A living laboratory on a sandbar
Sable Island, a crescent of sand just 42 kilometers long, drifts in the North Atlantic. Called the "Graveyard of the Atlantic" for the more than 350 ships it has wrecked, its most famous residents are the descendants of horses introduced in the mid-18th century. While romantic legends tell of horses swimming ashore from wrecks, historical records point to a more deliberate introduction. The most accepted theory is that the Boston merchant Thomas Hancock transported around 60 horses, seized from deported Acadians around 1760, to the island to be ranched. Of the various livestock introduced, only the horses survived.
For over 250 years, this isolated population of Equus ferus caballus has lived without human interference or predators. This has turned the island into a unique natural experiment. The horse population has fluctuated, with recent estimates ranging between 400 and 550 individuals. They are entirely unmanaged, receiving no supplemental food or veterinary care. In 1960, the Canadian government granted them full legal protection from human interference, and their status as "naturalized wildlife" was reinforced when Sable Island became a National Park Reserve in 2011.
Rapid adaptation to a harsh environment
Survival on a windswept sandbar has driven noticeable evolutionary changes in the Sable Island horses. They are small and stocky, typically standing 13 to 14 hands (132 to 142 cm) high, with shaggy coats and full manes that protect them from harsh Atlantic winters. Their short pasterns are an adaptation for easier movement across sand and rough ground.
Their diet consists primarily of marram grass, the dominant plant that also stabilizes the island's dunes. The constant grazing on sand-covered vegetation wears down their teeth, often leading to starvation in older animals. The horses get fresh water from ponds that form in depressions between dunes, and have even been observed digging for it during dry periods. Researchers study the population's unique genetics, which show low diversity and high inbreeding as expected from a small founding group. These horses allow researchers to observe natural selection and population dynamics in a closed system, free from predation and interspecies competition. The only other terrestrial mammal is the grey seal; Sable Island hosts the world's largest breeding colony.