Evolutionary Patterns in Advanced Snakes

One prevalent view of phylogenetic events in advanced snakesholds that the fangs evolved along at least two pathways one(e.g. elapids) from ancestors with enlarged anterior and theother (e.g. viperids) from ancestors with enlarged posteriormaxillary teeth. Selective forces driving these changes arepresumed to arise from the increasing advantages of teeth andglands in venom injection. In this paper another plausible viewof these events is proposed. First fangs of both elapids and viperids likely evolved fromreal maxillary teeth. In non-venomous snakes, differences intooth morphology and function suggest that there may be somedivision of labor among anterior and posterior maxillary teeth.Anterior maxillary teeth, residing forward in the mouth likelyserve the biological role of snaring and impaling prey duringthe strike. They are also conical frequently recurved and lacka secretion groove. On the other hand posterior teeth becauseof their geometric position on the maxilla and mechanical advantages,tend to serve as aids in preingestion manipulation and swallowingof prey. They are often blade shaped and occasionally bear asecretion groove along their sides. Although both front andrear maxillary teeth of nonvenomous snakes may be elongatedthis is likely to serve these different functional roles andhence they evolved under different selective pressures. Whenfangs evolved they did so several times independently but fromrear maxillary teeth. In support one notes a) the similar positionpostorbital of venom and Duvernoy s glands b) similar embryonicdevelopment of fangs and rear maxillary teeth c) secretion groovewhen present, is found only on rear teeth and d) similar biologicalroles of some rear teeth and fangs. For ease in clearance ofthe prey during the strike the fangs are positioned forwardin the mouth accomplished in viperid snakes by forward rotationof the maxilla and elapids by rostral anatomical migration tothe front of the maxilla. Second, the adaptive advantage first favoring initial rear toothenlargement likely centered not on their role in venom injectionbut rather on their role in preingestion manipulation and swallowing.However once enlarged, teeth would be preadapted for later modificationinto fangs under selection pressures arising from advantagesof venom introduction. This has implications for the function and evolution of associatedstructures. Besides possibly subduring or even killing of preythe secretion of Duvernoy's gland may be involved in digestionor in neutralizing noxious or fouling products of the prey.The presence or absence of constriction need not be functionallytied to absence or presence of venom injection. The phylogeneticpathways outlined herein were likely traveled several timesindependently in advanced snakes.     

Effects of dissociating agents on the fine structure of embryonic chick thyroid cells

Heads of the boid snakes Python sebae and Python molurus were dissected and the arthrology, myology and dentition studied. Living specimens of these species were observed and their feeding behavior analyzed by means of high- and regular-speed motion pictures. Camera speeds of up to 400 frames per second permitted examination of the jaw movements during the striking and seizing of prey. Motion picture studies conducted at regular speeds provided information on cranial movements during the swallowing of prey. The morphology of the head was correlated with observed movements in an attempt to analyze the functional and adaptive implications of the jaw apparatus. The cranial apparatus was discussed in terms of a linkage or kinematic chain whose constrainment and degrees of freedom were examined and compared with the jaw linkage of lizards. It was concluded that the very rigidly constrained mechanism in lizards is in remarkably sharp contrast to the very loose apparatus in snakes. Motions of various cranial bones were analyzed with particular attention given the mechanical factors involved. In full protraction the maxillae and palatines are lifted and rotated outward about a longitudinal axis. These movements are important in orienting the teeth with respect to the prey and are related to seizing and swallowing.     

The evolution of venom-delivery systems in snakes

The Colubroidea represents approximately 2300 of the 2700 species of living snakes and includes all venomous taxa. Although many morphological studies of colubroid snakes have been carried over the last hundred years, the phylogenetic relationships within this group are poorly known. In this study, components of the venom-delivery system (VDS) were examined within the context of two conflicting phylogenetic hypotheses proposed in 1988 by Cadle and in 1998 by Kraus & Braun. The results suggest that several major morphological changes occurred early in colubroid evolution: a Duvernoy's gland evolved, the posterior maxillary teeth became specialized relative to the anterior maxillary teeth, and the attachment of the pterygoideus muscle moved forward to a position associated with the posterior maxillary teeth. These innovations may have allowed the great radiation of colubroid snakes that led to the Colubroidea representing such a large percentage of living snakes. More recently, three separate lineages of colubroids have independently evolved highly specialized front-fanged VDSs with large and complex venom glands, venom gland compressor muscles, and tubular fangs.  © 2003 The Linnean Society of London, Zoological Journal of the Linnean Society, 2003, 137 , 337−354.     

