Macaques Eat Leaves, Fruit, Seeds and More, and the Menu Changes with the Season
Many people picture a monkey holding a piece of fruit. A study on Yakushima, a warm, forested island in southern Japan, tested that picture. A researcher followed a group of macaques of the local subspecies for 18 months and recorded what each adult was doing when it fed. Over a full year, 35.0 percent of the feeding records of identified foods were leaves and shoots, 30.2 percent were fleshy fruit, 13.2 percent were seeds and 5.5 percent were flowers. The rest included animal matter, such as insects (about 10 percent), and fungi (about 4.6 percent) [1].
So fruit and seeds together were a little over 40 percent. That is a big share, but it is not "mostly fruit." Also, the main food changed through the year: seeds, leaves, fruit and animal matter each took a turn as the most common food at different times [1].
Two cautions. These percentages count feeding records, not the weight of food swallowed. And they come from one troop in one lowland forest, so another place, such as a snowy mountain, could look quite different. That is what the next two sections explore.
In Winter, Gut Microbes Seem to Get Better at Digesting Leaves and Bark
When fruit and seeds are unavailable, the monkeys must rely on low-quality foods such as leaves and bark [2]. Leaves are tough and take a lot of work to digest. A monkey cannot break down much plant fiber by itself, but tiny organisms living in its gut can help.
A Kyoto University team studied this on Yakushima for a year. They watched what the macaques ate, analyzed the microbes in fresh droppings, and tested in the lab how well those microbes could ferment foods the monkeys eat. They found that the microbes' ability to ferment food was higher in the harsh winter, when the monkeys relied on leaves. The increase was linked to eating mature leaves, not simply to eating more fiber in general [2].
In other words, a macaque changes more than just its food. Its gut community appears to shift along with the menu. The researchers suggest that leaf eating may favor microbes that can handle fiber and also certain natural plant chemicals, though that part is a suggestion rather than a settled result [2].
In a Snowy Valley, Winter Droppings Held DNA from Insects, Snails and Fish
Yakushima is mild. What about macaques that live where winter is truly cold? Kamikochi is a high mountain valley in Nagano, in Japan's Chubu Sangaku National Park, and the macaques there live in one of the coldest places any wild non-human primate lives. Researchers collected 38 droppings during three sampling periods (December 2017, February 2018 and spring 2019) and read the DNA left from the animals' food [3].
DNA from freshwater animals turned up often. Eighteen droppings contained aquatic insects or other small stream animals, nine contained freshwater snails, and seven contained DNA from brown trout [3]. Streams fed by groundwater stay at about 5 °C even in winter, so some spots stay open and full of life when everything else is under snow [3].
How would a monkey get a fish? The authors think the macaques most likely caught live trout in shallow pools along the stream edge. Scavenging dead fish is possible, but the team saw no dead fish during their winter sampling. They also say that catching live fish is probably rare for these monkeys [3].
How Far Can We Trust Droppings? What the DNA Does and Does Not Show
DNA in a dropping tells us that an animal was eaten. It does not tell us how much. Seven of 38 droppings with fish DNA do not mean fish were a big part of the winter diet, and the study did not measure how much of the diet the stream animals made up [3]. The researchers also did not measure the nutrition of the items themselves, though they note that invertebrates generally provide more energy, protein and fat per unit of mass than fruit or leaves [3].
Kamikochi is also just one place. Whether other snowy-mountain macaque groups use streams like this was not tested in this study, so it is safest to call it a striking example, not a rule for all Japanese macaques.
So What Is a Japanese Macaque's Diet? A Flexible One That Changes with Place and Season
Putting the three studies together gives one answer to the opening question. Macaques are not simply fruit eaters. On Yakushima, they eat a mix, and the main food rotates through the year [1]. When fruit is gone, they turn to leaves and bark, and their gut microbes seem to adjust to help [2]. In a snowy valley, they find food in streams too [3]. These studies used different methods in different places, so their numbers cannot be compared directly.
How to Look into Macaque Food Yourself
At a zoo, watch one animal from the visitor area for five minutes and tally the foods you can identify: fruit, leaves, seeds, other. That is the same idea as "feeding records." Notice how it differs from weighing food. For a deeper look, the Milner et al. paper in Scientific Reports is free to read, and its figures show which animals were found in which winter's droppings.