17. Animal Senses
Horses
Horses evolved about 50 million years ago. They have been domesticated for the past 6,000 years. That means for around 4,994,000 years, horses’ senses were developed solely to best serve them as grazing animals pursued by predators. They have the same five senses humans have, yet they have evolved quite a different Umwelt, sensory world, with those same five senses.
Horses have the largest eyes of any land mammal. They are lateral-eyed, one placed on both sides of the head. This gives a 350-degree range of vision, but the 10% it cannot see is consequential. Horses cannot see directly in front of them, so they need a slight head shift to see there. The second blind spot is directly behind them, making them skittish when something is coming up from behind (such as a person). Their day and night vision are both well-developed. They have dichromic color vision, giving them a world of greens, blues, and yellows. They can detect even the slightest movement, preparing them for flight, fight, or forage. Their eyes adjust slowly to strong changes in light intensity (such as stable lights being turned on in the dead of night), and they generally prefer dim light.
Hearing is a finely tuned sense for horses. To receive the sound, they have ears with 16 muscles that they can twist and turn, giving the horse near 360-degree sound reception without moving its head. They can hear frequencies 1300Hz above our hearing but cannot hear some low frequencies we can sense. Astoundingly, they can hear your heartbeat from four feet away and distant sound as far as 2.8 miles away. With this acute sensitivity, they don’t like loud, intrusive sounds like rock music being played in the stables. They prefer quiet music or nothing. In one “experiment,” talk radio was played continuously in the stable, and the rate of gastric ulcers in the horses (and maybe people) increased markedly.
Horses have an exquisite sense of touch. A tiny fly can light anywhere on its body, and the horse is aware. Touch is essential in their social relationships, with the grooming of other horses and muzzle touching (handshakes) being significant factors in the harmony of the herd. A bite on the butt can also send an unpleasant tactile message. Their most subtle sense of touch is their whiskers growing around their face. Each whisker is supplied with blood and nerves to take a tactile reading of anything it touches. The whiskers can be valuable in sensory experience in the dark and the blind spot in front of them. In some equine circles, trimming horses’ whiskers for cosmetic reasons has become the fashion. This practice can lead to confusion, stress, and an increased chance of injury to the horse. The practice has been banned in Germany, Belgium, and in many international equine associations.
A horse has a long nose, and that space is put to good purpose. When a horse breathes in copious amounts of air being sent to its large lungs, all along the nasal passage, the air is thoroughly scanned for odors. The receptors send the information to a large, well-developed olfactory bulb in the brain for analysis and storage in memory. Under the nasal cavity are more olfactory organs called Jackobson’s organs. They primarily receive pheromone information. When a horse curls up its upper lip and lifts its head, the Flehems response, the horse sends an unusual scent on a quick trip to Jackobson’s organs for analysis. This information is highly valuable in social relations and may explain why horses can readily read human emotions.
Most horses can be picky eaters, carefully choosing which plants to eat while grazing. They have the ability not only to taste but also to determine the nutrient value of the food. This also gives them the ability to avoid most poisonous plants.
The next time you walk up to a horse (and you certainly should — they are magnificent beings), think about the sensory array the creature has aimed at you and how much s/he already knows about you.
Mark W McGinnis 