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. © 2009 Wiley‐Liss, Inc.     

Specializations of the Body Form and Food Habits of Snakes

Viperid snakes have stouter bodies, larger heads, and longerjaws than snakes in other families; there are no major differencesbetween the two subfamilies of vipers in these features. A suiteof morphological characters that facilitates swallowing largeprey finds its greatest expression among vipers, but certainelapid and colubrid snakes have converged upon the same bodyform. The number of jaw movements required to swallow prey islinearly related to the size of a prey item when shape is heldconstant. Very small and very large prey are not disproportionatelydifficult for a snake to ingest. Vipers swallow their prey withfewer jaw movements than do colubrids or boids and can swallowprey that is nearly three times larger in relation to theirown size. Proteolytic venom assists in digestion of prey, andmelanin deposits shield the venom glands from light that woulddegrade the venom stores. Ancillary effects of the morphologicalfeatures of vipers, plus the ability to ingest a very largequantity of food in one meal, should produce quantitative andqualitative differences in the ecology and behavior of vipersand other snakes.     

Influence of the venom delivery system on intraoral prey transport in snakes

We compared intraoral prey transport in venomous snake species from four families (two atractaspidids, nine elapids, three colubrids, 44 viperids) with that in eight non-venomous colubrid species, most feeding on similar mammalian prey. The morphology of the venom delivery system suggests that intraoral prey transport performance should be slightly decreased in atractaspidids, unmodified in most elapids and venomous colubrids, and increased or unmodified in vipers, as compared to that in non-venomous colubrid snakes. Our measurements of relative intraoral prey transport performance show that differences among families do not match expectations based on morphology or past studies. Decreased performance in Atractaspis results from reduction and loss of teeth on the medial palatal elements and dentaries, but affects only early phases of ingestion. Although joint and bone features of elapids and colubrids are similar, intraoral prey transport performance is significantly lower in elapids than in colubrids. Predicted enhancement of intraoral prey transport performance in vipers as compared to colubrids was not borne out by measurements, presumably because palatopterygoid movement during intraoral prey transport is reduced in many viper species to limit fang erection. Absence of significant performance differences between colubrids and viperids might suggest that evolution of the viperid venom delivery system was subject to little selection pressure from intraoral prey transport. Another possibility is that there are trade-offs between intraoral prey transport and strike performance in vipers related to relative skull mass and jaw fragility. Immobilizing prey prior to intraoral transport places less demand on transport performance in vipers. In this model, the conservative kinesis and greater robustness of the colubrid palate has greater potential for transporting live prey with less risk of injury.     

Feeding in Atractaspis (Serpentes: Atractaspididae): a study in conflicting functional constraints

African fossorial colubroid snakes of the genus Atractaspis have relatively long fangs on short maxillae, a gap separating the pterygoid and palatine bones, a toothless pterygoid, and a snout tightly attached to the rest of the skull. They envenomate prey with a unilateral backward stab of one fang projected from a closed mouth. We combined structural reanalysis of the feeding apparatus, video records of prey envenomation and transport, and manipulations of live and dead Atractaspis to determine how structure relates to function in this unusual genus of snakes. Unilateral fang use in Atractaspis is similar to unilateral slashing envenomation by some rear-fanged snakes, but Atractaspis show no maxillary movement during prey transport. Loss of pterygoid teeth and maxillary movement during transport resulted in the inability to perform. 'pterygoid walk' prey transport. Atractaspis transport prey through the oral cavity using movement cycles in which mandibular adduction, anterior trunk compression, and ventral flexion of the head alternate with mandibular abduction and extension of head and anterior trunk over the prey. Inefficiencies in manipulation and early transport of prey are offset by adaptability of the envenomating system to various prey types in both enclosed and open spaces and by selection of prey that occupy burrows or tunnels in soil. Atractaspis appears to represent the evolutionary endpoint of a functional conflict between envenomation and transport in which a rear-fanged envenomating system has been optimized at the expense of most, if not all, palatomaxillary transport function.     

