Brad R. Moon

Are ontogenetic shifts in diet linked to shifts in feeding mechanics? Scaling of the feeding apparatus in the banded watersnake Nerodia fasciata.

SUMMARY The effects of size on animal behaviour, ecology, and physiology are widespread. Theoretical models have been developed to predict how animal form, function, and performance should change with increasing size. Yet, numerous animals undergo dramatic shifts in ecology (e.g. habitat use, diet) that may directly influence the functioning and presumably the scaling of the musculoskeletal system. For example, previous studies have shown that banded watersnakes (Nerodia fasciata) switch from fish prey as juveniles to frog prey as adults, and that fish and frogs represent functionally distinct prey types to watersnakes. We therefore tested whether this ontogenetic shift in diet was coupled to changes in the scaling patterns of the cranial musculoskeletal system in an ontogenetic size series (70–600 mm snout–vent length) of banded watersnakes. We found that all cranial bones and gape size exhibited significant negative allometry, whereas the muscle physiological cross-sectional area (pCSAs) scaled either isometrically or with positive allometry against snout–vent length. By contrast, we found that gape size, most cranial bones, and muscle pCSAs exhibited highly significant positive allometry against head length. Furthermore, the mechanical advantage of the jaw-closing lever system remained constant over ontogeny. Overall, these cranial allometries should enable watersnakes to meet the functional requirements of switching from fusiform fish to bulky frog prey. However, recent studies have reported highly similar allometries in a wide diversity of vertebrate taxa, suggesting that positive allometry within the cranial musculoskeletal system may actually be a general characteristic of vertebrates.

The functional meaning of “prey size” in water snakes (Nerodia fasciata, Colubridae)

The evolutionary success of macrostomatan (enlarged-gape) snakes has been attributed to their ability to consume large prey, in turn made possible by their highly kinetic skulls. However, prey can be “large” in several ways, and we have little insight into which aspects of prey size and shape affect skull function during feeding. We used X-ray videos of broad-banded water snakes (Nerodia fasciata) feeding on both frogs and fish to quantify movements of the jaw elements during prey transport, and of the anterior vertebral column during post-cranial swallowing. In a sample of additional individuals feeding on both frogs and fish, we measured the time and the number of jaw protractions needed to transport prey through the buccal cavity. Prey type (fish vs. frog) did not influence transport kinematics, but did influence transport performance. Furthermore, wider and taller prey induced greater movements of most cranial elements, but wider prey were transported with significantly less anterior vertebral bending. In the performance trials, heavier, shorter, and wider prey took significantly more time and a greater number of jaw protractions to ingest. Thus, the functional challenges involved in prey transport depend not only upon prey mass, but also prey type (fish vs. frog) and prey shape (relative height, width and length), suggesting that from the perspective of a gape-limited predator, the difficulty of prey ingestion depends upon multiple aspects of prey size.

Testing an inference of function from structure: snake vertebrae do the twist.

The zygapophyses and zygosphene–zygantrum articulations of snake vertebrae are hypothesized to restrict or eliminate vertebral torsion. This hypothesis is apparently based solely on the inference of function from structure, despite the limitations of such inferences, as well as contradictory observations and measurements. In this study, I observed and measured axial torsion in gopher snakes, Pituophis melanoleucus. To examine the structural basis of axial torsion, I measured the vertebral articulation angles along the body and the insertion angles of five epaxial muscles. To examine torsion in a natural behavior, I digitized video images and measured the degree of apparent axial torsion during terrestrial lateral undulation. Finally, I measured the mechanical capacity of the vertebral joints for actual torsion over intervals of 10 vertebrae in fresh, skinned segments of the trunk. Vertebral articulation angles vary up to 30° and are associated with variation in torsional capacity along the trunk. The freely crawling P. melanoleucus twisted up to 2.19° per vertebra, which produced substantial overall torsion when added over several vertebrae. The vertebral joints are mechanically capable of torsion up to 2.89° per joint. Therefore, despite the mechanical restriction imposed by the complex articulations, vertebral torsion occurs in snakes and appears to be functionally important in several natural behaviors. Even in cases in which mechanical function appears to be narrowly constrained by morphology, specific functions should not be inferred solely from structural analyses. J. Morphol. 241:217–225, 1999.

