Biologists have long observed that when animals colonize and evolve on isolated islands, small animals tend to become larger while large animals tend to become smaller. Recent research by Monterey Bay Aquarium Research Institute, USA, postdoctoral fellow Craig McClain suggests that a similar trend affects animals as they adapt to life in the deep. Over the last few decades this trend has become known as the island rule. Scientists still debate the ruleâs applicability and possible causes. In keeping with the island rule, the current study suggests that both size trends may occur simultaneously, depending on the size of the animal being studied.