Article

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

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Abstract

In many organisms, juveniles have performance capabilities that partly offset their disadvantageous sizes. Using high-speed video recordings and imaging software, we measured the scaling of head morphology, axial morphology, and defensive strike performance of Pantherophis obsoletus across their ontogeny to understand how size and morphology affect performance. Head measurements were negatively allometric whereas the cross-sectional area (CSA) of epaxial muscles displayed positive allometry. The greater relative muscle CSA of larger ratsnakes allows them to produce higher forces relative to their mass, and those forces act on a relatively smaller head mass when it is thrust forward during striking. Maximum strike accelerations of 70-273.8 ms-2 and velocities of 1.08-3.39 ms-1 scaled positively with body mass but differed from the geometric predictions. Velocity scaled with mass0.15 and acceleration scaled with mass0.17 . Larger snakes struck from greater distances (range = 4.1-26 cm), but all snakes covered the strike distances with similarly short durations (84 ± 3 ms). The negatively allometric head size, isometry of anterior mass, and positively allometric muscle CSA enable larger P. obsoletus to strike with higher velocities and accelerations than smaller individuals. Our results contrast with the scaling of strike performance in an arboreal viper, whose strike distance and velocity were independent of body mass. When strike distance is modulated, all other performance capacities are affected because of the interdependence of acceleration, velocity, duration, and distance.

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... Recently, broader use of high-speed videography has found that other snake species beyond the vipers can strike at similar performance levels, adults and juveniles, in defense, or feeding (Penning et al. 2016;Ryerson and Tan 2017). Additionally, this broader exploration of snake strikes has led to several interesting avenues, including differences in field strikes compared to laboratory strikes, the effects of temperature on strike performance, and the effects of ontogeny on strike performance (Penning et al. 2020;Ryerson 2020;Whitford et al. 2020). With this wealth of recent work, it is clear that many snakes share the ability to perform at high levels, but the factors that impact performance are widespread and may manifest in ways that are ultimately linked to fitness. ...
... Strike kinematics were not impacted by differences in SVL or body mass. Strike velocity increased with strike distance (1.14 6 0.07, P ¼ 0.005) as has been noted for other snakes (Herrel et al. 2011;Penning et al. 2020;Ryerson 2020;Whitford et al. 2020). Maximum gape angle was predicted by strike category. ...
... However, very few of these works attempt to bridge the morphology and behavior gap. Those that do often focus on static elements of morphology as a means of predicting performance, such as using the size of skeletal elements to infer maximum gape and therefore maximum prey size (Hampton and Moon 2013), or using elements of body size to predict strike performance (Penning et al. 2020;Ryerson 2020). In this work we have attempted to integrate the tooth morphology of the B. constrictor with its strike behavior to better understand the role of tooth shape during the process of prey capture. ...
Article
Synopsis Snakes, with the obvious exception of the fangs, are considered to lack the regional specialization of tooth shape and function which are exemplified by mammals. Recent work in fishes has suggested that the definition of homodont and heterodont are incomplete without a full understanding of the morphology, mechanics, and behavior of feeding. We investigated this idea further by examining changes in tooth shape along the jaw of Boa constrictor and integrating these data with the strike kinematics of boas feeding on rodent prey. We analyzed the shape of every tooth in the skull, from a combination of anesthetized individuals and CT scanned museum specimens. For strike kinematics, we filmed eight adult boas striking at previously killed rats. We determined the regions of the jaws that made first contact with the prey, and extrapolated the relative positions of those teeth at that moment. We further determined the roles of all the teeth throughout the prey capture process, from the initiation of the strike until constriction began. We found that the teeth in the anterior third of the mandible are the most upright, and that teeth become progressively more curved posteriorly. Teeth on the maxilla are more curved than on the mandible, and the anterior teeth are more linear or recurved than the posterior teeth. In a majority of strikes, boas primarily made contact with the anterior third of the mandible first. The momentum from the strike caused the upper jaws and skull to rotate over the rat. The more curved teeth of the upper jaw slid over the rat unimpeded until the snake began to close its jaws. In the remaining strikes, boas made contact with the posterior third of both jaws simultaneously, driving through the prey and quickly retracting, ensnaring the prey on the curved posterior teeth of both jaws. The curved teeth of the palatine and pterygoid bones assist in the process of swallowing.
