Marauding across the tropical forest floor, aggressive army ant colonies harbor hidden enemies within their ranks. The impostors look and smell like army ants, march with the ants, and even groom the ants. But far from being altruistic nest-mates, these creatures are parasitic beetles, engaged in a game of deception. Through dramatic changes in body shape, behavior, and pheromone chemistry, the beetles gain their hostile hosts' acceptance, duping the ants so they can feast on the colony brood. This phenomenon did not evolve just once. Instead, these beetles arose at least a dozen separate times from non-ant-like ancestors. This discovery, published online on March 9, 2017 in Current Biology, provides evidence that evolution has the capacity to repeat itself in an astonishingly predictable way. The article is titled “"Deep-Time Convergence in Rove Beetle Symbionts of Army Ants.” "These beetles represent a new and really stunning system of convergent evolution," says study co-author and evolutionary biologist Joseph Parker, Ph.D., of Columbia University and the American Museum of Natural History. "It's an elaborate symbiosis, which has evolved in a stereotyped way, multiple times from free-living ancestors." The ant-mimicking beetles all belong to the Staphylinidae, or rove beetles, but don't mistake them for close relatives: the last common ancestor of the beetles in the study lived 105 million years ago, at about the time that humans split from mice. "What's exceptional is that this convergent system is evolutionarily ancient," says Dr. Parker. Although most other convergent systems, such as Darwin's finches, three-spined stickleback, and African lake cichlid fish, are a few million years old at most, this newly discovered example extends back into the Early Cretaceous.
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