Although the species is able to adapt to agricultural landscapes and urban areas, it is sensitive to severe habitat disturbance and clear-cutting. Where the species persists, so too does hunting and trapping for the illegal pet trade, for biomedical research and export, and for human consumption. Southern pig-tailed macaques are often persecuted as crop pests and other frequent human-macaque conflicts, leading to continued negative public perception of the species. Together with the observed extremely high annual infant mortality rate in human-impacted areas, possibly due to pollutants, and the inferred likely decrease in genetic diversity of populations in highly fragmented landscapes, this paints a concerning outlook for the long-term survival of this species.
SOUTHERN PIG-TAILED MACAQUE
Geographic Distribution and Habitat
Southern pig-tailed macaques (Macaca nemestrina), also known as Sunda or Sundaland pig-tailed macaques, are native to Brunei, Indonesia, Malaysia, and Thailand, and have also been introduced to areas of Singapore and the Natuna Islands. They are well at home in the dense tropical rainforests of southeast Asia, usually occupying lowland, coastal, swamp, and montane forests. While they are found in the highest population densities in primary forests, they can also live in secondary forests, and are even found living in agricultural areas, such as oil palm plantations. While they are found from sea level to 6,200 feet (1,900 m) in elevation, they tend to prefer the higher elevations and dry grounds of hills and slopes.
The closely related northern pig-tailed macaque species, M. leonina, used to be considered a subspecies of the southern pig-tailed macaque, M. nemestrina, but they were only recently widely considered to be two distinct species. Much research prior to this taxonomic change refers only to “pig-tailed macaques,” without specifying southern or northern populations, so the differences between the two species’ ecology and behavior are not well understood. There is also no well-defined, precise geographic boundary between the two species, and they are in fact known to hybridize in a small area of southern Thailand and several nearby islands.
Size, Weight, and Lifespan
Males are larger than females, about 20–23 inches (50–58 cm) in length compared to the females’ 15–19 inches (38–48 cm). The tail adds another 5–10 inches (13–25 cm) in length, which is quite short compared to most primates. Males weigh between 12 and 26 lbs (5–12 kg), and females weigh 10–13 lbs (4.5–6 kg). They can live upwards of 26 years in the wild and 35 in captivity.
Southern pig-tailed macaques are buff-brown in color, with a darker back and crown. Their eyes are a light amber. Males have mane-like hair framing their faces that the females lack. Males also have larger canine teeth, usually almost twice as long as females. Their face and belly are a light cream color, and they often have a yellowish tinge. They have short, hairless, or nearly-hairless tails that they carry half-erect, giving them a pig-like appearance, hence their name. Infants are born with a solid black coat but begin to develop their adult coloring when they are about three months of age.
Southern pig-tailed macaques are frugivorous, with fruits making up about 74% of their diet. They supplement their diet with leaves and buds (11%), flowers (1%), invertebrates (12%), and other items. Some favorite foods of the macaques are figs, tapioca roots, corn, durian fruit, and papaya. Their diet can sometimes render them agricultural pests, as they have been known to raid farms until a favorite crop, such as corn, is completely consumed and the crop ruined. They have been observed cooperatively raiding gardens and fields, with one monkey acting as “lookout,” raising an alarm call if a human is seen.
Behavior and Lifestyle
Southern pig-tailed macaques are quadrupedal (walking on all fours) and mostly terrestrial, tending to stick to the ground and even fleeing on the ground instead of in the trees. The main exception is foraging, as they tend to do this in the trees. Unlike most primates, they love water and swim readily. They spend most of their waking time moving (61%, based on one study), followed by resting (19%), foraging (16%), and social behavior (4%). Southern pig-tailed macaques like to sleep in tall trees (more than 66 feet, or 20 meters, tall on average), as a way to evade predators. They usually sleep in trees close to their last feeding site, so as to minimize the risk of predation while traveling from feeding to sleeping sites.
The southern pig-tailed macaque’s Latin name, nemestrina, is based on the Latin Nemestrinus: “the god of groves.”
Southern pig-tailed macaques have a home range size of 250–740 acres (100–300 hectares), although they don’t use all parts of their range equally. In fact, their home range can overlap by as much as 50% with other group’s, indicating that the entire area is not constantly defended. However, if two groups are in the same place at the same time, they attempt to drive the other group out. Daily movement ranges from about half a mile to two miles (0.8–3 km) per day, depending on weather and fruit availability.
They live in multi-male multi-female groups, with the male to female sex ratio varying from about 1:3 to 1:8. Total group size can vary wildly, with observed groups having varied from 15 to 81 individuals in size, although they tend to have between 15 and 40 individuals on average. Group size seems to be correlated with habitat, with hill forests groups having the largest sizes and lowland forest groups having the smallest. However, data about group sizes are based on a limited number of populations. Large groups sometimes split up into small sub-groups of about two to six individuals while foraging, to reduce competition. The subgroups stay relatively close to one another, and stay in contact through vocalizations.
