Belly Up (FunJungle Series #1) by Stuart Gibbs

Twelve-year-old Theodore ""Teddy"" Roosevelt Fitzroy has murder on his hands and trouble on his tail. He believes that Henry, the hippopotamus at the brand-new Fun Jungle, has been murdered. The zoo's top brass claim the hippo went belly up the natural way, but Teddy and his feisty friend Summer McCraken have other ideas. Could the culprit be FunJungle's animal-hating head of operations? Or is it FunJungle's owner-Summer's dad-a man who is much more concerned about money than animal welfare? The deeper Teddy and Summer dig, the more danger they're in, because when it comes to hippo homicide, the truth can't be caged!