Crab-eating macaque

The crab-eating macaque, known scientifically as *Macaca fascicularis*, is a highly adaptive primate native to Southeast Asia.

They are also frequently called long-tailed macaques or cynomolgus macaques. These primates are classified as synanthropic, meaning they thrive in environments close to human settlements. This ability to live near people allows them to inhabit diverse areas like primary rainforests, secondary forests, and coastal mangroves. Because they are so versatile, they play many different roles in human society. They are seen as sacred animals in some cultures, while others view them as pests or vermin.

Socially, these macaques live in organized matrilineal groups. These groups can range in size from 10 to 85 individuals. The species exhibits female philopatry, which means females stay within their birth groups for their entire lives. In contrast, males undergo emigration at puberty, leaving their natal groups to find new ones. Their diet is both omnivorous and frugivorous, meaning they eat both animals and fruit. They are famous for foraging for crabs on beaches.

Remarkably, they are the only old-world monkeys known to use stone tools for daily foraging. In certain tourist areas, they even display complex behaviors like robbing and bartering with humans.

The physical appearance of the crab-eating macaque is quite distinct. They are sexually dimorphic, meaning males and females look different. Adult males typically weigh between 7 and 12 kilograms, while females are smaller, weighing between 3 and 6 kilograms. Their height ranges from 40 to 65 centimeters for males and 30 to 50 centimeters for females. Their most notable feature is a tail that is as long as their head and body combined. Their fur, or pelage, is usually greyish or brownish with a white underbelly.

Most adults have beards around their faces, though older females often have fuller beards than males.

Taxonomically, the species has a complex history of subspecies. While there were once ten recognized subspecies, the status of the Philippine long-tailed macaque is currently under dispute. The most widespread is *M. f. fascicularis*, found in Indonesia, Malaysia, the Philippines, and several other nations. Other subspecies are isolated on various islands. For example, *M. f. aurea* is found in Myanmar, Laos, and Thailand. Some subspecies, like *M. f. umbrosa*, live on deep-water fringing islands, while others, like *M. f. karimondjiwae*, inhabit shallow-water islands.

The evolutionary history of the macaque genus began in northeastern Africa about 7 million years ago. The *fascicularis* group likely shared a common ancestor with other species about 2.5 million years ago. Scientists believe the group originated from ancient hybridization between the *sinica* and *silenus* lineages roughly 3.5 million years ago. This happened during periods of changing sea levels in the early Pleistocene.

Fossil evidence shows they inhabited the Sunda Shelf at least 1 million years ago. They are unique because they are one of the few terrestrial mammals to cross the Wallace line, reaching the Lesser Sunda Islands.

Human interaction has significantly impacted the species. In 2000, the crab-eating macaque was included on a list of the 100 most invasive species. On the island of Mauritius, they are considered an invasive alien species. Some argue they compete with native species like the Mauritian flying fox or spread invasive plant seeds. However, many researchers argue that human-driven habitat destruction is the primary cause of bird extinctions on the island.

The perception of them as "weeds" or pests is often used to justify their use in biomedical research. Currently, they are the most traded primate species and the most popular species used in scientific studies.

Due to these various pressures, the crab-eating macaque faces serious survival challenges. In 2022, the IUCN Red List officially classified them as Endangered. Their survival is tied to the balance between their natural habitats and the expanding human world. Whether they are being studied in a lab or foraging in a mangrove forest, they remain a vital part of the Southeast Asian ecosystem. Their ability to adapt to human presence makes them a central figure in discussions about biodiversity and conservation.