Geographic Distribution and Habitat
The Heck’s macaque (Macaca hecki) is endemic to the Indonesian island of Sulawesi. This volcanically active island, which looks something like a lopsided Komodo dragon from above, consists of four distinct peninsulas fanning out from a mountainous center. This unique combination of features that tends to isolate populations partially accounts for the island being one of the most biologically diverse places in the world. Indeed, Heck’s macaques are only one of seven macaque species that call Sulawesi home.
Heck’s macaques live at the base of the long, narrow peninsula known as the Isthmus of Palu, which S-curves from the island’s center in a northeasterly direction. Their range follows this peninsula from the cities of Palu to Gorontalo. Within this range, they inhabit a mix of lowland and montane forest—both primary and secondary—as well as some grassland.
Though the seven macaque species endemic to Sulawesi inhabit their own portions of the island, there is frequently overlap in the ranges of those that live next to each other. Such is the case with Heck’s macaques and their neighbors, Tonkean and Gorontalo macaques. While each one is a species in its own right, the existence of hybrids in the overlapping zones means that they are capable of mating with one another—and do! The extent and significance of this information is still being assessed.
Size, Weight, and Lifespan
Sulawesi macaques all are similar in size, with males being larger—sometimes even twice the size—of females.
Male Heck’s macaques weigh an average of around 24 pounds (11.2 kg) and measure between 21 and 26 inches (53–66 cm) long not including their tail, while females are only 15 pounds (6.8 kg) on average and measure between 16 and 24 inches (42–60 cm) long. Despite their different builds, males and females sport tails of similar lengths: between 0.7 and 1.4 inches (18–36 mm).
Male Heck’s macaques have slightly bigger heads than females, perhaps to accommodate their larger canine teeth. In the world of primates, a disparity in canine size can influence hierarchical relationships between the sexes. How this disparity plays out for Heck’s macaques is still a subject awaiting research, however.
Based on what we know of other Sulawesi macaques, Heck’s macaques most likely live between 20 and 30 years in the wild.
When a species’ population is reduced in size limiting the genetic diversity of the species.
The offspring of two plants or animals of different species or varieties.
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Sulawesi macaques tend to look quite similar. Their differences are so subtle that the average person most likely couldn’t pick them out of a lineup.
The key differences relate to their fur. Crested black macaques, the most famous variety of Sulawesi macaques, have jet black fur covering their entire bodies; a signature tuft of hair flares up from the top of their foreheads. In contrast, Gorontalo macaques, their neighbors to the west along the Isthmus of Palu, have brown fur but retain the prominent “coronal head crest,” as researchers call it.
So, what about Heck’s macaques?
Heck’s macaques’ fur gets more complicated. Instead of the entire body being uniform, their coats are a variety of colors. Their forearms are dark brown, but their shanks can be either brown, pale gray, or pale brown. The fur on their back retains most of the black fur seen in crested black macaques. In Heck’s macaques, the head crest is significantly less prominent than its Gorontalo and crested black neighbors.
For some added perspective, if we continue west along the peninsula, Tonkean macaques sport fur coats uniform in color, closer to what is seen for Gorontalo and crested black macaques. But, like Heck’s macaques, they lack the head crest.
Thus, along the Isthmus of Palu, Heck’s macaques, with their more complex markings, may well be the easiest species of macaque to identify.
The diets of Heck’s macaques have not been thoroughly studied at this time, but data collected on other Sulawesi species give us an idea of what these monkeys may eat.
Sulawesi macaques eat fruit. Immature leaves and arthropods act as reliable sources of protein and other nutrients as well. However, northern varieties of Sulawesi macaques, like Heck’s macaques, appear to follow a less folivorous (leaf-based) diet than their southern counterparts.
As Sulawesi macaque’s habitats are converted to farmland, the monkeys are relying more and more on human-grown crops, such as papaya, maize, yam, banana, and cacao.
