| Adapatation of New World vultures and storks that uses evaporation-- and excrement-- in the process of cooling off. Though these large birds can spread their wings as a means of absorbing or dissipating heat, sometimes evaporation from a non-feathered part of their body can provide the fastest cooling. Since birds don't have sweat glands, they have to resort to other means. While smaller birds may try panting or the gular flutter, the turkey vulture and its brethren have a highly effective, if less dignified method, of evaporative cooling, which I'll explain below.
You can try it too, but as there are certain difficulties, both anatomical and social, this practice is best done at home. In an easily-cleaned room. Better yet, outdoors. A waterproof tarp may be in order. Ready to play along? Okay, five simple steps to urohydrosis:
- Remove your pants. Birds don't have this step to contend with, but for this experiment, you'll need bare legs.
- Remove your underpants, too.
- Urinate on your bare legs.*
- Are you noticing a cooling effect? That's evaporation in action. As the heat from arterial blood passes by the surface of the legs, it evaporates the liquid on the surface, and thus cools the blood.
- Allow your veins to re-circulate the cooled blood to lower your core temperature.
*Technically, birds don't have a urinary tract, so what comes out may be a combination of urine and feces. The important part is that what comes onto the legs is liquid. As an added advantage, the uric acid works as an antiseptic on the legs, which is helpful if you're a carrion feeder always stepping into something full of bacteria.
Aren't you glad you have sweat glands?
Sources:
Maxine Annabell, "Turkey Vultures," Vultures and Condors, 1998-2000, <http://www.lairweb.org.nz/vulture/turkey.html> (2 April 2002)
Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye, "Temperature Regulation and Behavior," The Birder's Handbook (1988), excerpted in: Darryl Wheye, Birds of Stanford Web Site, 18 February 2000, <http://www.stanfordalumni.org/birdsite/text/essays/Temperature_Regulation.html> (2 April 2002) | 
