The Roar by Clayton

It's the future again and things have been really messed up by the animal plague. As a result, everything outside the wall has been razed to get rid of the animals. The earth's population has been piled on top of each other, most of them living in relative squalor in semi-flooded moldy apartments. The first generation born after the plague have a variety of mutations ranging from webbed feet to telepathy. When the government starts a fitness and video game program all teens are required to do, Mika doubts their intentions. However, he believes that by playing the game, he'll be able to find his twin sister, who was supposedly drowned a year ago. Nothing is what it seems.