|
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.
Citation
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.