Millions of years ago, a small, unremarkable fish called the Mexican tetra started swimming into the caves of eastern Mexico. In the all-encompassing darkness of these limestone caverns, the tetras’ eyes, which take a lot of energy to build and maintain, were useless luxuries. Over several generations, the cave fish lost them entirely. Today, they are born with small eyes that gradually waste away as they get older.
The tetras’ eye sockets, however, don’t go to waste; they can use them to store fat. Blind cave fish are stockier than their cousins that live on the surface, and some have fat-filled humps. “You dissect them and you see that their body cavity is full of visceral fat that surrounds their organs,” says Misty Riddle, from Harvard Medical School. Some of them eat more than their sighted relatives, but even when the two groups are fed the same amounts of food, the blind ones put on more weight. These are sensible adaptations for living in caves, where food is scarce and starvation is always just around the corner.
Now, Riddle and her colleague Ariel Aspiras have discovered another bizarre metabolic trick that may help the cave fish cope with a world of little food. They’ve relinquished the tight control that most animals exert over the levels of glucose (sugar) in their blood. Instead, they allow those levels to fluctuate wildly, in a way that’s reminiscent of type 2 diabetes. And yet, these fish have none of the health problems that diabetic humans experience.