Glowing like a horse
Dennis Mitchell: "Margaret, you are all sweaty" Margaret Wade: "Dennis, girls don't sweat. Horses sweat, boys perspire and girls glow" Dennis Mitchell: "Margaret, you are glowing like a horse". -- Dennis the Menace / Hank Ketcham
Horses and humans sweat but most other mammals do not. Sweating lowers the body’s surface temperature by evaporating off the surface of the skin. The heat drawn by evaporation is removed from the surface, thereby cooling it. But as anyone who has been skiing in a poorly-ventilated jacket can tell you, this does not work well if the sweat is not allowed to evaporate. Indeed, most mammals have fur, which would trap the sweat not allowing it to evaporate quickly. They use alternative cooling mechanisms, like evaporative cooling from the respiratory tract, or panting. The horse’s solution is to mix in its sweat a protein called latherin which acts as a surfactant. This means it lowers the surface tension of the water in the sweat, allowing the water to it wet the horse’s coat hairs better and allowing for faster evaporation. Latherin acts like it’s name suggests: it is basically a kind of naturally produced soap, and racehorses are known to lather up during a race.
Horses are also known to foam at the bit. In an article published today in PLoS ONE, Rhona McDonald and her colleagues at the Universities of Glasgow and Manchester show that lathering up and foaming at the bit are two facets of the same phenomenon as latherin is also produced by the horse’s salivary glands. Horses’ food is unusually dry, and latherin in the salivary glands serves to make it nice and mushy, turning oats into oatmeal. Here is an interesting case of adaptation of the same protein to different functions: helping initial digestion, and helping the cooling mechanism, through the same biophysical principle of a surfactant agent.
We use artificial surfactants, such as soap to clean ourselves. That includes toothpaste, which is mostly detergent, explaining the foam that we generate while brushing our teeth. It may also be that the latherin acts as a tooth cleaning agent for the horse: a third use. But that possibility is is not mentioned in the paper. Maybe the authors did not want to look a gift horse in the mouth.
Update: this post has been selected as Blog Pick of the Month for June 2009 by EveryOne, PLoS-ONE’s community blog.
McDonald, R., Fleming, R., Beeley, J., Bovell, D., Lu, J., Zhao, X., Cooper, A., & Kennedy, M. (2009). Latherin: A Surfactant Protein of Horse Sweat and Saliva PLoS ONE, 4 (5) DOI: 10.1371/journal.pone.0005726