supported by for his continued mentorship and contagious enthu- grant awards to Elizabeth L. Brainerd from the US siasm for the natural world. Another special thanks National Science Foundation [grant numbers to Elizabeth Brainerd for her constant academic and 1655756 and 1661129]. Reaction forces and rib functions in snakes 229

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