Size-dependent predation by snakes: selective foraging or differential prey vulnerability?

I staged replicate encounters between unrestrained lizards andsnakes in outdoor enclosures to examine size-dependent predationwithin the common garden skink (Lampropholis guichenoti). Yellow-facedwhip snakes (Demansia psammophis) forage widely for activeprey and most often consumed large skinks, whereas death adders(Acanthophis antarcticus) ambush active prey and most oftenconsumed small skinks. Small-eyed snakes (Rhinoplocephalusnigrescens) forage widely for inactive prey and consumed bothsmall and large skinks equally often. Differential predationmay reflect active choice by the predator, differential preyvulnerability, or both. To test for active choice, I presentedforaging snakes with an inert small lizard versus an inertlarge lizard. They did not actively select lizards of a particularbody size. To test for differential prey vulnerability, I quantifiedvariation between small and large lizards in behavior thatis important for determining the outcome of predator—prey interactions. Snakes did not differentiate between integumentarychemicals from small and large lizards. Large lizards tendto flee from approaching predators, thereby eliciting attackby the visually oriented whip snakes. Small lizards were moremobile than large lizards and therefore more likely to passby sedentary death adders. Additionally, small skinks were more effectively lured by this sit-and-wait species and less likelyto avoid its first capture attempt. In contrast, overnightretreat site selection (not body size) determined a lizard'schances of being detected by small-eyed snakes. Patterns ofsize-dependent predation by elapid snakes may arise not becauseof active choice but as a function of species-specific predatortactics and prey behavior.     

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

The kinetic skull is a key innovation that allowed snakes to capture, manipulate, and swallow prey exclusively using their heads using the coordinated movement of eight bones. Despite these unique feeding behaviors, patterns of evolutionary integration and modularity within the feeding bones of snakes in a phylogenetic framework have yet to be addressed. Here, we use a dataset of 60 μCT‐scanned skulls and high‐density geometric morphometric methods to address the origin and patterns of variation and integration in the feeding bones of aquatic‐foraging snakes. By comparing alternate superimposition protocols allowing us to analyze the entire kinetic feeding system simultaneously, we find that the feeding bones are highly integrated, driven predominantly by functional selective pressures. The most supported pattern of modularity contains four modules, each associated with distinct functional roles: the mandible, the palatopterygoid arch, the maxilla, and the suspensorium. Further, the morphological disparity of each bone is not linked to its magnitude of integration, indicating that integration within the feeding system does not strongly constrain morphological evolution, and that adequate biomechanical solutions to a wide range of feeding ecologies and behaviors are readily evolvable within the constraint due to integration in the snake feeding system.     

Phylogenetic relationships of colubroid snakes based on mitochondrial DNA sequences

We used a 694 bp length of the mitochondrial ND4 gene from 40 genera to infer phylogenetic relationships among colubroid snakes. The goals of this study were to identify conserved subsets of ND4 sequence data that could be used to address (1) which nominal higher-level colubroid taxa are monophyletic, and (2) the relationships among the monophyletic lineages identified. Use of transversions only proved the most reliable and efficient means of retrieving colubroid relationships. Transversion parsimony and neighbour-joining analyses identify similar monophyletic higher-level taxa, but relationships among these lineages differ considerably between the two analyses. These differences were affected by the inclusion/exclusion of (1) transitions, (2) autapomorphies, and (3) the boid outgroups. Saturation effects among the transitions, uninformativeness of autapomorphies for clustering taxa, and long-branch and base-compositional problems among the boids lead us to regard the tree resulting from transversion parsimony analysis rooted with Acrochordus as the best current estimate of colubroid phylogenetic relationships. However, several aspects of this proposed phylogeny need further testing (e.g. the apparent diphyly of Natricinae is especially controversial). Relationships retrieved using all colubroid taxa are not obtained when sparsely or unevenly sampled experimental subsets of taxa are used instead, suggesting that long-branch problems can severely compromise elucidation of colubroid relationships if limited taxonomic sampling strategies are followed. We discuss the importance of this finding for previous molecular attempts to assess colubroid relationships. Our analyses confirm the historical validity of several nominal colubroid families and subfamilies, establish polyphyly of a few, but generally fail to resolve relationships among the monophyletic taxa we identify. More conservative character information will be required to confidently resolve the last issue.     

