ORTHOSTATIC INTOLERANCE OF VIPERID SNAKES

Whereas many terrestrial vertebrates can assume upright postures for l ong periods, some terrestrial snakes are unable to do so because blood circulation fails. Measurements of carotid arterial bloodflow during head-up tilt in four viperid species demonstrate that blood flow to th e head diminishes markedly when the body deviates from horizontal post ure and ceases entirely at tilt angles greater than or equal to 30 deg rees. In this respect, large ground-dwelling vipers are similar to aqu atic snakes, which also experience circulatory dysfunction when tilted upright in air. On the other hand, postural changes do not seriously impede blood circulation in arboreal species of snakes that routinely employ vertical postures. Examination of body size and habits among vi perid species suggests that gravitational hemodynamics play a potentia lly important role in limiting the behavior and ecological radiation o f these snakes.