... Thus, there is a possibility that large-bodied vipers can partially circumvent the deleterious effects of low body temperature on strike performance. However, several studies on the scaling relationships between strike performance and snake size indicate that larger snakes can accelerate more rapidly, and, in some species, can also attain higher maximum velocities (Herrel et al. 2011;Penning et al. 2019). The increased strike performance at larger body sizes is the result of a negative allometric relationship between head size and body size, while the dominant epaxial muscles used during a strike scale either isometrically or positively with body size. ...
... Like a boxer's jab (Kimm and Thiel 2015), defensive strikes should pose a risk to the predator while limiting the snake's own exposure to risk. Defensive strikes are likely also used as feints, or a means to keep a putative predator at a distance Penning et al. 2019). In contrast, the purpose of the predatory strikes is to capture prey. ...
... This movement has a substantial effect on the quantification of strike kinematics (Ryerson 2020). Almost all kinematic studies include the distance between the head of the snake and the target at the onset of the strike (typically referred to as strike distance) as an explanatory variable, and this distance often affects the maximum velocity and acceleration achieved by the strike (Herrel et al. 2011;Penning et al. 2019). As strike velocity is typically linearly related to strike distance, any change in distance will have a direct effect on the speed of a strike. ...
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The outcomes of predator-prey interactions between endotherms and ectotherms can be heavily influenced by environmental temperature, owing to the difference in how body temperature affects locomotor performance. However, as elastic energy storage mechanisms can allow ectotherms to maintain high levels of performance at cooler body temperatures, detailed analyses of kinematics are necessary to fully understand how changes in temperature might alter endotherm-ectotherm predator-prey interactions. Viperid snakes are widely distributed ectothermic mesopredators that interact with endotherms both as predator and prey. Although there are numerous studies on the kinematics of viper strikes, surprisingly few have analyzed how this rapid movement is affected by temperature. Here we studied the effects of temperature on the predatory strike performance of rattlesnakes (Crotalus spp.), abundant new world vipers, using both field and captive experimental contexts. We found that the effects of temperature on predatory strike performance are limited, with warmer snakes achieving slightly higher maximum strike acceleration, but similar maximum velocity. Our results suggest that, unlike defensive strikes to predators, rattlesnakes may not attempt to maximize strike speed when attacking prey, and thus the outcomes of predatory strikes may not be heavily influenced by changes in temperature.
... Despite the recognized importance of the links between ontogeny and ecology, there is a surprising lack of investigation on the interaction of ontogeny and performance, particularly in ectothermic vertebrates (Herrel and Gibb, 2006). The theoretical framework for changes in muscle physiology as underlying performance were first described by Hill (1950), who predicted that velocity should scale independent of mass, and that acceleration should scale as mass -1 (Penning et al., 2019). However, many empirical investigations have instead found that velocities and accelerations may actually increase throughout ontogeny, often as the result of changing morphology (Richard and Wainwright, 1995;Meyers et al., 2002;Penning et al., 2019). ...
... The theoretical framework for changes in muscle physiology as underlying performance were first described by Hill (1950), who predicted that velocity should scale independent of mass, and that acceleration should scale as mass -1 (Penning et al., 2019). However, many empirical investigations have instead found that velocities and accelerations may actually increase throughout ontogeny, often as the result of changing morphology (Richard and Wainwright, 1995;Meyers et al., 2002;Penning et al., 2019). Changes in the musculature powering the movement may be the underlying cause (Meyers et al., 2002), or that a relatively smaller mass is being moved in relation to the size of the entire body (Young, 2010;Penning et al., 2019). ...
... However, many empirical investigations have instead found that velocities and accelerations may actually increase throughout ontogeny, often as the result of changing morphology (Richard and Wainwright, 1995;Meyers et al., 2002;Penning et al., 2019). Changes in the musculature powering the movement may be the underlying cause (Meyers et al., 2002), or that a relatively smaller mass is being moved in relation to the size of the entire body (Young, 2010;Penning et al., 2019). ...
