Lemurs depend on leaves because their local fruit lacks protein
No matter how delightful a good salad may be, most of us can’t live on leafy greens alone. Primates, including humans, definitely consume plants in our diets, but we eat other items like nuts, fruit or meat to round our our nutritional needs. Lemurs break this pattern though, and skip everything but leaves when they go looking for food. As it turns out, the lemurs might just think that the local produce on in Madagascar just isn’t worth eating in the first place.
To be clear, fruit does grow in Madagascar. Studies have named at least 125 native species, and humans have introduced everything from oranges to avocados. With seemingly a number of choices in their habitat, scientists wondered why more lemurs didn’t follow the model of other primates and make these fruits a bigger part of their diets. Scientists wondered if the fruit supply was historically unstable due to events like cyclones, forcing lemurs to shape their diets around more reliable leaves. After all, even the local fruit bats, Eidolon dupreanum, have to switch to eating nectar for portions of the year when their favorite foods are unavailable.
Not enough nitrogen
When scientists started looking closer at what foods were available to lemurs, they started finding that the quality of the fruit was the real issue. While fruit could be found in the lemurs’ habitat, it didn’t seem to provide much protein per bite. To quickly compare the nutritional content of the fruit available to lemurs and other primates around the world, researchers started measuring nitrogen concentrations, since nitrogen is a key ingredient in many proteins. Fruit eaten by various monkeys from around the world was sampled, and all of it had more valuable nitrogen per bite than the fruit available to lemurs in Madagascar.
To make up for this gap in their diets, it seems that lemurs had to learn to skip the fruit and eat more leaves. Leaves don’t always provide a lot of calories per bite though, and so this required further energy-conserving adaptations, like eating leaves around the clock, or hibernating to use less energy in the first place. Now that the importance of leafy-greens is better understood, conservation efforts can be better designed to ensure that critically endangered lemurs have access to the slightly unintuitive foods they’ve come to depend on.
Source: Lemurs are weird because Madagascar's fruit is weird, Phys.org