A musth elephant, wild or domesticated, is extremely dangerous to both humans and other elephants. In zoos, bull elephants in musth have killed numerous keepers when normally friendly animals have become uncontrollably enraged. In contrast to normal dominance behavior, bulls in musth will even attack and kill members of their own family, including their own calves. Zoos keeping adult male elephants need extremely strong, purpose-built enclosures to isolate males during their musth, which greatly complicates the expense of attempting to breed elephants in zoos; most zoos that keep a single elephant or a small herd typically have only females for this reason.