Article
The rapid strike of snakes has long been of interest in terms of mechanical performance. Recently, several nonvenomous taxa have been found to strike with the same incredible strike velocity and acceleration as the high-performing vipers. However, little is known regarding how these patterns change through ontogeny. Here I present ontogenetic strike data on ten ball pythons (Python regius) over a three year time period, from birth to sexual maturity. I found that performance declined rapidly over the first 18 months in nearly all kinematic measures. This puts the adult data out of the currently developing trend of high performance being maintained across the diversity of snakes. The underlying cause of the decline in performance is unclear, but there are several avenues of behavior, morphology, biomechanics, and ecology to be investigated.
... For example, Dangles et al. (2007) found that juvenile crickets stimulated by touch had faster response times which led to increased escape distances compared to adults. Taken together the constraints and selective pressures an organism faces through ontogeny will likely change the patterns of variation in ecologically relevant behaviors such as feeding (Herrel et al. 2011;Penning et al. 2020;Ryerson 2020). ...
... Studies of invertebrate locomotion have found support for the Hill model (Katz and Gosline 1993;Quillin 1999;Nauen and Shadwick 2001), while studies of vertebrate feeding have found mixed support for both models (Richard and Wainwright 1995;Burnette and Gibb 2013). The mixed results for the effects of ontogenetic changes in locomotion and feeding come from studies examining vertebrates and locomotion (Carrier 1996;Herrel and Gibb 2006;Herrel et al. 2011;Penning et al. 2020;Ryerson 2020) and escape and jump performance among invertebrates (Queathem 1991;Dangles et al. 2007;Sutton et al. 2016). Few have assessed how feeding in invertebrates changes through ontogeny to determine if it is reduced, similar to previous predictions and the patterns of scaling to determine which model (Hill or Richard and Wainwright) they may support. ...
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... I assessed head size using 11 parameters of the head (Fig. 1): (1) Width at the nares, (2) width at the eyes, (3) width at the jaw joint, (4) width at the end of the head, (5) head length; (6) mandible length; (7) eye diameter; (8) height at the nares, (9) height at the eye, (10) height at the jaw joint, and (11) height at the end of the head. The measurement locations were chosen following previous measures of snake head dimensions (Vincent et al. 2004a(Vincent et al. , 2004bHerrel et al. 2011;Penning 2017;Penning et al. 2020), as well as ease of measurement in the field. Linear measurements were chosen over landmark-based approaches (e.g., geometric morphometrics) for two reasons: (1) linear measurements can be directly compared with several over recent works on scaling of head morphology in snakes (Herrel et al. 2011;Penning 2017;Penning et al. 2020) and (2) preliminary tests of landmarkbased approaches using digital images were more prone to measurement error than the linear caliper-based approach, as a result of the small head size of the individuals. ...
... The measurement locations were chosen following previous measures of snake head dimensions (Vincent et al. 2004a(Vincent et al. , 2004bHerrel et al. 2011;Penning 2017;Penning et al. 2020), as well as ease of measurement in the field. Linear measurements were chosen over landmark-based approaches (e.g., geometric morphometrics) for two reasons: (1) linear measurements can be directly compared with several over recent works on scaling of head morphology in snakes (Herrel et al. 2011;Penning 2017;Penning et al. 2020) and (2) preliminary tests of landmarkbased approaches using digital images were more prone to measurement error than the linear caliper-based approach, as a result of the small head size of the individuals. After 1 year, the juvenile snakes were released to the site of their mother's capture, in August 2018. ...
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1. The Standardised Major Axis Tests and Routines (SMATR) software provides tools for estimation and inference about allometric lines, currently widely used in ecology and evolution. 2. This paper describes some significant improvements to the functionality of the package, now available on R in smatr version 3. 3. New inclusions in the package include sma and ma functions that accept formula input and perform the key inference tasks; multiple comparisons; graphical methods for visualising data and checking (S)MA assumptions; robust (S)MA estimation and inference tools.
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Summary 1. For gape-limited predators such as snakes, it should be possible to predict the relationship between maximum prey size and body size from the relationship between maximum prey size and gape size and between gape size and body size. Such predictions were generated for Water Snakes, Nerodia sipedon L., using a data subset and then tested with a larger data set. 2. Gape size was computed based on jaw length and width and cyclical regression was used to identify prey of maximum size for snakes of a given gape or mass. 3. Predicted and observed maximum prey cross-section-snake mass allometry were in good agreement. Predicted maximum prey mass-snake mass allometry somewhat exceeded observed allometry which did not differ from 1. 4. Observed minimum prey size-snake size allometry was significantly greater than 0, indicating that larger snakes drop small prey from their diets. 5. Gape size-body size allometry in two other natricine snakes ( Thamnophis sirtalis , Storeria dekayi ) suggest that patterns of ontogenetic change in prey size should differ among species in predictable ways. 6. Sex differences in gape size-snake size allometry suggest that sex differences in maximum prey size should increase with increasing snake size, even when linear measures of head dimensions do not.