Forming a more scientific understanding of Heck’s macaques’ diets will be paramount for their successful conservation in the wild.
Behavior and Lifestyle
Macaques are known for being some of the most adaptable nonhuman primates on the planet. Twenty-three species are scattered across the Old World, surviving in a diverse set of environments—including busy cities! The macaques of Sulawesi, however, represent a particularly unique set, not to mention a unique opportunity for research into primate evolution. After all, how often do we get the chance to study seven different species of the same type of monkey that have all evolved in such a uniquely intriguing environment?
Unfortunately, that opportunity has not yet been seized. So far, only one of the seven species, the crested black macaque (Macaca nigra), has received a fair amount of scientific and media attention. Those remaining have been studied here and there, or not at all.
Heck’s macaques fall closer to the “not at all” end of the spectrum. Little is known about their ways of life—and with their habitat decreasing, time may be running out to study them under fairly natural circumstances. What we can guess about Heck’s macaques’ way of life is mostly pieced together using data collected on the various other Sulawesi species, plus what we know about macaques in general. However, this only tends to raise more intriguing questions than clear and scientific answers.
Macaques tend to be semi-terrestrial, meaning they spend some of their time in the trees and some of it on the ground. Sulawesi macaques are also semi-terrestrial. However, some species may spend more of their time among the branches while others may prefer the forest floor. Gorontalo macaques, for instance, spend up to 96% of their day in the trees. They’re practically arboreal at that point! Does this behavior carry over to their Heck’s macaque neighbors to the east? Maybe! But we simply won’t know until proper studies are conducted.
Macaques are known as social and gregarious creatures, but they behave according to strict social hierarchies. Individuals tailor their behaviors based on where they fall in the hierarchy, the nuances of which are different for every species. When a macaque breaks social code, others may lash out at them. For what it’s worth, however, macaques are not often as overtly aggressive as they may seem from the outside. They use complex, though sometimes subtle, strategies to mitigate aggressive conflicts. Additionally, adversaries typically reconcile after fights. Crested black macaques are known for being particularly theatrical, and put on dramatic displays when in disagreement. Intimidation through theatrics is an effective way for many primates to prevent conflict from escalating further into violence, thereby benefiting the entire group. It would be interesting to know if other Sulawesi macaques, including Heck’s macaques, exhibit similar behaviors or not.
Establishing a more scientific understanding of Heck’s macaques’ behaviors and lifestyles will be paramount for their successful conservation in the wild.
Heck’s macaques have yet to be tracked in the wild, so their daily activities and group dynamics remain unknown for the time being. Based on what we do know about other Sulawesi macaque species, they probably live in multi-male/multi-female groups ranging in membership anywhere from 5 to 30 members. A group spends its day foraging, traveling, and socializing. Between activities, they rest.
During the day, groups or individuals may split off from the larger group to run their own errands—as has been observed in Gorontalo macaques. But by evening all group members return to sleep the night in the safety of the larger group.
Groups of crested black macaques are led by a single dominant male who delegates his position to other males, lower in the hierarchy, when they break off into smaller groups to forage. Though other males are technically vying for the alpha’s spot, male-to-male relationships generally remain quite peaceful. Fights tend to be more theatrical than violent, with minimal—if any—bloodshed.
Occasionally, male crested black macaques form coalitions in order to oust the current leader of their group. Once a group’s leader is supplanted, he can never regain his former post. Depending on circumstances, he might remain with his current group and fall peacefully into line or choose to leave and join another group where he will have the chance of moving up the ranks again. Female crested black macaques enjoy the protection of males and are generally peaceful, preventing and resolving their conflicts with grooming and other prosocial gestures.
Though our knowledge of other macaque species can certainly help us to imagine the lives of Heck’s macaques, only more thorough research can give us a scientific understanding of the unique lives they lead in the wild. In the end, we may find they operate completely differently from their cousins. Now, wouldn’t that be interesting?
