Why do cows lie and horses don't when they are both prey animals?

Answer 1

This has to do with the different protective strategies used by horses versus cows. Horses are runners - their primary defense is to escape and evade predators. To do this, horses need to be on their feet when attacked, which is why evolution favored horses that could sleep standing up. I will note that horses do have to lay down a bit - generally about a half hour out of every day when adults. In contrast, cows are "huddle and hunker down" defenders - they will put their calves in the center of the herd and form a standing protective circle around the calves. Predators facing a circle of horned, strong, unhappy cows without the ability to get into an overhead position will generally turn away from the herd and try again later. Therefore, it wasn't as important for cattle to be able to run away quickly, and cattle sleep like most mammals do - flat on the ground.