Alexandra Deufel

Striking Patterns in Booid Snakes

Striking is kinematically variable in booid snakes but generally fits one of three patterns. Analysis of slow speed, short-exposure video records of more than 200 strikes in 17 juveniles and subadults of seven species of booids shows that they first hit the prey either with the mandibles (MAN strikes) or with the mandibles and upper jaw together [driving scissors (DSC) strikes], rarely with the upper jaw [palatomaxillary (PMX) strikes]. In MAN strikes, inertia carries the braincase over the prey in an arc whose radius is partly defined by the tip of the mandible. In DSC strikes, both jaws slide over the prey. In both of these strike patterns, much of the impact of prey contact appears to be directed through the mandibles and their suspensoria and adductor muscles. Ventral flexion of the whole head and neck initiates the first constricting coil shortly after contact with the prey and may be aided by reaction forces generated by the preys inertia. In PMX strikes, on the other hand, the braincase incurs a higher proportion of impact forces and ventral flexion of the head is often delayed. The predominant use of MAN and DSC strikes suggests that these kinematic patterns in combination with tooth geometry may reduce the probability of tooth breakage at the time of jaw impact with the prey. The same tooth geometry increases the probability of snaring in the interval between contact and immobilization by constriction.

Do Booids Stab Prey

Measurements of anterior maxillary tooth form combined with video analysis of predatory strikes show that booid snakes do not stab their teeth into prey as previously proposed. The recurved shape of the teeth combined with the direction of travel of the upper jaw at prey contact cause the teeth to slide over the prey. Maxillary tooth penetration normally occurs as the prey recoils against the tooth tips. Snaring is a better descriptor of maxillary tooth function in booids.

Drinking in Snakes: Resolving a Biomechanical Puzzle

Snakes have long been thought to drink with a two-phase buccal-pump mechanism, but observations that some snakes can drink without sealing the margins of their mouths suggest that buccal pumping may not be the only drinking mechanism used by snakes. Here, we report that some snakes appear to drink using sponge-like qualities of specific regions of the oropharyngeal and esophageal mucosa and sponge-compressing functions of certain muscles and bones of the head. The resulting mechanism allows them to transport water upward against the effects of gravity using movements much slower than those of many other vertebrates. To arrive at this model, drinking was examined in three snake species using synchronized ciné and electromyographic recordings of muscle activity and in a fourth species using synchronized video and pressure recordings. Functional data were correlated with a variety of anatomical features to test specific predictions of the buccal-pump model. The anatomical data suggest explanations for the lack of conformity between a buccal-pump model of drinking and the performance of the drinking apparatus in many species. Electromyographic data show that many muscles with major functions in feeding play minor roles in drinking and, conversely, some muscles with minor roles in feeding play major roles in drinking. Mouth sealing by either the tongue or mental scale, previously considered critical to drinking in snakes, is incidental to drinking performance in some species. The sponge mechanism of drinking may represent a macrostomatan exaptation of mucosal folds, the evolution of which was driven primarily by the demands of feeding.

Effects of Size, Condition, Measurer, and Time on Measurements of Snakes

Abstract:  We tested the precision and accuracy of common measurements of snakes by repeated measurement of the heads and trunks of 10 preserved snakes and 10 live snakes by two groups of five people over 10-wk periods. The measurements produced values with variances and ranges related to the nature of the variable, the measurer, and the snake, but accuracy could not be determined. Reporting sizes of snakes to high levels of accuracy is therefore unwarranted. Measurements of head variables on preserved and anesthetized live snakes had similar levels of variance that approximate half the variance of the same measures on live, unanesthetized snakes. Conversely, measurements of snout–vent length (SVL) on both preserved and unanesthetized live snakes had about twice the variance of the same measures made on anesthetized snakes. Measurers differed for all measurements of preserved snakes and for all head measurements of live, unanesthetized snakes, more experienced measurers generally yielding higher precision. Conversely, measurers did not differ for most measures of anesthetized snakes. Our data support suggestions that the most repeatable measures of SVL are made on anesthetized snakes. Lengths of the head and lower jaw can be measured with relative precision on a snake in any condition. Head width and supralabial length have both inter- and intrameasurer variances high enough to make them unreliable measures of head size. We conclude that features of live snakes most commonly measured vary because they have no exact size. We therefore suggest a new convention for reporting sizes of snakes.

Functional morphology of the palato-maxillary apparatus in “Palatine dragging” snakes (Serpentes: Elapidae: Acanthophis, Oxyuranus)

Elapid snakes have previously been divided into two groups (palatine erectors and palatine draggers) based on the morphology and inferred movements of their palatine bone during prey transport (swallowing). We investigated the morphology and the functioning of the feeding apparatus of several palatine draggers (Acanthophis antarcticus, Oxyuranus scutellatus, Pseudechis australis) and compared them to published records of palatine erectors. We found that the palatine in draggers does not move as a straight extension of the pterygoid as originally proposed. The dragger palato‐pterygoid joint flexes laterally with maxillary rotation when the mouth opens and the jaw apparatus is protracted and slightly ventrally during mouth closing. In contrast, in palatine erectors, the palato‐pterygoid joint flexes ventrally during upper jaw protraction. In draggers, the anterior end of the palatine also projects rostrally during protraction, unlike the stability of the anterior end seen in erectors. Palatine draggers differ from palatine erectors in four structural features of the palatine and its relationships to surrounding elements. The function of the palato‐pterygoid bar in both draggers and erectors can be explained by a typical colubroid muscle contraction pattern, which acts on a set of core characters shared among all derived snakes. Although palatine dragging elapids share a fundamental design of the palato‐maxillary apparatus with all higher snakes, they provide yet another demonstration of minor structural modifications producing functional variants. J. Morphol. 2010.

Burrowing with a kinetic snout in a snake (Elapidae: Aspidelaps scutatus)

Of the few elongate, fossorial vertebrates that have been examined for their burrowing mechanics, all were found to use an akinetic, reinforced skull to push into the soil, powered mostly by trunk muscles. Reinforced skulls were considered essential for head‐first burrowing. In contrast, I found that the skull of the fossorial shield‐nosed cobra (Aspidelaps scutatus) is not reinforced and retains the kinetic potential typical of many non‐fossorial snakes. Aspidelaps scutatus burrows using a greatly enlarged rostral scale that is attached to a kinetic snout that is independently mobile with respect to the rest of the skull. Two mechanisms of burrowing are used: (1) anteriorly directed head thrusts from a loosely bent body that is anchored against the walls of the tunnel by friction, and (2) side‐to‐side shovelling using the head and rostral scale. The premaxilla, to which the rostral scale is attached, lacks any direct muscle attachments. Rostral scale movements are powered by, first, retractions of the palato‐pterygoid bar, mediated by a ligament that connects the anterior end of the palatine to the transverse process of the premaxilla and, second, by contraction of a previously undescribed muscle slip of the m. retractor pterygoidei that inserts on the skin at the edge of the rostral scale. In derived snakes, palatomaxillary movements are highly conserved and power prey capture and transport behaviors. Aspidelaps scutatus has co‐opted those mechanisms for the unrelated function of burrowing without compromising the original feeding functions, showing the potential for evolution of functional innovations in highly conserved systems.