It’s hard to be a primate who lives

It’s hard to be a primate who lives in northern climes and doesn’t wear clothes. Resources are scarce, and you have to seize every advantage you can to stay alive and swinging. That may be why one group of monkeys has evolved an impaired tasting gene. Their worse sense of taste means they can better take advantage of the foods around them—especially the crops their human neighbors grow.

Japanese macaques, or Macaca fuscata, are also called snow monkeys. They live farther north than any other non-human primate. To stay warm, they huddle together or take dips in hot springs. Sometimes their home forests are washed away by tsunamis. All things considered, they do OK. But researchers in Japan recently discovered a genetic mutation that might give some of these macaques a boost.

The mutation is in one gene in a group called the TAS2Rs. In mammals such as you, the macaque, or a mouse, these genes code for receptors that pick up bitter flavors. Depending on where we find bitterness, we may choose to eat certain foods and reject others—because something that tastes bitter might also be toxic. One of these bitterness-sensing genes recognizes chemicals called glucosinolates (which are in cruciferous plants, like broccoli and horseradish) and limonin (in citrus). This is the gene in which some certain Japanese macaques have a mutation.

Along with the natural bitter chemicals in a kale salad, the same gene lets animals taste a synthetic chemical called PTC. Humans also have different versions of this gene that make us more or less sensitive to PTC. Remember doing an experiment in biology class where you put a little strip of paper in your mouth, then raised your hand if you could taste anything on it? That was PTC.

The researchers discovered that the genetic mutation in macaques also made them less sensitive to these bitter flavors. But they couldn’t find the mutation in the Japanese macaque’s close relative, the rhesus macaque. It wasn’t evenly spread out among Japanese macaques, either. The mutation only appeared in Macaca fuscata monkeys from one region of Japan, the Kii peninsula.

In a new study, Kyoto University researcher Nami Suzuki-Hashido and colleagues did an extensive survey of Japanese macaques to find out more about the mutation. Out of several hundred macaques in 17 different local populations, the bitterness mutation only appeared in Kii macaques. About 29% of the Kii monkeys have the mutation.

The researchers found that at the cellular level, this mutation creates receptors that don’t work at all. They confirmed the effect using experiments where captive monkeys drank from water bottles with varying amounts of PTC (the bitter chemical from biology class). By choosing between two bottles, the monkeys showed the researchers whether they noticed a bitter taste or not. And sure enough, monkeys with the genetic mutation were less discerning.

Finally, the scientists looked at the DNA surrounding the mutated gene. They saw the signature uniformity of DNA that has spread through a population quickly, without enough time to pick up errors here and there. This suggested that the mutation first appeared recently, sometime within the past 13,000 years, and that macaques with the mutation have an evolutionary advantage.

Why is it good for a monkey not to taste bitterness? The authors point to a wild fruit called the tachibana, which was the first citrus to grow in Japan. Macaques will peel these green fruits, if they’re big enough to bother with, and eat the insides. But when given a small tachibana that’s mostly the bitter peel, they toss it. A monkey that doesn’t taste bitter flavors as strongly might be able to eat the small fruits, too. The tachibana is thought to have landed in Japan about 2,800 years ago—via the Kii peninsula.

Over the past several hundred years, farming has spread citrus plants more widely across Japan. It’s also brought bitter cruciferous vegetables such as cabbage and radish. Japanese macaques often eat some parts of these plants, the authors write. Any monkey that doesn’t mind munching through bitter rinds and leaves probably has an advantage, since it can find these foods growing conveniently in fields even when other food is scarce. “In Japan, it is a serious problem that monkeys go to farms and steal the fruits and vegetables,” says senior author Hiroo Imai.

For the Kii macaques, then, a piece of botched DNA turned out to be a boon. The mutation seems to have swept through the population because it opens up a whole new world of foods to eat. The monkeys may have to steal these foods from farms, but it’s a crime that doesn’t taste bitter at all.