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Summary Previous studies have suggested that piscivorous snakes generally use sideways head sweeping to capture prey in order to minimize drag on the skull and/or to avoid pushing prey items away from the mouth. However, some aquatic species of garter snakes (genus Thamnophis) have been reported to use fast forward strikes to capture fish and amphibians. To characterize fast forward striking as a mode of piscivory in snakes and compare its use among specialist and generalist species, the aquatic specialists Thamnophis couchii and T. rufipunctatus, and a terrestrial generalist, T. sirtalis, were filmed at 250 fps (frames per second) while preying on minnows. Both T. couchii and T. rufipunctatus oriented visually toward prey items and struck forward rapidly with peak head velocities that approached speeds attained by fast striking booid, colubrid and viperid species on land. In contrast, T. sirtalis did not orient visually toward specific prey items and displayed strikes that were four to six times slower than those of specialist species. Aquatic specialists used moderate amounts of cranial rotation during jaw opening and achieved maximum gape within 20 ms of jaw opening. In the generalist T. sirtalis, jaw opening took 40–60 ms and was due almost entirely to mandibular rotation. Significant differences in the prey capture kinematics of the two aquatic specialists are consistent with the hypothesis that T. couchii is an open water hunter, whereas T. rufipunctatus is an ambush predator. Thamnophiine snakes display a diversity of aquatic prey capture styles that reflect different behavioral and mechanical solutions to the problem of feeding in water.
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Although several researchers have recognized the experimental potential of thermoregulation following feeding, exploitation of that potential has been limited largely to studies of captive animals. Postprandial thermophily (PPT) in the laboratory has been reported in many snake species, but may not be important in others (Peterson, Gibson, & Dorcas 1993). There is obviously no consensus about the generality of the phenomenon in snakes (Lillywhite 1987), and it is unclear whether this reflects real differences among species or is an artefact of studying captive animals. Only two studies of PPT have been conducted on free-living snakes (Beck 1996; Brown & Weatherhead 2000), and only Brown & Weatherhead (2000) tested simultaneously for the presence of PPT in the laboratory and in the field. They found that northern water snakes (Nerodia sipedon sipedon (Linnaeus)) did not increase their Tb significantly, and did not thermoregulate more carefully following feeding in either the laboratory or the field. However, their study also revealed that environmental operative temperatures (Tes) within the preferred Tb range (Tset) of northern water snakes were widely available in the snakes’ habitat, thus making it very easy for the snakes to maintain their preferred Tb. To assess the generality and importance of PPT in the wild, we need information on the postprandial thermoregulatory behaviour of species that face thermally challenging environments. Black rat snakes in eastern Ontario provide such an opportunity. In addition, we are unaware of any study that has formally examined habitat selection in relation to digestion of a meal in snakes, despite the tenet that habitat selection is one of the primary ways in which snakes adjust their Tb (Reinert 1993). Thus, it is important to study not only PPT in free-living snakes, but also to document the role that habitat selection plays in this phenomenon.
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Segments of the spinalis portion of the M. semispinalis-spinalis (SSP) were examined in 107 snakes representing 94 species, 85 genera, and 11 families. Allowing for slight variation within individuals and species, the following generalizations can be made. (1) Three major types of segments of the SSP were found in Typhlops, booids, and colubroids. (2) Within each type, differences in the segmental length of the spinalis result primarily from different lengths of the anterior tendons. (3) Specializations in habitat and locomotor modes usually account for variations in the segmental lengths of the spinalis. (4) Constrictors seem to have undergone selection for increased flexibility which is gained by having relatively more vertebrae and, in some cases, shorter muscle segments.