Convergence in trophic morphology and feeding performance among piscivorous natricine snakes

Piscivory has independently evolved numerous times amongst snakes, and therefore these animals provide a powerful opportunity to test for convergent evolution in a vertebrate feeding system. In this study, we integrate performance trials with comparative methods to test the hypothesis that piscivory drives convergence in trophic morphology and feeding performance among natricine snakes. Within and across species, increasing the relative length of the quadrate bone in the skull is positively and strongly linked to a reduction in the time needed to swallow large fish prey. Thus, our feeding experiments suggest that a longer quadrate bone enables snakes to better conform their head shape to the shape of the prey during swallowing. Ancestral diet reconstructions and phylogenetically corrected multiple regression analyses further reveal that evolutionary increases in piscivory are coupled to the evolution of relatively longer quadrates, and hence improved feeding performance on fish prey in these animals. The exploitation of similar dietary niches drives the evolution of convergent trophic morphologies and feeding performances in natricine snakes.     

Feeding Strategies in Marine Snakes: An Analysis of Evolutionary, Morphological, Behavioral and Ecological Relationships

Analysis of 1,063 stomach contents from 39 species of sea snakesindicates that about one-third of the shallow, warm, marine,Indo-Australian fish families are preyed upon by sea snakes.Families of eels and gobies are taken by the greatest numbersof snake species. Most species of sea snakes feed on fish familieswhose members are relatively sedentary, dwelling along the bottom,within burrows or reef crevices. With one exception, a fishegg-eating specialization found uniquely in the Aipysurus-Emydocephaluslineage, the dietary habits of sea snakes cannot be categorizedaccording to the snakes' three phylogenetic lineages. Eels,mullet-like, rabbitfish-like and goby-like fish forms are takenby all three lineages. Two or three snake species are generalists,and numerous ones specialize on eels, goby-like fish or catfish.There are differences among sea snake species in the relationshipbetween snake neck girth and the maximum diameter of the prey;in the relationships of both snake gape measurements and fanglength, to the type of prey taken; and in the relationship ofsnake shape and body proportions to the prey selected. Severalmodes of feeding have been observed among sea snakes: feedingin nooks and crannies in the bottom or in reefs, cruising nearthe bottom, and feeding in drift lines. Analysis of percentdigestion of stomach contents and projections backward to thetimes of prey capture provides evidence for feeding periodicity.The greatest amount of diet overlap is for two species of seasnakes which do not both occur at the same locality. Where speciesdo co-occur, diet overlap index values are lower. The numbersof species present as well as their relative abundances varyamong localities as does the relative importance of generalists,eel-eaters, egg-eaters and other specialized feeders.     

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.     

Terrestrial feeding in the Mudskipper Periophthalmus (Pisces: Teleostei): A cineradiographic analysis

Mudskipping gobies (Periophthalminae) are among the most terrestrial of amphibious fishes. Specializations associated with terrestrial prey capture and deglutition have been studied in Periophthalmus koelreuteri by light and X-ray cinematography which permits direct visualization of pharyngeal jaw movement during deglutition. Anatomical specializations of the pharyngeal jaws are described and include depressible teeth, a large ventral process on ceratobranchial five, and muscular modifications.
Multiple terrestrial feedings occur by Periophthalmus without a return to the water, and cineradiography reveals that the buccal cavity is often filled with air during terrestrial excursions in contrast to some previous hypotheses. Transport of the prey into the oesophagus occurs primarily by anteroposterior movement of the upper pharyngeal jaw. The lower pharyngeal jaw plays a limited role in food transport and may serve primarily to hold and position prey. The bite between upper and lower pharyngeal jaws occurs between the anterior teeth, and both jaws are protracted together during raking of food into the oesophagus. Functional specializations correlated with terrestrial feeding include obligatory use of pharyngeal jaws for swallowing even small prey items and positioning of the prey in the pharynx by pharyngeal jaw and hyoid movements alone.
This analysis of terrestrial feeding allows hypotheses of design constraints imposed by the aquatic medium on fishes to be raised and tested.     

