yes it is depression I have had pigs for years and each time a cage mate or even a playmate died they do go into decline, having just lost ginger a few weeks ago his cage mate harvey was moping but have succesfully paired him up with another pig dobby, so far so good he has picked up but is still not the old harvey, going back 5 years I had 2 brothers sylvester and tweety, together since birth, aged 4 sylvester died and tweety went 4 days later he just gave up on life, so I definatly believe they suffer just as we do , I also lost a dog charlie in july his brother max has only just started eating properly and being a little more playful. he is 14 on 9th so am going to be feeling down myself that day as obvioulsy it would have been Charlies birthday as well, but all animals feel a loss and act diffrently I believe, they need time as Max proved to me