The evolution of tail length in snakes associated with different gravitational environments.

Gravity imposes potentially important constraints on blood circulation in tall or elongate animals during upright posture or climbing. Upright postures create vertical gradients of gravitational (=hydrostatic) pressures within circulatory vessels. In terrestrial animals, this pressure potentially induces blood pooling and oedema in dependent (inferior) tissues and, secondarily, decreases blood volume reaching the head and vital organs. Arboreal snakes exhibit a suite of adaptations for countering the effects of gravity on blood circulation, including relatively non-compliant tissue compartments in the tail. However, patterns of tail length related to arboreal habitats and gravity have not been previously studied. Here, we test the hypothesis that arboreal snakes have relatively longer tails than non-climbing species, and we interpret the results within the context of adaptation to gravity stress., Length data were obtained for 226 snake species, 139 genera and 15 snake families assigned to three broad gravitational habitat categories (G-habitats: stenotopically arboreal, eurytopically arboreal/terrestrial and non-scansorial). We constructed a composite phylogenetic tree and quantified the relationships between snake morphology and G-habitat using conventional regression analysis of species data and regressions of phylogenetically independent contrasts., Mean relative tail length ( RTL) increased and mean relative snout-vent length decreased with increasing arboreality among the three G-habitats. However, mean total body length ( TL) was not different between the two arboreal G-habitats., Snakes with longer RTLs have a higher percentage of elongate blood vessels contained within the relatively non-compliant integument of the tail, which mitigates postural blood pooling experienced during climbing. TL in adult female arboreal snakes is hypothesized to be an evolutionary trade-off between demands for a longer tail to enhance cardiovascular performance and the length of body cavity to provide space for young., Our results complement previous studies correlating RTL with arboreal habitat use, in addition to physiological experiments quantifying the effects of gravity on blood flow in snakes. These results provide evidence that long RTLs of arboreal snakes function, at least in part, as an adaptive response to cardiovascular stresses on blood circulation imposed by gravity. Selection on tail length related to other factors seems less likely, but is in need of further investigation. [ABSTRACT FROM AUTHOR]