Over the past 70 million years, plants have evolved all kinds of clever strategies to spread their seeds far and wide to ensure the survival of their species – but the invention of fruit might just be their most brilliant move. We've all seen birds, fruit bats and monkeys snacking on fruit. But did you know that some fish do it too? Meet the pacu, a freshwater fish with human-like teeth that lives in the Amazon. During the rainy season, when the rainforest floods and trees drop fruit into the water, pacus swim inland and snack on the fallen bounty. But eating fruit underwater isn't as easy as it sounds. Without hands to hold fruit steady in the flow, fruit-eating fish often end up pushing them away accidently with every bite they take. To successfully grab and crush fruit in this turbulent terrain, pacus need a unique set of teeth: sharp enough to pierce slippery fruits quickly as they grab hold, yet blunt enough to crush tough skins and break down fibrous pulp. The possibility that fruit-eating fish might have teeth that were both sharp and blunt piqued the curiosity of Jack Rosen, a PhD student at the University of Louisville, USA, and his colleagues. They wondered how the teeth of specialized fruit-eating fish compare with those of fish that eat pretty much anything and full-time meat eaters like the razor-toothed piranha.
To find out, the team produced complex 3D X-ray images of six different fish – one that predominantly eats fruit, some with mixed diets and a classic carnivore. Then they 3D-printed the different jaws and mounted them on a mechanical rig to simulate real bites. By having the printed jaws bite down on different foods, such as cherries, kiwis and even tough macadamia nuts, the researchers could measure two key things: how much force it took for the teeth to initially pierce and grip the food, and the amount of energy used to crush it.
What they found was fascinating. The fruit-loving pacu could bite into food with much less force than the fish that consume a wide range of foods – surprisingly, pacu bite forces were more similar to the piranha's famously sharp bite. The fruit-eating fish had narrow, sharp points in the center of each tooth that helped to pierce and grab the fruit quickly before it floated away. But when it came to crushing the food, pacus outperformed the piranha by a wide margin. In fact, both the fruit-eating fish and those with a diverse diet generated more total crushing energy than the carnivorous fish. Adjacent to those central dagger-like points in pacu teeth were broad lateral surfaces strong enough to break down tough, nutty textures and leathery fruit casings. This tooth shape echoes what we see in fruit-eating mammals such as monkeys and bats, demonstrating remarkably similar tooth shapes across fruit-eating vertebrates.
So, it turns out the pacu has a pretty amazing set of chompers: they have teeth that are grippy enough to snag fruit floating in the current and strong enough to smash through tough and fibrous skins and pulp. But those teeth aren't just for eating: they play a role in shaping the rainforest. As they munch on fruit and move through flooded forests, pacus end up spreading seeds far and wide, helping the rainforest thrive. Over time, plants and pacus have shaped each other: fruits became tastier and easier to find, while the pacu evolved just the right set of teeth to enjoy them: not too pointy, not too dull but just right. A bite that would make Goldilocks proud.
