People aren't the only ones who can get depressed. Pets can, too - especially if they've lost a human or animal companion or experienced a change in environment. So how do you know if your four-legged friend is feeling down? Put the following nine signs on your radar and talk to your vet if you notice any of them. Much like with humans, there are numerous options for treating depression in pets, and the sooner you recognize the signs, the sooner your pet can start getting treatment.