Horses survived mass extinction by adapting to forests

Wild horses survived the mass extinction event that knocked out most of Eurasia’s megafauna – including the mammoth – by moving into forests and turning black, new research shows.

A study led by Edson Sandoval-Castellanos of the Universidad Nacional Autónoma de México and published in the journal Nature Ecology and Evolution combined geographical modelling and genetic analysis to answer the question of how horses managed to survive the Pleistocene-Holocene mass extinction 11,700 years ago. 

The event saw the end of 70% of genera comprising animals weighing over 44 kilograms, but wild horses such as the tarpan (Equus ferus ferus) weathered the storm – and remained in Europe until 1909.

The stubborn survival of the wild horses is especially intriguing because they have a relatively restricted diet, yet fared better than many generalist feeders, such as the mammoth. Sandoval-Castellanos and his colleagues set out to test the two hypotheses most commonly advanced to explain the phenomenon.

The period is marked ecologically by the development and spread of forests, and the consequent decrease in grassland habitats. Horses are well adapted to open land, so the first hypothesis suggests that sufficient grassland persisted to allow a horse population to remain viable.

The second hypothesis suggests that the horses gradually adapted to the spread of woodland and forest environments and learned to live among the trees.

To test the theories Sandoval-Castellanos and his team drilled through available data to model the presence of plant species – some found in grassland environments and others found in forests – and align them with dated horse remains.

The researchers found that as the Pleistocene era gradually morphed into the Holocene, the co-occurrence of horses and forest plants increased.

The shift from open ground to tree cover was accompanied by a change to coat colour – a trait that earlier research has shown to be genetically determined. By analysing the genomes of 27 wild horses that lived at various times during the mass extinction period, the researchers found that in the oldest, grassland-dwelling examples the allele associated with coat colour produced a brown or bay pelt.

In more recent, forest-dwelling animals, however, the allele coded for a black coat.

The researchers acknowledge that the small sample size available to them affects the reliability of their findings, but suggest that black coats may have provided a selective advantage by helping to conceal the animals from predators in the dark and shadowy woods.

Many species of mammals, they add, tend to favour dark coats in closed environments and lighter ones in open lands.