Lemurs are weird animals. Found only in Madagascar, they're primates (like apes, gorillas, and us), but unlike all other primates, they do things like hibernate and sleep in caves—that is, when they sleep, because they don't follow the normal nocturnal/diurnal pattern. Oh, and unlike many primates, most of them aren't frugivores. Meaning, they don't eat fruit. Their diet is much heavier on leaves than other primates.
A new study in the journal Scientific Reports suggests a reason why: The fruit available on the island doesn't have enough protein in it to meet their dietary needs, so they evolved a diet that didn't include it. The researchers, an international team led by Giuseppe Donati of Oxford Brookes University, combed through 79 different studies to analyze the nitrogen content (a necessary component of protein) of fruits cross the world and compare the rates of primate communities who eat fruit in different regions.
The higher the protein content of the fruits found in an area, they discovered, the more the animals relied on them as a food source. The fruits of Madagascar are also lower in nitrogen than fruits elsewhere, and in turn, the number of lemur communities in Madagascar that eat fruit is significantly lower than the number of primates in the Western Hemisphere, Asia, or elsewhere in Africa that eat fruit. (Only two genera of lemur subsist mainly on fruit, while elsewhere in the world, even primates that eat leaves still enjoy a good fruit salad now and then.)
"Lemurs are equal parts ridiculously cool and totally bizarre in that they represent the extremes and the extremely strange in the primate world," the Field Museum's Abigail Derby Lewis, a senior conservation ecologist, said in a press release. And studying their dietary patterns suggests why they've evolved to be so strange in comparison to their other primate relatives. Unable to get protein from fruit, they had to eat more leaves. To eat more leaves, their sleep schedules had to accommodate round-the-clock eating, which would explain their odd sleep patterns. And to conserve energy, they go into hibernation.
Lemurs aren't the only primates that go for leaves over fruit. So do howler monkeys. A March 2017 study found that primates that do eat fruit tend to have bigger brains. Nutritious fruit might not be the sole factor determining how primate species evolve, but it's clear that having access to it matters significantly.