The functional morphology of hooding in cobras.

Many snakes, particularly cobras, form as part of a defensive display, a hood, an active lateral expansion of their neck skin and underlying musculature and ribs. We identified muscle groups possibly involved in hooding based on their attachments on the specialized ribs of the neck. We then used a combination of morphology, kinematic analysis, morphometrics, electromyography and muscle stimulation to test hypotheses about the functional basis of hooding. We confirmed that hood protraction and erection is an active process that begins cranially and extends caudally, often in stages, through the combined action of several sets of muscles. One set of axial muscles (levator costae and supracostalis lateralis superior) coursing along a line of action to rib displacement are the prime erectors acting to lift the hood. However, a second set of muscles connecting ribs to skin primarily keep the skin taut, rather than to displace the ribs relative to the vertebrae. A third set of muscles coursing between ribs function primarily to transmit forces between adjacent ribs rather than to move ribs. The maintenance of the erect hood requires continued muscle activity. Hood relaxation is due to both active muscle contraction of a fourth set of axial muscles and to passive recoil events in the costovertebral ligaments. The shape of the fully erect hood is reflective of the morphometrics of the underlying ribs, while the duration and kinematics of hood erection and relaxation are related to the behavioral context of the display.

The forked tongue and edge detection in snakes (Crotalus oreganus): an experimental test.

Many stimulus-detection systems are lateralized to allow for simultaneous comparison of paired stimuli. It has been hypothesized that the deeply forked tongue of snakes and some derived lizards functions as a chemical edge detector where cues gathered by each tine are kept separate to provide two points of lateral odor assessment by the central nervous system via vomeronasal input. While following a chemical trail, one time can be on the trail, the other off, and such differential information prompts the snake to turn back to the trail. The authors tested this hypothesis in rattlesnakes within a predatory context by unilaterally severing the vomeronasal nerves. If edge detection is used by snakes during prey trailing, then unilateral denervation should disrupt trailing ability. The authors found no change in the seven separate trailing parameters measured. Therefore, they found no support for the edge detection hypothesis as it applies to prey trailing behavior. Instead, the deeply forked tongue may represent a chemosensory specialization to increase odor-sampling area, with snakes and derived lizards detecting only the concentration of chemical trails.

Mechanisms controlling venom expulsion in the western diamondback rattlesnake, Crotalus atrox.

Although many studies have documented variation in the amount of venom expended during bites of venomous snakes, the mechanistic source of this variation remains uncertain. This study used experimental techniques to examine how two different features of the venom delivery system, the muscle surrounding the venom gland (the Compressor Glandulae in the rattlesnake) and the fang sheath, could influence venom flow in the western diamondback rattlesnake, Crotalus atrox. Differential contraction of the Compressor Glandulae explained only approximately 30% of the variation in venom flow. Lifting (compression) of the fang sheath as occurs during a normal strike produced marked increases in venom flow; these changes were closely correlated and exceed in magnitude by almost 10 x those recorded from the Compressor Glandulae alone. These results suggest that variation in these two aspects of the venom delivery system--both in terms of magnitude and temporal patterning--explain most of the observed variation in venom injection. The lack of functional or mechanical links between the Compressor Glandulae and the fang sheath, and the lack of skeletal or smooth muscle within the fang sheath, make it unlikely that variation in venom flow is under direct neural control. Instead, differential venom injection results from differences in the pressurization by the Compressor Glandulae, the gate keeping effects of the fang sheath and enclosed soft-tissue chambers, and by differences in the pressure returned by peripheral resistance of the target tissue.

Biomechanics (Communication arising): prey attack by a large theropod dinosaur.

Prey-capture strategies in carnivorous dinosaurs have been inferred from the biomechanical features of their tooth structure, the estimated bite force produced, and their diet. Rayfield et al. have used finite-element analysis (FEA) to investigate such structure-function relationships in Allosaurus fragilis, and have found that the skull was designed to bear more stress than could be generated by simple biting. They conclude that this large theropod dinosaur delivered a chop-and-slash 'hatchet' blow to its prey, which it approached with its mouth wide open before driving its upper tooth row downwards. We argue that this mode of predation is unlikely, and that the FEA results, which relate to an 'overengineered' skull, are better explained by the biomechanical demands of prey capture. Understanding the mechanics of predation is important to our knowledge of the feeding habits of carnivorous dinosaurs and for accurate reconstruction their lifestyles.

Ultrastructure of duvernoy's gland from the wandering garter snake, Thamnophis elegans vagrans (Serpentes, Colubridae).

In addition to the supralabial glands (strips of glandular tissue lying along the maxilla), most snakes of the family Colubridae possess an enlarged oral gland lying behind the eye and emptying near the rear maxillary teeth, the Duvernoy's gland. Duvernoy's gland is most probably homologous to the venom gland of viperid and elapid snakes, and occasionally has been implicated in cases of human envenomation. Although of possible medical concern, there is reason to believe that secretion from this gland serves a biological role different from that of the venom gland, namely a role primarily in digestion rather then largely in rapid prey immobilization. The parenchyma of the Duvernoy's gland comprise two cell types, a serous cell containing numerous, electron-dense secretory granules, and myoepithelial cells. There are no mucous cells in the parenchyma; instead cells of this type are located exclusively in the lining epithelium of the main duct. Numerous unmyelinated nerves pass between secretory acini. Observations of the supralabial gland reveal that this gland, in addition to serous cells, also contains mucous cells and a putative third cell type we designate as an intermediate cell. In cellular morphology, Duvernoy's gland is closest to the venom gland of elapids, and least like the venom gland of viperids. Compared to the venom glands in both families of venomous snakes, Duvernoy's gland lacks a large luminal secretory reservoir. Emptying of Duvernoy's gland is thought to involve release of secretion granules into the lumen, and movement of the secretory product from there may be supplemented by mechanical pressure exerted externally by nearby contracting striated mucles. These differences in structure and mechanism of secretion release are taken as evidence that although they are homologous, the two types of glands, Duvernoy's and venom glands, are functionally distinct.

Ultrastructure of the lung of the rattlesnake, Crotalus viridis oreganus.

Scanning and transmission electron microscopic observations were made on the rattlesnake lung, which has the form of a cigar-shaped bag enclosing a large axial air chamber. The lungs were fixed by tracheal instillation of fixative to preserve the structural features of inflated lungs. An open tracheal groove along the ventral aspect of the lung is the only structural "airway" present. The wall of the lung has two histologically distinct regions: anteriorly, a respiratory portion, where up to three generations of septa subdivide the wall into cup-shaped gas-exchange chambers, termed faveoli; and posteriorly, a simple, thin-walled saccular portion. The epithelium lining the internal surface of the lung is composed of several cell types: (1) ciliated cells; (2) type I pneumonocytes; (3) type II pneumonocytes, secretory cells characterized by the presence of lamellar bodies; and (4) serous epithelial cells, secretory cells characterized by the presence of homogeneous, densely staining secretory granules. However, the distinctiveness of the secretory cell types in the snake lung is blurred because intermediate-appearing cells have both the lamellar body and homogenous type of secretory granule. The nonepithelial components of the pulmonary wall and septa consist of blood vessels and lymphatics, smooth muscle cells and fibroblasts, embedded in a matrix of extracellular connective tissue fibers. Tubular myelin figures were observed in the faveolar lining layer.