Latherin, a soaplike protein in horse sweat and saliva, helps spread sweat over the coat, maximizing evaporation of water for heat loss, and causing the foam that we see when horses sweat profusely. Latherin is also found in saliva, which might explain the foam often seen around a bitted horse’s lips. New research sheds light on this unique feature of equine physiology.

“Horses stand out, not only in their ability to dissipate large amounts of heat, which they must do as large flight animals, but also in their ability to chew and process dry foods,” said Malcolm W. Kennedy, PhD, an evolutionary biologist at the University of Glasgow, and co-author of a recent paper on latherin.

Using recent advances in methods for studying proteins, the research team concluded that latherin is unique to equids–possibly evolving as a digestive aid, and then as a means for heat dissipation. When a horse overheats, the protein is released, promoting the rapid movement of sweat water from the skin, through the dense pelt, and to the surface for heat dissipation.

Kennedy and his team also think the detergent, slippery qualities of latherin in saliva helped early equids to cope with dry forage.

“Equids have evolved to digest drier food,” Kennedy explained. “For example, on the Serengeti plains, once the rains have passed, the ruminants (antelopes and wildebeest) move across first, eating the fresh grasses. Zebras follow and consume the dry, nutrient-poor grass left behind, which they need in large quantities.”

Finally, latherin could also be one of the culprits in human allergies to horses. The researchers tested the immune response of eight people who are allergic to horses. The patients’ antibodies bound to the latherin protein, as well as to the known horse allergen, Equ c 1, suggesting latherin itself could also be a potential allergy trigger.