Sometimes a crabby neighbor is a good neighbor. Scientists in Australia have observed that a male fiddler crab will come to the aid of a neighbor under attack--but only if there's something in it for him. While common in human warfare, this kind of coalition building had never been clearly observed in the animal world. "Now, we've found it in an invertebrate with a very simple brain," says Pat Blackwell, a biologist at Australian National University in Canberra. On a beach, fiddler crabs live in burrows surrounded by about eight inches of territory. Males must fend off "floaters"--homeless males that try to move in. Whenever a neighboring crab comes to the rescue, says Blackwell, the invader is bigger than the crab it is trying to evict--and the rescuer is bigger than the invader. According to Blackwell, the helper crab has a stake in the outcome. "If he loses his little neighbor, he's got to fight with this new guy and create new borders," she says. "He might as well fight with him now."