Proboscis monkeys are unlike any other primates. The males' noses are almost four inches long - because females find it sexy, naturally. Plus, they're the only primates to throw up their half-digested food and eat their meals all over again.
Grazing animals like cows and sheep practice something known as rumination, which means they eat their food once, partially digest it in their stomachs until it's nice and soft, then barf back up the larger bits of food and eat them again. The extra chewing helps break down the more stubborn cud and allows them to gain access to even more nutrients. It's a perfectly valid - if disgusting - dietary strategy, but until now it was thought that no primates practiced rumination.
But now we've observed the practice in proboscis monkeys, an unusual species found on the island of Borneo. They get their names because of the males' over-sized noses, which play a role in attracting females that I honestly prefer not to contemplate. (In all seriousness, it's thought a larger nose allows the monkey to make louder, deeper sounds that make it sound bigger than it really is, which can be useful in asserting dominance during mating.)
Researchers observed 200 of these monkeys, and 23 of them regurgitated their food for further eating. The monkeys had a pretty nifty technique - they would suck in their stomachs and stick out their tongues before barfing, which kept all the cuds in their mouth. They're the first primates we've observed that actually ruminate as a dietary strategy and not just as a pathological behavior - gorillas and humans have both been known to do this, but it's not considered "normal" behavior for them.
Biology Letters via LiveScience.