Many of the differences we appreciate in the behavior of our domestic donkeys and horses are attributable to this solitary versus group social organization. | Photo: iStock

—Alex, via e-mail

A.That is great topic. In fact, one of my most well-used and favorite teaching slide sets is entitled Donkeys are Not Horses. I first assembled it to outline and explain the biological basis of the many differences in donkeys’ social and reproductive behavior. I and my colleagues learned early on that if you try to breed donkeys as you would horses, things often don’t progress very well. That led to studying free-roaming donkeys in Brazil. I still remember the embarrassing moment when I realized how silly it was of me not to have figured it out sooner. These species’ behavior is in fact very different for very good evolutionary reasons. Over the years my interest and the slide set have grown to include other differences than reproductive that can be equally confusing and frustrating.

Many of the behavior differences stem from differences in how the various equid species evolved. Donkeys and asses and Grevy’s zebras that evolved mostly in arid regions where forage and water were sparse naturally have a primarily territorial social organization and system of breeding. In contrast, horses (e.g., Przewalski’s horse and the common zebra) evolved in environments favoring a social herd organization. Horse herds are composed of multiple harem bands of one mature stallion, multiple mares, and their juvenile offspring, as well as bachelor bands of mostly or all males of various ages. Harem stallions herd and guard their families, staying with them at all times, but they also maintain relationships with other bands in the herd, such that they share defense of the herd as a whole in times of major