Chelsea Wald in Slate:
The six horses in a 2002 study were “known weavers.” When stabled alone, they swayed their heads, necks, forequarters, and sometimes their whole bodies from side to side. The behavior is thought to stem from the social frustration brought on by isolation. It can be seen in a small percentage of all stabled horses, and owners hate it—they think it causes fatigue, weight loss, and uneven muscle development, and it looks disturbing. People had tried stopping the weaving by installing metal bars that limit a horse’s movement, but the study found that a different modification to the stable worked surprisingly well: a mirror. “Those horses with the mirror were rarely [observed] weaving,” the researchers reported. A later study even found that the mirror worked just as well as the presence of another horse.
Studies have shown that mirrors can improve the lives of a variety of laboratory, zoo, farm, and companion animals. Isolated cows and sheep have lower stress reactions when mirrors are around. With mirrors, monkeys alone or in groups show a healthy increase in social behaviors such as threats, grimaces, lip-smacking, and teeth chattering, and laboratory rabbits housed alone are also more active. Mirrors in birdcages reduce some birds’ fear. Gordon Gallup invented the test that shows whether an animal recognizes itself in the mirror: He marked primates’ faces and ears with dye and watched whether they used a mirror to investigate the spots. If they did, it revealed that the animals understood that the faces in the mirror were their own. But he thinks that most animals probably think of their reflections as another animal. The calming effect in some cases could come partly from the reflection’s apparent mimicking. “The animal confronting its own reflection in a mirror has complete control over the behavior of the image, and therefore the image is always attentive and ready to reciprocate when the animal is,” he and Stuart Capper wrote in 1970. In other words, the mirror image is sort of like a friend who always does exactly what you want.