Evolution of the Toxoglossa venom apparatus as inferred by molecular phylogeny of the Terebridae

Toxoglossate marine gastropods, traditionally assigned to thefamilies Conidae, Terebridae, and Turridae, are one of the mostpopulous animal groups that use venom to capture their prey.These marine animals are generally characterized by a venomapparatus that consists of a muscular venom bulb and a tubularvenom gland. The toxoglossan radula, often compared with a hypodermicneedle for its use as a conduit to inject toxins into prey,is considered a major anatomical breakthrough that assistedin the successful initial radiation of these animals in theCretaceous and early Tertiary. The pharmacological success oftoxins from cone snails has made this group a star among biochemistsand neuroscientists, but very little is known about toxins fromthe other Toxoglossa, and the phylogeny of these families islargely in doubt. Here we report the first molecular phylogenyfor the Terebridae and use the results to infer the evolutionof the venom apparatus for this group. Our findings indicatethat most of the genera of terebrids are polyphyletic, and onespecies ("Terebra" (s.l.) jungi) is the sister group to allother terebrids. Molecular analyses combined with mapping ofvenom apparatus morphology indicate that the Terebridae havelost the venom apparatus at least twice during their evolution.Species in the genera Terebra and Hastula have the typical venomapparatus found in most toxoglossate gastropods, but all otherterebrid species do not. For venomous organisms, the dual analysisof molecular phylogeny and toxin function is an instructivecombination for unraveling the larger questions of phylogenyand speciation. The results presented here suggest a paradigmshift in the current understanding of terebrid evolution, whilepresenting a road map for discovering novel terebrid toxins,a largely unexplored resource for biomedical research and potentialtherapeutic drug development.     

Morphometries of the ectopterygoid in advanced snakes (Colubroidea): a concordance of shape and phylogeny

A principal components analysis was performed on 20 measurements of the ectopterygoids of 85 colubroid snakes. This sample encompassed all the shape variation previously recognized in the ectopterygoid of colubroids. Simple proportions which correlate with the first two principal component axes are: relative ectopterygoid length with the first and five measures of absolute or relative head shape with the second. Using these simple proportions colubroid ectopterygoids can be sorted into four shape classes each concordant with a taxonomic grouping: relatively long with broad notched head—Crotalinac; relatively long with narrow, flat head—Viperinae; relatively short with broad, notched head—Colubridae; and relatively short with narrow, flat head—Elapidae. This concordance has an error of about 10⁰₀. We propose that these shape classes reflect phylogenetically old and conservative functional differences in the palatomaxillary complexes of the four taxonomic groupings. Other aspects of ectopterygoid shape are described and provisional character state phylogenies for some aspects of the ectopterygoid are presented. Finally, the bearing of our data on the systematics of aparallactines, micrurines, sea snakes, Azemiops and certain other genera are discussed.     

Evolution of asynchronous motor activity in paired muscles: effects of ecology, morphology, and phylogeny

Many studies of feeding behavior have implanted electrodes unilaterally(in muscles on only one side of the head) to determine the basicmotor patterns of muscles controlling the jaws. However, bilateralimplantation has the potential to achieve a more comprehensiveunderstanding of modification of the motor activity that maybe occurring between the left and right sides of the head. Inparticular, complex processing of prey is often characterizedby bilaterally asynchronous and even unilateral activation ofthe jaw musculature. In this study, we bilaterally implant feedingmuscles in species from four orders of elasmobranchs (Squaliformes,Orectolobiformes, Carcharhiniformes, Rajoidea) in order to characterizethe effects of type of prey, feeding behavior, and phylogenyon the degree of asynchronous muscle activation. Electrodeswere implanted in three of the jaw adductors, two divisionsof the quadratomandibularis and the preorbitalis, as well asin a cranial elevator in sharks, the epaxialis. The asynchronyof feeding events (measured as the degree to which activityof members of a muscle pair is out of phase) was compared acrossspecies for capture versus processing and simple versus complexprey, then interpreted in the contexts of phylogeny, morphology,and ecology to clarify determinants of asynchronous activity.Whereas capture and processing of prey were characterized bystatistically similar degrees of asynchrony for data pooledacross species, events involving complex prey were more asynchronousthan were those involving simple prey. The two trophic generalists,Squalus acanthias and Leucoraja erinacea, modulated the degreeof asynchrony according to type of prey, whereas the two behavioralspecialists, Chiloscyllium plagiosum and Mustelus canis, activatedthe cranial muscles synchronously regardless of type of prey.These differences in jaw muscle activity would not have beendetected with unilateral implantation. Therefore, we advocatebilateral implantation in studies of cranial muscle functionin fishes, particularly when investigating behaviors associatedwith processing complex prey. Incorporating this methodologywill provide a more detailed understanding of the coordinationand evolution of paired-muscle function in the feeding apparatusrelative to behavioral and ecological performance.     