Gape size, its morphological basis, and the validity of gape indices in western diamond‐backed rattlesnakes (crotalus atrox)

Maximum gape is important to the ecology and evolution of many vertebrates, particularly gape‐limited predators, because it can restrict the sizes and shapes of prey that can be eaten. Although many cranial elements probably contribute to gape, it is typically estimated from jaw length or jaw width, or occasionally from a combination of these two measures. We measured maximum gape directly for 18 individuals of the western diamond‐backed rattlesnake, Crotalus atrox. We measured each individuals body length, several external cranial dimensions, several cranial osteological dimensions from cleaned skeletons, and we calculated gape index values from two published gape indices (GI). Cranial bone lengths and gape circumference showed negative allometry with snout–vent length (SVL), indicating that small individuals have relatively larger heads and gapes than their larger conspecifics. We then used Akaikes Information Criterion to determine which external and osteological measurements were the best predictors of gape. Body size (SVL) was the best predictor of maximum gape overall; however, when SVL was excluded from the analysis, quadrate (QL) and mandible lengths (MdLs) were the best predictors of maximum gape using both external and osteological measurements. Quadrate length probably contributes directly to gape; however, the importance of MdL to gape is less clear and may be due largely to its allometric relationships with head length and SVL. The two published GI did not prove to be better indicators of actual gape than the jaw and QLs in this study, and the gape values they produced differed significantly from our empirically determined gapes. For these reasons, we urge caution with the use and interpretation of computed GI in future studies. The extensive variation in quadrate and mandible morphology among lineages suggest that these bones are more important to variation in gape among species and lineages than within a single species. J. Morphol., 2013.

Minimal shortening in a high-frequency muscle

SUMMARY Reducing the cost of high-frequency muscle contractions can be accomplished by minimizing cross-bridge cycling or by recycling elastic strain energy. Energy saving by contractile minimization has very different implications for muscle strain and activation patterns than by elastic recoil. Minimal cross-bridge cycling will be reflected in minimal contractile strains and highly reduced force, work and power output, whereas elastic energy storage requires a period of active lengthening that increases mechanical output. In this study, we used sonomicrometry and electromyography to test the relative contributions of energy reduction and energy recycling strategies in the tailshaker muscles of western diamondback rattlesnakes (Crotalus atrox). We found that tailshaker muscle contractions produce a mean strain of 3%, which is among the lowest strains ever recorded in vertebrate muscle during movement. The relative shortening velocities (V/Vmax) of 0.2-0.3 were in the optimal range for maximum power generation, indicating that the low power output reported previously for tailshaker muscle is due mainly to contractile minimization rather than to suboptimal V/Vmax. In addition, the brief contractions (8-18 ms) had only limited periods of active lengthening (0.2-0.5 ms and 0.002-0.035%), indicating little potential for elastic energy storage and recoil. These features indicate that high-frequency muscles primarily reduce metabolic energy input rather than recycle mechanical energy output.

The Ontogeny of Contractile Performance and Metabolic Capacity in a High-Frequency Muscle*

High‐performance muscles such as the shaker muscles in the tails of western diamond‐backed rattlesnakes (Crotalus atrox) are excellent systems for studying the relationship between contractile performance and metabolic capacity. We observed that shaker muscle contraction frequency increases dramatically with growth in small individuals but then declines gradually in large individuals. We tested whether metabolic capacity changed with performance, using shaker muscle contraction frequency as an indicator of performance and maximal activities of citrate synthase and lactate dehydrogenase as indicators of aerobic and anaerobic capacities, respectively. Contraction frequency increased 20‐fold in 20–100‐g individuals but then declined by approximately 30% in individuals approaching 1,000 g. Mass‐independent aerobic capacity was positively correlated with contractile performance, whereas mass‐independent anaerobic capacity was slightly but negatively correlated with performance; body mass was not correlated with performance. Rattle mass increased faster than the ability to generate force. Early in ontogeny, shaker muscle performance appears to be limited by aerobic capacity, but later performance becomes limited equally by aerobic capacity and the mechanical constraint of moving a larger mass without proportionally thicker muscles. This high‐performance muscle appears to shift during ontogeny from a metabolic constraint to combined metabolic and mechanical constraints.

