Ever wondered why your horse lathers up when it sweats? It's all because of a special protein in the sweat appropriately named latherin.
The unusual properties of latherin have been the subject of research by a team led by Rhona McDonald, from the Department of Ecology and Evolutionary Biology at the University of Glasgow.
The researchers, whose findings have just been published in the open-access journal PLoS One, said horses were unusual in producing protein-rich sweat for controlling their body temperature.
Latherin is a major component of horse sweat and acts as a surfactant. It produces a significant reduction in water surface tension at low concentrations, and therefore probably acts as a wetting agent to help evaporative cooling through a waterproofed pelt.
"This detergent-like activity is associated with the formation of a dense protein layer ... at the air-water interface," the researchers wrote.
The protein - found in the horse, zebra, onager and ass - is similar to a family of proteins found previously only in the mouths and associated tissues of mammals.
Laterin was also detected in horses in the salivary gland.
"Equids therefore may have adapted an oral/salivary mucosal protein for two purposes peculiar to their lifestyle, namely their need for rapid and efficient heat dissipation and their specialisation for masticating and processing large quantities of dry food material."
The researchers point out that sweating is also a mechanism used by humans, but is otherwise rare in mammals.
"Horses, however, have a thick, waterproofed, hairy pelt that would normally impede the rapid translocation of sweat water from the skin to the surface of the hair necessary for evaporative cooling.
"To solve this, horses appear to have evolved [this] surface-active, detergent-like protein that they release at unusually high concentrations in their sweat (human sweat is instead high in salt but low in protein).
"This protein, latherin, presumably acts by wetting the hairs to facilitate water flow for evaporation, the side effect of which is the lathering that is often observed on the pelts of sweating horses, especially where rubbing occurs."
The researchers said interest in biological surfactants has been steadily increasing since the 1960s when they first attracted attention as hydrocarbon dispersal agents with low toxicity and high biodegradability.
Recent studies have shown further potential for biological surfactants for their antimicrobial activity or anti-adhesive agents against germs.
"Such a dual function would make sense for latherin given that the pelt of a horse could be readily colonised by microorganisms potentially harmful to both skin and the hair itself, particularly following saturation sweating that would provide ample resources for the proliferation of micro-organisms.
"Interestingly," they wrote, "human skin secretions are also thought to have a surfactant-like role in enhancing the spreading of sweat water for cooling, though by a mechanism suited to an effectively hairless surface."
The authors said depositing large quantities of protein through and over the pelt of horses would seem to present the risk of providing a nutrient resource for micro-organisms.
"It could therefore be that latherin's surface activity may also directly affect the surfaces of microbes or impede their adhesion and establishment on hair and skin.
Latherin is distantly related to proteins that are directly anti-microbial, but the researchers had been unable to detect any interaction between latherin and typical bacteria. The possibility that latherin's surface-activity presents a danger to mammalian cells needed to be addressed, they said.
"A simple small molecule detergent would normally be extremely damaging to cellular membranes, but it is possible that proteins can be designed to perform detergent-like functions without endangering cell membranes.
Latherin, they said, appeared to represent a remarkable adaptation of a salivary protein for heat dissipation from skin by horses and their ilk, and a readily available protein with which to study intrinsic surface activity of a naturally folded protein from a mammal.