The influence of prey-scent stimuli on predatory behavior of the North American copperhead Agkistrodon contortrix (Serpentes: Viperidae)

Viperid snakes strike, envenomate, and release mammalian preyto prevent being harmed by the prey; snakes must then trackprey in the process of strike-induced chemosensory searching.Because rattlesnakes prefer to track and consume envenomatedprey, it would seem that the scent of envenomated tissue iskey to the tracking process. After striking rodents, rattlesnakesalso retain a specific chemical search image of prey items.I examined this behavioral pattern in copperheads (Agkistrodoncontortrix) from three US populations with documented dietarybiases toward mammals (Kansas), lepidopteran larvae (Texas),and amphibians (Louisiana), respectively. Experiments were conductedto assess whether copperheads form a specific search image ofnon-envenomated mouse, hornworm, and frog prey items. Additionalexperiments tested the relative importance of envenomated tissueto prey scent. Results indicate that copperheads do not forma specific search image of prey items. Preference for non-envenomatedprey items is in the order mouse > hornworm > frog forall three populations; therefore, the innate behavioral preferencefor types of prey does not match the dietary biases noted inthe literature. Envenomated mice and hornworms were preferredto all nonenvenomated prey items, but most trials involvingenvenomated frogs did not suggest envenomated prey preference.Overall, these results suggest that when the snakes search forprey, envenomated tissue stimuli are more important to snakesthan scents arising from the prey itself. Searching and consumptionbehaviors seem to be independent, suggesting that strike-inducedchemosensory searching and consumption are more complicatedbehavioral processes than previously recognized.     

Presence of gular and parietal pits in Atretium schistosum (Serpentes, Colubridae), a singular trait not exclusive to psammophine snakes

The sporadic occurrence of localised pits on parietal plates was recently discovered in different colubrid genera of the subfamily Psammophiinae; these were considered to play a role in sensory perception. In the present study, we describe the presence of similar structures in Atretium schistosum, another colubrid snake reportedly not belonging to the Psammophiinae. As this species is suspected of being phylogenetically distantly related to psammophine snakes, some hypotheses are provided to explain (1) the putative function of these pits, (2) their sporadic occurrence, and (3) to suggest when they may have evolved in the colubroid snake clade.     

Post-Strike Behavior of Timber Rattlesnakes (Crotalus horridus) During Natural Predation Events

The complexity of natural environments is an important component of animal behavior, and laboratory environments often cannot reproduce that complexity. Strike‐induced chemosensory searching (SICS) is a robust phenomenon among venomous snakes that has been studied extensively in the laboratory. To date, observations of this behavior in the field have been limited largely to anecdotes; the extent to which post‐strike behaviors in the laboratory accurately reflect what occurs in nature has not been examined. In this study, I use time‐lapse video equipment in the field to record the predatory behavior of timber rattlesnakes (Crotalus horridus). This represents the first quantitative analysis of post‐strike predatory behaviors associated with natural feeding events. As in the laboratory, stereotyped post‐strike behaviors were only observed after successful strikes, and not after missed strikes. Snakes in the field were observed to proceed through the same basic behavioral stages that have been documented in the laboratory: striking prey, releasing prey immediately after strike, post‐strike immobility, location of the chemosensory trail, trail following, and prey swallowing. However, the duration of post‐strike immobility, trail location, and prey swallowing was substantially longer in field than in laboratory studies. Additionally, post‐strike immobility was significantly longer when snakes struck large prey (prey over 100 g) than when they struck small prey. Overall, these results indicate that the behavioral challenges associated with SICS may be more robust than laboratory studies have indicated.