Animals congregate in huge colonies partly out of necessity and partly for the security that numbers provide. Icelandic puffins form nesting colonies of more than a million, tucked in between hundreds of thousands of other seabirds, which provides shared information about food sources and reduces the odds of individual birds being attacked. But, colonies are also useful for predators. 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 perfect multi-generational caregiving options or the opportunity to build enormous cities – such as the acre-wide, multi-million-citizen colonies built by leaf cutter ants in Costa Rica.