For years, biologists have suspected that the answer has to do with disease. Many diseases are species-specific, infecting only animals of the same species. Perhaps the risks of catching a species-specific disease by eating your same-species neighbor outweigh the benefits of such a nutritionally complete meal. A study of so-called “tiger salamanders” explored this theory. Tiger salamanders are interesting because if they grow up in over-crowded conditions, they can change into so-called “cannibal morphs”–big salamanders with extra-wide mouths, perfect for munching other salamanders. The study found that cannibal salamanders were likely to die earlier than their non-cannibal peers, precisely because of the risks of catching a disease from their dinner. What’s more, cannibal morphs showed a marked preference for dining on salamanders of a different species, rather than dining on their own. So what’s the moral? Don’t eat your neighbor. It’s hazardous to your health.