Sunday, January 10, 2016

Death by constriction, a fourth possible cause

A juvenile Burmese Python constricting a rat.
The evolution of constriction  was undoubtedly very important milestone in the evolution and radiation of snakes. Killing large prey quickly and reducing the chance of injury to a snake allowed snakes to subdue otherwise unobtainable, larger prey. Constricting snakes exert pressure by coiling around and squeezing their prey, usually killing it before swallowing. The process of constriction takes energy and time, and risks injury to the snake. A snake's ability to constrict and kill quickly is important because it impacts feeding success, and thus growth and fitness. Constriction pressures are generated by forces from the snake’s axial musculature applied to the prey. These forces are proportional to the cross-sectional area of active muscle, and therefore to the snake's diameter. In a new paper Penning et al. (2015) describe the ontogeny of constriction performance in Reticulated and Burmese Pythons. The authors also discuss the implications for the cause of prey death during constriction.

The study found both species constrict prey vigorously using coils of 1–4 loops. Reticulated Pythons exerted maximum pressures of 8.27–53.77 kPa, with larger individuals exerting significantly higher peak pressures than smaller individuals. Burmese Pythons constricted with maximum pressures of 18.0–42.93 kPa, with larger individuals also exerting significantly higher peak pressures than smaller individuals  The species or the number of loops in a coil did not significantly affect peak
pressure in either species.

Constriction pressures exerted by both pythons scale differently from those of other snakes, many of the highest pressures were probably enough to force blood into the brain at high pressure in mammalian prey. In addition to suffocation, circulatory arrest and spinal dislocation, the authors propose the ‘red-out effect’  as a fourth possible mechanism of prey death by constriction. The redout effect describes the effect of negative gravity on jet pilots during extreme flight manoeuvres, in which vision becomes reddened by uncontrollable blood flow to the brain and eyes. When fighter pilots experience negative gravitational accelerations (G-forces), they incur a rush of blood to the brain that causes rapid loss of consciousness. Constriction pressures above the venous blood pressure of the prey will impede blood flow and oxygen delivery to tissues.  Pressure from constriction  dramatically higher than the prey’s blood pressure could force blood away from the site of constriction and into the extremities, including the head and brain. Blood being pushed into the brain during peak constriction could cause the same red-out effect described above for pilots, and could cause extensive ruptures in cranial blood vessels.

Penning, D. A., Dartez, S. F., & Moon, B. R. (2015). The big squeeze: scaling of constriction pressure in two of the world's largest snakes, Python reticulatus and Python molurus bivittatus. Journal of Experimental Biology,218(21), 3364-3367.

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