For a fish, one of the surest ways to avoid being eaten is to be bigger than its predator’s mouth. But size can be costly, and many fish make the most of their mass by growing into awkward, spiky shapes that are hard to swallow. Now, researchers have found evidence that the positions of a fish's spine help guide where its body expands over evolutionary time. Many fish have defensive spines extending from fins on the tops and bottoms of their bodies. Past experiments have shown that in the presence of predators, such fish evolve a greater distance between their backs and their bellies, becoming “deeper” or “taller” in shape. But other fish have spines in different places, and some have no spines at all. In a new study published today in Proceedings of the Royal Society B, scientists reasoned that for a fish trying to outgrow its predator’s mouth, the best dimension to expand is the dimension in which it has spines. To test their hypothesis, the researchers measured museum specimens representing 347 families of fish. Those with no spines tended to have average shapes, while those with spines on their backs or bellies had deep bodies. The few who had spines on the sides of their bodies tended to have wide shapes. The findings suggest that defensive spines and body shape evolve together to turn fish into the most awkward mouthful possible.