For some animals, living in the midst of huge colonies of their own kind is the most secure and rewarding housing arrangement. Icelandic puffins form nesting colonies of more than a million, providing shared information about food sources and reducing the odds of attacks on individual birds. But colonies are useful for predators, too. Social spiders in Ecuador work together to capture prey 20 times the size an individual might subdue on its own. For others, communal living provides multi-generational care-giving options or the opportunity to build enormous cities like the acre-wide multi-million-citizen colonies built by leaf cutter ants in Costa Rica, or Australia's Great Barrier Reef, built entirely by tiny corals.