Humans aren’t the only animals who exercise. Baby Lake Malawi cichlids—a group of 10-centimeter-long striped fish native to East Africa—open and close their mouths up to 260 times per minute to develop a short jaw and a long retroarticular process, a critical bone for jaw opening, researchers report today in the Proceedings of the Royal Society B. Both of those features are an advantage for scraping algae from rocks. Some species of young cichlids “exercise” less, only gaping about 180 times per minute. They develop a long jaw and a short retroarticular process, which are also advantageous for feeding by sucking prey into their mouth. When researchers manipulated the baby fish gaping behavior, they produced changes in bone shape that were similar to those driven by genes, suggesting that the fishes’ environment can influence development as much as their DNA does.