Muscle Physiology and the Evolution of the Rattling System in Rattlesnakes

The rattle appears to have a been a key innovation in the adaptive radiation of rattlesnakes (Greene, 1988, 1992). However, despite the importance of the rattle to the biology of rattlesnakes, the evolution of the rattle remains poorly known. The rattle is associated with a suite of anatomical, physiological, and behavioral specializations (e.g., Greene, 1992; Rome et al., 1996; Schaeffer et al., 1996) that together represent an integrated system. Several of these specializations appear to vary among pitvipers and, therefore, have great potential for helping to resolve the evolution of the rattling system. The physiological specializations associated with tail vibration and rattling can help distinguish among possible ancestral states in the evolution of the rattling system. Currently, only limited behavioral and morphological data are available for studying rattle evolution. Nevertheless, several recent morphological and behavioral studies, discussed below, have highlighted recent progress, as well as major gaps, in our knowledge of rattle evolution. Some of these gaps may be filled by combining physiological data with the morphological and behavioral results. Here I discuss the potential bearing of some morphological, physiological, and behavioral features on the evolution of the rattling system. Anatomy and Mechanics of the Rattle.-Although the structure of the rattle in its current state is well known (Zimmerman and Pope, 1948; Klauber, 1972), the form of the rattle early in its evolution remains unknown. The starting condition for the origin of the rattle may be reflected in the cornified tail tips of several extant species of pitvipers (Garman, 1888; Greene, 1992). However, the particular type of cornified tail tip that may have been ancestral to the rattle is unresolved because none of the cornified tail tips in extant pitvipers approaches the complexity of the rattle (Klauber, 1972; Greene, 1988) and because the relationships between rattlesnakes and other pitvipers remain unresolved. Slow vibrations of a two-lobed tail cap, similar to a rattle button, probably did not enhance sound production in early rattlesnakes (Sisk and Jackson, 1997). However, the louder sounds produced by faster rattling (Rowe and Owings, 1996) suggest that high frequencies of vibration were at least as important to sound production as the structure of the incipient rattle. This inference in turn suggests that early tail vibration behaviors involved high twitch frequencies. However, few studies have addressed the relationship between tail vibration behaviors and vibration frequencies. The Origin of Rattling Behavior.-Two behavioral contexts have been hypothesized for the origin of the rattling system. The traditional hypothesis for the origin of the rattle is that it evolved as a warning device against predators or other potentially dangerous animals, such as large grazing mammals that could trample the snakes (Hay, 1887; Barbour, 1922, 1926). Use of the rattle only as a warning device in all living rattlesnakes supports this hypothesis (Klauber, 1972; Greene, 1988). The quiet sounds of small rattles (e.g., Sistrurus species; Cook et al., 1994) provide evidence against the warning hypothesis for the origin of the rattle (Schuett et al., 1984). In early rattlesnakes, small or structurally simple rattles may have been completely inaudible to large predators or grazing mammals. Instead, Schuett et al. (1984) argued that the rattle may have evolved as a device to enhance the visual attractiveness of caudal luring, in which the tip of the tail is held off the ground and twitched slowly to attract prey. However, Tiebout (1997) noted that three lines of evidence refute the caudal luring hypothesis for origin of the rattle. First, no incipient rattle-like structures are known in other snakes, including nonrattlesnake pitvipers and other snakes that use caudal luring to attract prey. Second, rattles larger than the (silent) button appear not to be used in caudal luring. Third, sound production by a small or simple rattle may have been enhanced by tail vibration against the substratum rather than in a tail held off the ground. Fast, audible tail vibration against the ground is used as a defensive behavior in many snakes, including nonrattlesnake pitvipers, diverse colubrids, and some basal snakes (Greene, 1988, 1992). In terms of character state changes, the origin of the rattle as a caudal luring device and its subsequent shift to a warning device would require at least two evolutionary steps in the use of the rattle. In contrast, inferring that the rattle originated as a warning device would require only one evolutionary step, which better fits the available data on rattle use only as a warning device in extant rattlesnakes. The phylogenetic and behavioral evidence indicating that rattling evolved from defensive tail vibration would be strengthened by mechanistic evidence that links tail vibration, rather than caudal luring, to rattling. The Bearing of Muscle Physiology on Rattle Evolution.Both caudal luring and antipredator tail vibration occur in diverse snakes and appear to be ancestral features in rattlesnakes (Greene, 1988, 1992). Which of these behaviors was associated with the evolution of the rattling system? Physiological evidence can be used to distinguish between caudal luring versus defensive tail vibration as the ancestral behavior to rattling. Extant rattlesnakes have tailshaker muscles that are highly specialized for sustaining fast contractions of 20-95 Hz for minutes to hours (Conley and Lindstedt, 1996; Rome et al., 1996; Schaeffer et al., 1996). Hoyvever, typical reptilian muscle can sustain only slow movements and fatigues quickly during fast contractions (Bennett, 1978, 1982; Lillywhite, 1987). Caudal luring involves slow, intermittent twitches (Greene, 1988). Therefore, caudal luring should not require any physiological specializations of the tail musculature. In contrast, antipredator tail vibrations and rattling are fast and often sustained for long periods. Further497

