Talking animals may populate Disney movies and corny 1960s sitcoms, but in the real world animals can’t simply tell us when they’re hurting. But a growing body of science is devoted to the assessment and measurement of pain in animals.

Just as you might grimace when the doctor presses something and asks, “Does this hurt?” the facial expressions of mice, rats and rabbits change when they are in pain. A “grimace scale” used with mice in research laboratories measures changes in facial expressions such as narrowing of the eyes and bulging of the cheeks.

Lessons from the first study of facial expressions of pain in animals quickly moved from the lab to the veterinary clinic when results were published two years ago. And the premise of the mouse grimace scale was confirmed earlier this year with the publication of a similar study of rabbits.

The rabbit scale also measures changes in whisker movement, ear position, cheek bulging, nose bulging and narrowed eyes.

"The only way you can alleviate pain is to be able to identify it, and to understand how much pain an animal is in," study author Matthew Leach of Newcastle University in the United Kingdom tells the science journal Nature.

Animals that evolved as prey animals, even animals as large as horses, will do their best to mask pain and compensate for injury, says veterinarian Shari Silverman. Predators, after all, focus on the weak and the lame.

There are several behavioral clues to look for with horses, she says. A horse suffering severe pain from a bout of colic, for example, may appear anxious and unusually sweaty. They may paw at the ground, kick or bite at their abdomen or groan or grind their teeth.

Less severe pain may show itself through loss of appetite, reluctance to move and general grumpiness.

An animal in pain suffers physiological, immunological and behavioral changes that can “confound experimental results,” according to the National Research Council Committee on Recognition and Alleviation of Pain in Laboratory Animals.

The ability to assess pain in research animals isn’t just about the treatment of animals; it contributes to better science.

How animals express pain
Knowing that an animal is in pain pushes humans to alleviate that discomfort, and it also helps advance the science of understanding it.