Equid Adaptations to Cold Environments

Over the past 20 million years from mid-Miocene, the horse family has successfully radiated across the Northern Hemisphere, displaying extraordinary adaptations (including morphological, physiological, behavioural, and genetic) to the cooling world. From early tridactyl horse, like Hipparion to modern Equus, we review the evolutionary history to reveal how external drivers like climatic and environmental changes contributed to their continually evolving cold adaptation. We also reason that sudden population declines happened after the last glacial period, resulting in a significant recession of equids from the world’s cold zones, could have resulted from mixed effects of factors such as temperature rise, and anthropogenic overkill. We group the modern equids by their geographical occurrences across temperature and precipitation gradients, which yields three unique niche clusters—“hot and arid”, “cold and arid”, and “cold and moist”. Then, we delve into the latter two clusters, and examine the phenotypical and genotypical mechanisms that allow their persistence in the cold parts of the world, and at the cold part of the year. Across the current geographical ranges, equids display traits in active habitat-use strategies, diets, and behaviours to facilitate adaptations to the cold. In addition, we briefly consider the genetics of cold adaptation in equids. Our findings highlight the need for more species-specific studies to be conducted on wild equids, especially on cold adaptation. We also present our thoughts on how the evolutionary history, physiology, and changing climate might impact current and future wild equid conservation efforts.