Why do cows mount other cows?

Cows of European origin were often kept apart from bulls during the past two millenia with the result that herdsmen had to judge by female behavior when to bring them together for mating. Since cow-cow mounting is an accurate sign that the time is right for taking the cow being mounted (“mountee”) to be bred, and that estrus is imminent in most mounting cows (“mounters”), selection for this behavior was probably coincidental.