Japanese snow monkeys have learned to fish for food

The Japanese macaque (Macaca fuscata) is found on the main islands of Japan, except Hokkaido, and is the most northerly occurring non-human primate in the world. In addition, in parts of its range it inhabits sub-alpine regions of the Japanese Alps (elevation 1500–1600 m), which become extremely cold and harsh during winters. Since the monkeys have little access to their preferred diet of fruits, seeds, flowers and leaves when snow covers the ground, scientists have wondered what they eat in order to survive.

Previous research involving the DNA analysis of fecal samples from macaques living in Chubu Sangaku National Park during winter revealed evidence not only of the bark and leaves of bamboo (that stand out above the snow), but also of freshwater fish, mollusks and aquatic arthropods. It is very rare for monkeys to eat fish, unless the fish are dead or dying, trapped in receding pools. A research team led by scientists from Shinshu University therefore decided to investigate how and where the macaques are catching fish to supplement their winter diet. 

Between mid- and late January, 2022, the researchers entered the Kamikochi area of the Chubu Sangaku National Park in the Japanese Alps on a daily basis. They located troops of monkeys by tracking their footprints in the snow, and then followed them and observed their behavior along the Azusa River. The team used several cameras, including a shoulder-mount camcorder, to record macaque behavior. In this way, they followed three different troops of macaques, one for five days, one for four days and one for three days. 

In addition, 12 infrared sensor cameras were set up as trail camera traps to record macaque behavior, for two months from the end of January 2022. The cameras were positioned in three areas – a small wetland and two small, spring-fed streams. The cameras recorded 1,122 instances of macaque behavior, with each recording lasting one minute. The researchers analyzed all photographic footage and divided the different behaviors into categories. 

The results, published in Scientific Reports, identified 14 cases of macaques hunting for actively swimming fish, and then consuming them. Their behavior along the river also included grabbing drifting invertebrates from the surface of the water, turning over pebbles and capturing food from beneath, and feeding on riverine plants. When fishing, a monkey commonly chased a fish into shallow water, where it attempted to hide between the rocks. The monkey then held the fish down in the stream with both hands and grabbed it with the mouth. 

The river and associated small streams are supplied with warmer water from many groundwater springs and active volcanoes in the area, such that their waters keep a stable temperature of around 5–6 °C, despite the snow and cold conditions. The streams thus flow throughout the winter and are accessible to the monkeys. 

The catching and feeding behavior recorded in this study appears to be an extension of existing, documented behaviors of feeding on vegetation that grows above the surface of the snow, as well as on aquatic plant matter. These types of vegetation also often carry insects, a food source that is readily utilized by the macaques. The streams containing aquatic plants are also home to many cold-water salmonids, so it is a small step for the macaques to transition from locating insects in streams to locating fish. 

“Video analysis has also shown that, during these behaviors, when the fish are nearby, [the monkeys’] attention is diverted to the fish. We interpreted this sequence of behavior as a pre-adaptation to fish-eating, and that fish-eating behavior may have evolved through these several stages,” said Masaki Takenaka and Koji Tojo, both from the Department of Biology at Shinshu University.

These “pre-adaptations” are the proposed mechanism for the evolution of the novel fish-hunting and eating behaviors observed, hypothesize the authors. They state that the macaques do not choose to eat fish as a preferred food source, but pressures brought about by the harsh winter conditions and the shortage of other suitable foods make this unique fishing behavior a matter of survival. 

The researchers now wish to investigate how the fishing behavior is spread between members of the macaque group. Not all of them hunt for fish, or accept fish as a food, as evidenced by the fact that only 20 percent of fecal samples over three winters contained DNA from brown trout. 

“Having obtained evidence that Japanese macaques in the Kamikochi area catch and eat live fish, the next step for us in this research was to investigate how these fish-eating behaviors spread within the macaque group,” said Tojo. “Is it genetic? Is it a kind of culture that can be transmitted within the group?” 

Now, after identifying 200 individuals across four distinct groups, Tojo and the team seek to identify relationships based on these individuals and their fish-seeking (or fish-avoidance) behavior, to see how it may be passed on to other individuals in the group and to future generations.

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By Alison Bosman, Earth.com Staff Writer

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