QUESTION:
I run a small equine rescue and I recently took in two mares. They will not socialize with others horses since one of the mare’s is so dominant, and she acts like a Stallion, herding the other mare away from the other horses. If any horse comes near her and her stable mate, she attacks. Could this be a physical problem? They also cannot be separated since I have tried to separate them to ride one and they go insane. What can I do to help them get over this separation anxiety?

ANSWER: Any time there is a report of a mare acting like a stallion, we want to make sure that the mare does not have a granulosa-cell tumor.  These tumors can grow on the ovaries and actually secrete testosterone, which would account for the stallion-like behavior.  These tumors can be removed by a veterinary surgeon and removal usually results in a return to normal mare-like behavior.  This may also help to have her be less “herd-bound” to the other mare.  If her hormones are telling her that she is a stallion, than she may view the other mare as her mate, rather than just her herd-mate.