The science behind creating mini-horses
When 6-pound, 14-inch-tall mini-stallion Einstein was born in late April, the world watched with piqued curiosity. News organizations and bloggers galloped to cover the new Guinness title winner for smallest horse, which has everyone clamoring on YouTube to see him. So, he’s small, real small, but why?
“We have no idea why Einstein is so small,” says Judy Smith, one of Einstein’s breeders and owner of Tiz A Miniature Horse Farm, where Einstein was born.
Miniature horses have been bred for centuries, and in the last 40 years, the United States has created two American Miniature Horse registries. Although miniature horses can be perfectly healthy, some are born with health problems or less desirable traits, such as oversize heads or stumpy legs.
During the last few years, interest in mammals’ height and size variations has gained interest from geneticists.
Equine geneticists Samantha Brooks and Nathan Sutter from Cornell University are hunting for the genes that determine height and size.
“As researchers, we aren’t interested in controlling the body size of the horse,” Brooks said. “[We’re] more [interested] in understanding how the genes already in play exert their control over the development of the skeleton and the horse’s final size.”
In 2007, Sutter identified the most important factor controlling size in dogs, a gene called insulinlike growth factor 1. Sutter says it’s unlikely this growth factor gene is the only one that determines the mininess of a Chihuahua or the hulking stature of a Great Dane.
“Body size is a classic complex genetic trait, meaning that multiple genes plus environmental factors control the ultimate expression of the trait,” said Sutter, who is mapping additional genes that control dog size.
In the same vein, Brooks and Sutter will complete genomic studies in 1,300 horses from more than 50 breeds to find size genes in horses. The horses vary from very small (about 2.5 feet tall) to very large (6.5 feet tall at the shoulder). Though the researchers have only just begun to collect genetic data, they aim to identify particular gene sequences that contribute to the difference in size among horse breeds.
“Identifying even one or two key genes that control body size will be a significant discovery,” Brooks said.
Though manipulation is outside the scope of their study, Brooks and Sutter agree that breeders may become interested in using the knowledge gleaned from this study to manipulate horse size.
“There is intense interest on the part of horse breeders to create proportionally small equines like the American Miniature,” Sutter says.
The miniature horses and small ponies are primarily used as companion and show animals, much like dogs. Small horses also excel in physical therapy programs and as guide animals for the blind.
Selecting for desirable traits in animals is not new — the practice has gone on for centuries, even when scientists had no knowledge of genetics, says Thomas Famula, professor of statistics in animal science from University of California, Davis, who was not part of the study.
For years, Holstein dairy cows have been bred to produce milk, while Hereford cattle have been selected to make tender, juicy steaks. However, Famula points out, sometimes when we push too hard for a certain trait, we can run into trouble — we have bred chickens for meat so intensively, they now carry too much breast tissue to stand up straight.
The same situation is possible with miniature horses. The genes that determine skeleton growth in horses may also control other features. In the past, breeders trying to produce miniature horses have created deformed or sick animals, such as dwarfs with malfunctioning jaws and legs.
“In any situation where you are selectively breeding an organism for a single trait, you run the risk of uncovering unknown genetic defects,” Brooks said.
The biological limits of body size in the horse may be reached, placing its health at risk, but Brooks added that “conscientious breeders make efforts to maintain the health of their breeding stock.”
