[[abstract]]To adapt for arboreal life, tree-dwelling snakes have adopted many changes in morphology, physiology and behavior. Although arboreal snakes seems have better cantilever ability, this behavior has not been studies quantitatively. Similarly, their air sac was shown to be longer than that of terrestrial snakes in a figure of one paper. But, no other report has verified whether this is a general trend. Consequently, I tried to cheek if above two phenomena is true and inquire whether air sac possess a function of aiding snakes in spanning. I examined the relative position of viscus to abdomen scales of 4 snakes: Boiga krapelini, Dinodon ruforzonatum, Trimeresurus stejnegeri and Trimeresurus mucrosqumatus. Phylogeny played more important role in shaping the position and length of viscus. However, within the same taxa, arboreal snakes did possess longer air sac than terrestrial snakes. To test whether arboreal snakes have better cantilever ability than terrestrial snakes, I added 2 more species; Pareas formosanus and Naja atra, in the cantilever experiments. All the arboreal snakes(B. krapelini, T. stejnegeri and Pareas formosanus )showed a better cantilever ability than terrestrial snakes(D. ruforzonatum, T. mucrosqumatus and N. atra) despite the difference of phylogeny. I used only D. ruforzonatum to examine the role of air sac in cantilever behavior. Because this species adapted well in laboratory and was abundant in the field. To monitor the intrapulmonary pressure in the air sac, I implanted a tube in the air sac and connected it to a pulse transducer. In resting condition, the respiratory frequency, mean intrapulmonary pressure, and amplitude of pressure were all significantly larger than that when in moving on the ground or spanning gaps. Although intrapulmonary pressure increased clearly when a snake almost reached its maximal cantilever length, no statistical difference of respiratory frequency, mean intrapulmonary pressure, and amplitude of pressure was found between crawling and spanning condition. When I further deflated the intrapulmonary pressure by implanting another bigger rubber tube in the air sac, I found the snake could still span a same length as before. Therefore, the results did not support the idea that air sac may aid snakes to span gaps. Because D. rufozonatum is a terrestrial snake, further investigation will reveal whether this condition may also apply to arboreal snakes.
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