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Organisms spanning a 107-fold range in length of the body engage in aquatic propulsion—swimming; they do so with several kinds of propulsors and take advantage of several different fluid mechanical mechanisms. A hierarchical classification of swimming modes can impose some order on this complexity. More difficult are the issues surrounding the different kinds of propulsive devices used by different organisms. These issues can be in part exposed by an examination of how speeds and accelerations scale with changes in body length, both for different lineages of swimmers and for all swimmers collectively. Clearly, fluid mechanical factors impose general rules and constraints; just as clearly, these only roughly anticipate actual scaling. Indeed, collections of data on scaling can serve as useful correctives for assumptions about functional mechanisms. They can also reveal size-dependent constraints on biological designs.
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Despite being large, heavy-bodied snakes, puff adders (Bitis arietans) are capable of achieving strike velocities and accelerations similar to, or greater than, those of much smaller snakes (means of 2.6 and 72 m/sec(2), respectively). The mechanistic basis of the strike was examined using high-speed digital videography, coupled with electromyographic (EMG) analysis, of the two main extensors of the vertebral column, the semispinalis and longissimus. Although the strike involves the rapid extension of preformed body curves, the extensor muscles were not electrically active during body extension. The vertebral extensor muscles exhibited bursts of electrical activity before the onset of movement-and quantified features of these EMG signals were significantly related to kinematic aspects of the strike (e.g., acceleration)-however, this electrical activity terminated shortly (approximately 50 msec) before the onset of movement. It is hypothesized that the prestrike activity of the extensor muscles functions to place the (extensive) musculo-tendon complex of the snake's epaxial muscles under tension, and that the displacement of the body during the strike is due to the elastic recoil of this musculo-tendon complex. Incorporation of this type of elastic recoil would increase the power output of the vertebral extensors. Power amplification of the vertebral extensors may be an evolutionary necessity if a large, heavy-bodied snake, like B. arietans, is going to achieve rapid acceleration during the strike.
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Many investigators use the reduced major axis (RMA) instead of ordinary least squares (OLS) to define a line of best fit for a bivariate relationship when the variable represented on the X-axis is measured with error. OLS frequently is described as requiring the assumption that X is measured without error while RMA incorporates an assumption that there is error in X. Although an RMA fit actually involves a very specific pattern of error variance, investigators have prioritized the presence versus the absence of error rather than the pattern of error in selecting between the two methods. Another difference between RMA and OLS is that RMA is symmetric, meaning that a single line defines the bivariate relationship, regardless of which variable is X and which is Y, while OLS is asymmetric, so that the slope and resulting interpretation of the data are changed when the variables assigned to X and Y are reversed. The concept of error is reviewed and expanded from previous discussions, and it is argued that the symmetry-asymmetry issue should be the criterion by which investigators choose between RMA and OLS. This is a biological question about the relationship between variables. It is determined by the investigator, not dictated by the pattern of error in the data. If X is measured with error but OLS should be used because the biological question is asymmetric, there are several methods available for adjusting the OLS slope to reflect the bias due to error. RMA is being used in many analyses for which OLS would be more appropriate.
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The stride frequency at which animals of different size change from one gait to another (walk, trot, gallop) changes in a regular manner with body mass. The speed at the transition from trot to gallop can be used as an equivalent speed for comparing animals of different size. This transition point occurs at lower speeds and higher stride frequencies in smaller animals. Plotting stride frequency at the trot-gallop transition point as a function of body mass in logarithmic coordinates yields a straight line.
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Although of prime ecological relevance, acceleration capacity is a poorly understood locomotor performance trait in terrestrial vertebrates. No empirical data exist on which design characteristics determine acceleration capacity among species and whether these design traits influence other aspects of locomotor performance. In this study we explore how acceleration capacity and sprint speed have evolved in Anolis lizards. We investigate whether the same or different morphological traits (i.e., limb dimensions and muscle mass) correlate with both locomotor traits. Within our sample of Anolis lizards, relative sprint speed and acceleration capacity coevolved. However, whereas the variation in relative acceleration capacity is primarily explained by the variation in relative knee extensor muscle mass, the variation in relative sprint speed is correlated to the variation in relative femur, tibia, and metatarsus length as well as knee extensor muscle mass. The fact that the design features required to excel in either performance trait partly overlap might explain the positive correlation between the variation in relative sprint speed and acceleration capacity. Furthermore, our data show how similar levels of sprint performance can be achieved through different morphological traits (limb segment lengths and muscle mass) suggesting that redundant mapping has potentially played a role in mitigating trade-offs.