Southern pig-tailed macaque groups have a dominance hierarchy, with male hierarchies being determined by strength and female hierarchies by genetic lineage. When a new male joins the group, he enters it as the lowest-ranking male, and has to improve his rank through competition. Generally speaking, the alpha female leads the group, while the alpha male manages conflict in the group and defends it. Males are socially dominant over females. This dominance hierarchy results in a fair amount of aggression within a group. Higher-ranking males often display aggression to lower-ranking males, and to new males seeking to join their groups. Females sometimes band together to attack lower-ranking males.
Southern pig-tailed macaques communicate through a huge variety of vocal calls, body postures, facial gestures, chemical cues, and touches. Interestingly, while very much capable of vocalizations and using them extensively, they are often silent in situations when other primates would usually be vocalizing, such as when fleeing. When they do vocalize, they can be heard at distances of up to 260 feet (80 m), and their vocalizations come in forms such as coos, squeals, barks, and growls.
Two individuals can display their tolerance of each other by grooming each other, kissing, and feeding together, behavior that is commonly displayed between high-ranking females, who are usually sisters. Southern pig-tailed macaques also have a unique set of social cues used to reconcile after an aggressive encounter. Dominant females mount subordinate ones, and in males, the subordinate male mounts the dominant one. Dominant females also reconcile by kissing subordinate females.
Estrus is very apparent in female southern pig-tailed macaques, because their anogenital region swells and turns bright pink when they are receptive to breeding. When she is ready, a receptive female presents her backside to a male, who responds by pushing his lips out and flattening his ears before mounting her. The highest-ranking males tend to monopolize receptive females, acting aggressively to lower ranking males who attempt to breed. If there are multiple females in estrus at the same time, however, the highest-ranking males cannot effectively control all copulation, and lower-ranking males get their chance to breed.
Interestingly, despite this dynamic, studies show that higher-ranking males do not tend to produce more offspring than lower-ranking males, and female rank is actually the more important predictor of reproductive success. Female rank also tends to determine the sex of the offspring: higher-ranking females produce more female offspring and lower-ranking females tend to produce male offspring. Because females inherit their rank and males fight for it, it is more beneficial for a low-ranking female to produce a son who has a chance of improving his rank, rather than a daughter who will inherit her mother’s low rank. Conversely, high-ranking females are more likely to produce a daughter who will inherit her mother’s high rank, rather than a son who will become low-rank when he joins a new group.
Breeding happens year-round, although prime breeding time seems to be between January and May. Gestation averages 172 days long, or almost six months. Females give birth every year or two, and continue to nurse their offspring for four or five months. Females become sexually mature around age three while males become mature at about 4–5 years of age. Males leave the group upon reaching maturity, while females stay with their natal groups. Their generation length is about 10–12 years.
In their first month, offspring are virtually attached to their mothers. After a month, they begin to explore their surroundings. It is at this time when they are most at risk: they may die of starvation or dehydration if separated for too long, or they may be kidnapped by other adult females. If they survive their first month, their mothers continue to provide most of their care by nursing, carrying, and protecting them through their first year of life. After that, their mothers continue to provide care as needed, such as through grooming and social support, either for their whole lives for female offspring, or until they leave the group for males.
Southern pig-tailed macaques live in the same forests as gibbons and siamangs, and white-handed gibbons (Hylobates lar) in particular compete with southern pig-tailed macaques for food and seem to be a nuisance to them. Natural predators are not documented but likely include large cats and snakes. While exact figures are not known, it is likely that, as frugivores, southern pig-tailed macaques are important seed dispersers for the plants they consume.
The International Union for Conservation of Nature (IUCN) assesses the southern pig-tailed macaque as Endangered (IUCN 2022) based on an ongoing population reduction of at least 50% in the past 33 years, which is likely to continue into the future if threats are not addressed. This population reduction is due to the ongoing conversion of their prime habitat to other land use forms, leading to permanent habitat loss and degradation. This includes conversion of lowland tropical rainforest to large-scale oil palm monocultures and other crops (e.g., durian, rubber), and for mining activities through clear felling; habitat degradation due to selective logging for timber extraction and the construction of roads and linear infrastructure, and other large-scale urban and industrial development projects, draining of peat swamps, and seasonal forest burning that will likely become more severe due to predicted extreme weather events in the region. The incidences of road casualties have also steadily increased over the past years.
Southern pig-tailed macaques are listed under Appendix II of the Convention on International Trade in Endangered Species (CITES), limiting their international trade. One promising 2006 study involved leading workshops in villages in southeast Asia, educating local people about endangered species, including pig-tailed macaques. The communities involved in the study made changes that resulted in reduced human-wildlife conflict, and thus less killing of the macaques. It is clear that in addition to widespread habitat protection, it is imperative that local people receive support in dealing with the sometimes nuisance southern pig-tailed macaques, improving the lives of both the humans and monkeys.
Written by K. Clare Quinlan, September 2020. Conservation status updated July 2022.