Sampling rates, aliasing, and the analysis of electrophysiological signals

Low analog to digital (A-D) sampling rates, including the Nyquist rate, are inadequate for direct display and analysis of many electrophysiological signals (i.e., without inverse Fourier reconstruction). Recent empirical studies have reported some results that do not follow from sampling theory, such as increasing spike frequency with increasing sampling rate, and thus require explanation. This study addresses the effects of A-D sampling rate on the frequency and amplitude of known artificial signals and of electromyograms. A-D sampling rate need not always be a multiple of the upper band limit because electromyograms, and many other electrophysiological signals, do not always contain frequency components near the upper band limit. Rather, sampling rate must be matched to particular signal frequencies. Furthermore, A-D sampling rate has different effects on frequency and amplitude. Higher sampling rates are required for accurate amplitude reproduction than for accurate frequency reproduction. Very high sampling rates significantly bias quantitative results by detecting low-level noise in signals. This bias is exacerbated in taped signals sampled at reduced tape speeds.

Debunking the viper's strike: harmless snakes kill a common assumption

To survive, organisms must avoid predation and acquire nutrients and energy. Sensory systems must correctly differentiate between potential predators and prey, and elicit behaviours that adjust distances accordingly. For snakes, strikes can serve both purposes. Vipers are thought to have the fastest strikes among snakes. However, strike performance has been measured in very few species, especially non-vipers. We measured defensive strike performance in harmless Texas ratsnakes and two species of vipers, western cottonmouths and western diamond-backed rattlesnakes, using high-speed video recordings. We show that ratsnake strike performance matches or exceeds that of vipers. In contrast with the literature over the past century, vipers do not represent the pinnacle of strike performance in snakes. Both harmless and venomous snakes can strike with very high accelerations that have two key consequences: the accelerations exceed values that can cause loss of consciousness in other animals, such as the accelerations experienced by jet pilots during extreme manoeuvres, and they make the strikes faster than the sensory and motor responses of mammalian prey and predators. Both harmless and venomous snakes can strike faster than the blink of an eye and often reach a target before it can move.

The big squeeze: scaling of constriction pressure in two of the world's largest snakes, Python reticulatus and Python molurus bivittatus.

ABSTRACT Snakes are important predators that have radiated throughout many ecosystems, and constriction was important in their radiation. Constrictors immobilize and kill prey by using body loops to exert pressure on their prey. Despite its importance, little is known about constriction performance or its full effects on prey. We studied the scaling of constriction performance in two species of giant pythons (Python reticulatus and Python molurus bivittatus) and propose a new mechanism of prey death by constriction. In both species, peak constriction pressure increased significantly with snake diameter. These and other constrictors can exert pressures dramatically higher than their preys blood pressure, suggesting that constriction can stop circulatory function and perhaps kill prey rapidly by over-pressurizing the brain and disrupting neural function. We propose the latter ‘red-out effect’ as another possible mechanism of prey death from constriction. These effects may be important to recognize and treat properly in rare cases when constrictors injure humans. Summary: Constriction performance increases with size in large pythons, and involves pressures that are high enough to stop the preys circulation and possibly disrupt neural function in the brain.