In establishing his reputation, Galvayne had the advantage of dealing with a large population of horses kept under similar environmental and management conditions. The teeth of horses kept stabled and fed the same kinds of feed are apt to wear in consistent patterns. Soon widely accepted in Europe, Galvayne's system went on to become the basis for aging techniques taught in veterinary colleges around the world. His ideas are still found in a standard veterinary reference, The Official Guide to Determining the Age of a Horse, published in 1966 by the American Association of Equine Practitioners. The book is illustrated with photographs of teeth that match Galvayne's descriptions, but because the examples came from slaughterhouse skulls, there is no way of corroborating that the dental configurations are from horses who actually fit the ages specified.
Quantifying the evidence
Though veterinarians and other professionals grumbled over the years that Galvayne's aging formulas didn't seem to apply 100 percent to all horses, anecdotal objections did little to shake the faith most horsemen had in the technique. Then, in 1994, J.P. Walmsley, a British veterinarian, examined the teeth of a number of registered Thoroughbred horses between five and seven years of age and discovered a greater deviation from Galvayne's dental patterns than he had expected. For example, some of the five-year-olds had the teeth of a four-year-old, while others had mouths that would pass for eight.
The following year, six British researchers did another dental study, in which a number of experienced equine practitioners were asked to age horses--all registered and thus of known age--on the basis of their teeth alone. Results showed that with immature horses (age five and under), dental growth patterns differed enough from the expected to cause the experts to err in their estimates by as much as two years and nine months. Mature horses between five and 10 years of age were even more likely to be misjudged: Eight- and 10-year-olds were estimated by some experienced clinicians to be between 13 and 15; one six-year-old was judged by one veterinarian to be 12; two eight-year-olds were pegged at 15 by several veterinarians. Some much older horses, on the other hand, passed for mere youths: A 17-year-old was judged to be nine by one practitioner, and a 15-year-old was aged as seven by another.
One reason for the uncertainty in aging came to light in 1996, when three Belgian researchers examined the teeth of 570 horses representing several different types of horses--Arabians, Belgian draft horses and trotters. All were registered and of known ages, ranging from two to 25 years. The researchers found that the rate of dental wear was markedly different for each of the breeds--slower in Arabs, faster in the draft horses, somewhere in between in the trotters. Tooth composition and enamel hardness, both heritable traits, made the horses "age" at different rates. The study concluded that accurate aging by dentition has to be based on breed-specific guidelines.
The perils of prediction
Jack Easley, DVM, an equine practitioner in Shelbyville, Kentucky, who writes and lectures widely on equine dentistry, agrees that aging horses using Galvayne's formula is, at best, an educated guess and hardly the exact science that we have been led to believe it is.
"Horsemen of all types, veterinarians included, have perpetuated these theories, generation after generation, just because it was the way it was always done and it was what they were taught in school," Easley says. "You can certainly estimate whether the horse is within a certain age range by looking at his teeth, but in general, aging horses by studying their incisors is difficult and inaccurate." He believes that veterinarians and horsemen should take the recent research as an opportunity to reassess established practices.
Breed differences, such as those outlined in the Belgian study, are but one reason for the inaccuracies, Easley says. Individual rates of tooth emergence and wear account for other aging errors. The sequence of dental maturation takes horses from toothlessness at birth to a complete set of 36 full-size permanent teeth by age five--give or take some months. At predictable intervals, starting at day six, 24 temporary "milk" teeth erupt from the gums and are later shed, to be replaced by larger, more durable permanent teeth. (Because only geldings and stallions usually grow canine teeth--also called "tushes"--and both sexes may display the vestigial premolars known as "wolf teeth," the count of permanent teeth ranges between 36 and 42.)
When a horseman pulls back the lips of an immature animal and examines the incisors, he's checking the number of temporary and permanent teeth in the lineup as an indication of the youngster's age. But just as with human children, immature horses produce and shed milk teeth and acquire permanent replacements following roughly, but not precisely, the same schedule. Unlike human teeth, which stop growing once fully emerged, equine permanent teeth continue to erupt throughout the horse's life: As chewing wears away the surface tooth, new tooth material emerges from beneath the gum to replace it.