Ecologists normally treat a species as a collection of carbon-copy critters, each eating the same foods. But animals often have quirky personal likes and dislikes a subtle form of biodiversity that could have big implications, says Daniel Bolnick, a graduate student in population biology at the University of California at Davis. He and his colleagues researched more than 100 species of insect, fish, bird, amphibian, reptile, and mammal and found individual food-choice variation in every case. Choosiness takes many forms. A butterfly will almost always prefer the type of flower it landed on previously. Sunfish specialize in eating either snails or invertebrates for a mechanical reason. Once they start crunching through the hard shells of snails, they build up their jaw muscles so much that they can no longer easily handle softer foods. Ignoring preferences could harm efforts to protect endangered species. "We know Cocos finches eat insects. To make sure the species survives, we might make sure there are insects for it to eat. But if some finches drink nectar and others eat fruit, we'll only end up feeding some individuals," Bolnick says. He suspects dietary variation can lead individuals into different ecological niches, propelling the evolution of new species.