and I think it's interesting enough to merit some discussion:
its last companion was Boo, who was aggressive and euthanized in 2013
Zoos euthanize (or "transfer" to dubious facilities where animals may be hunted or skinned) most males of larger mammal species before or at maturity. They're big and dangerous and hard to handle, but they're also not big tourist draws in the way that babies are, and with reproductive technology you don't really need to keep a male around to keep the mature females pregnant or even to run a captive breeding program.
It's a situation that really cuts through the bullshit about zoos as research facilities or breeding programs. You can't really understand species when you keep unnatural populations in unnatural conditions.
edit: To illustrate how skewed zoo/circus populations are, here are the elephants who have been retired to Elephant Sanctuary in TN:
http://www.elephants.com/meetElephants.php Note that they didn't cause the problem, I'm just using their herd to illustrate the larger point.
2nd edit: And lest anybody think Elephant Sanctuary's herd is not representative, here's PAWS' elephant facility in CA:
http://www.pawsweb.org/meet_elephants.html You should be noticing a theme.