Thermoregulation in Turkey Vultures. Vascular Anatomy, Arteriovenous Heat Exchange, and Behavior

The vasculature of skin and of peripheral heat exchangers in Turkey Vultures (Cathartes aura, mass 1.4 kg) was studied in relation to peripheral temperatures and behavior at ambient temperatures (Ta) from 10°C through 50°C. The head's unfeathered skin contains thin-walled, anastomosing veins near the epidermis. Metatarsal skin has a similar venous plexus, but cutaneous vasculature from the wing's underside does not. Skin temperature (Ts) at the crown was close to Ta but Ts at the nape was close to body temperature (Tb). At both skin sites Ts changed with Ta. Investigation of deeper vasculature showed that the head has bilateral ophthalmic retia that probably cool the brain and eyes, as in other birds. In the wing, the basilica vein branches to form a venae comitantes, a mesh of anastomosing veins that separately surrounds the radial and ulnar arteries. A bypass vein is also present. The temperature profiles along the 10-cm length of this humeral plexus became steeper with decreasing Ta. At its distal end, temperature fell to 22°C at 10°C Ta, with the steepest observed gradient being 3.0°C/cm. In each leg, the tibial and fibular arteries closely contact two to four venae comitantes to form a simple rete, as in other species. This helps conserve body heat in cold but, as in the wing, is probably bypassed during heat stress. Behavior ranged from neck and wing retraction with a pale head and feet at low Ta, to neck and wing extension with a deep red head and feet at high Ta. Panting, gular flutter, and urohidrosis accompanied these responses at the highest TaS.