Why do horse and cow lack a philtrum while other animals have it?
The short answer is that no one knows. The philtrum is the small depression or seam between the nose and lip and humans and other animals. Some evidence suggests that having a deep philtrum helps keep the nose moist, which could be useful for animals that use their sense of smell a lot. According to this theory, horses and cows don’t need a philtrum either because they don’t use their sense of smell much, or because they have other ways of keeping the nose moist if necessary.
– Jessica Abbott
