ROMMEL, S.A.*; PABST, D.A.; MCLELLAN, W.A.: Convergent evolution in the vascular systems of three marine mammals

Conservation of thermal energy in aquatic mammals is essential for the maintenance of high core temperatures in a heat conducting aquatic environment. Seals, dolphins, and manatees have gross morphological adaptations such as large size, modification of appendages, and blubber to conserve body heat. Additionally, they possess vascular countercurrent heat exchangers in their extremities. Nevertheless, the need for heat conservation is balanced by an equally important need for selective heat dissipation to protect susceptible organs, such as the testes and epididymides, from hyperthermic insult. Seals, dolphins, and manatees are ascrotal, increasing the potential for hyperthermic insult. However, specialized vascular designs enable the selective cooling of these thermally sensitive tissues. Vascular structures that prevent reproductive hyperthermic insult in seals, dolphins, and manatees are reviewed and physiological evidence to support these anatomical observations is presented. Each of these marine mammals possesses vascular shunts associated with the testes and epididymides. These shunts are parallel to the countercurrent heat exchangers, connecting superficial vascular beds and deep vascular plexuses. The thermo-vascular “circumventions” are convergent adaptations across three diving mammalian groups with independent evolutionary histories.