An arboreal eyelash viper (Bothriechis schlegelii)
resting on a branch in Costa Rica. Photograph by
Coleman M. Sheehy III.
Gravity is a pervasive force that can severely affect blood circulation in terrestrial animals, and these effects can be particularly pronounced in tall or long organisms such as giraffes and snakes. Upright postures create vertical gradients of gravitational pressures within circulatory vessels that increase with depth. In terrestrial animals, this pressure potentially induces blood pooling and edema in the lower-most tissues and decreases blood volume reaching the head and vital organs.
Since their evolutionary origins about 100 million years ago, snakes have diversified into a wide variety of aquatic, burrowing, terrestrial, and arboreal habitats where they experience various levels of gravitational stress on blood circulation. At the extremes, these stresses range from low to none in fully aquatic species living in essentially “weightless” environments, to relatively high in climbing species, especially arboreal forms specialized for climbing trees. As a result, arboreal snakes exhibit many adaptations for countering the effects of gravity on blood circulation, including relatively tight tissue compartments in the tail. However, patterns of tail length in relation to arboreal habitats and gravity have not been previously studied.
We obtained length data for 226 snake species representing almost all snake families to test the hypothesis that arboreal snakes have longer tails than do non-climbing species. We found that average tail length increased and average body length decreased with increasing use of arboreal habitats and that arboreal snake species had average tail lengths 3–4 times longer than those of non-climbing species. Snakes with longer tails have a higher percentage of elongate blood vessels contained within the relatively tight skin of the tail, which counters blood pooling experienced during climbing. Total body length appears to be constrained in arboreal species, and total body length in adult female arboreal snakes appears to be an evolutionary tradeoff that favors longer tail lengths over maximum production of offspring as arboreal habitat-use increases. Our findings provide evidence that long tails of arboreal snakes function, at least in part, as an adaptation to counter cardiovascular stresses on blood circulation imposed by gravity.
Sheehy, C. M., Albert, J. S., & Lillywhite, H. B. (2015). The evolution of tail length in snakes associated with different gravitational environments. Functional Ecology. Early On-line.