Good communication is key to living in large social groups in which conflicts require prevention, mitigation, de-escalation, and reconciliation. Being particularly social, as well as hierarchical, macaques have developed many nuanced ways by which they actively communicate with one another. Vocalizations, facial expressions, body postures, and hand gestures are some of the ways that primates communicate with each other. While many similarities exist, such behaviors vary greatly between species—possibly even between groups of the same species! No doubt Heck’s macaques, too, evolved these sorts of behaviors. However, the specific ways in which they do so are not well-studied at this time.
We know Heck’s macaques make barking sounds. These are loud, brief vocalizations typically associated with conflict; but the purpose of this call is not known. Adult males also make loud calls. High in pitch, a male often makes this call in situations of arousal and social tension. But again, it is not understood exactly what such a call is meant to communicate.
While it doesn’t help us to know the specifics, the complexity already seen in other Sulawesi macaques communication methods more than suggests that Heck’s macaques will display a similar complexity when they are finally researched.
At this time, the life histories of Heck’s macaques have not been duly studied in the wild. If they are anything like their crested black cousins, however, males and females are quite promiscuous, mating multiple times and with multiple partners. A male’s mating success, however, is largely affected by his rank in the dominance hierarchy.
Mating can occur at any time of the year. When a female is ready to mate, she enters her estrus cycle. Males know she is fertile when her hind quarters swell and brighten, enticing them to initiate copulation.
Once she is pregnant, her gestation period lasts for approximately six months. At the end, she births a single infant who she will nurse for one year before weaning. Her offspring reaches sexual maturity within four to six years, with females normally reaching it before males.
While a mature female remains with her natal group, a male leaves his. Often, newly matured males form bachelor groups. As a member of such a band, a male has the protection of numbers and the comforts of social contact while he searches for a mixed-sex group he can join, in which he can begin vying for dominance and eventually—hopefully—mate.
Once more, Heck’s macaques may lead surprisingly different lives than their crested black cousins. We cannot truly know what their lives consist of in the wild before thorough research is conducted.
Heck’s macaques likely disperse the seeds from the fruit they eat in their dung as they travel through the forest. This has not been researched, however. Forming a more scientific understanding of Heck’s macaques’ ecological role will be paramount to their successful conservation in the wild.
The International Union for Conservation of Nature lists the Heck’s macaque as Vulnerable (IUCN, 2015), appearing on the IUCN Red List of Threatened Species. Its prior assessment, in 2006, listed this species as Near Threatened, meaning that over the last decade the macques’ situation has worsened.
Sulawesi is part of what is known as the Wallacea region. A long history of unique geographical factors has made this region into one of the most endemically rich areas in the world—a fact that makes it, and all of its flora and fauna, especially vulnerable to even minute changes. The many primates—including Heck’s macaques—endemic to the island of Sulawesi are each threatened by the rapid expansion of human settlements that destroy their fragile habitats. Infrastructural and other development projects fragment or destroy the forests they rely on for food and space, putting the well-being of the entire ecosystem in jeopardy.
Forest fragmentation puts species—particularly primate species—at risk of developing genetic bottlenecks. When genes stop flowing between different groups of animals, the lack of genetic diversity gradually makes offspring more prone to contracting diseases, viruses, and parasites, or may cause them to develop other physiological problems that affect their well-being. As this problem persists generation after generation, animals gradually become less and less viable—less likely to mate and successfully rear another generation. For Gorontalo macaques, who rely more on trees than other Sulawesi macaques, undisturbed and continuous forests are particularly precious. Might the same be true for Heck’s macaques?
Even the protected forests in Sulawesi are increasingly threatened by human settlements and activities, including illegal gold mining operations. Due to the unlawful nature of their activities, gold miners frequently use mercury in their extraction of gold. When they heat the mercury during this process, it escapes as vapor into the atmosphere. When they do not properly dispose of the inevitable waste material, it seeps into the soil and water where it quickly becomes part of the food chain. Mercury is known for its negative effects on biology; inhaling or ingesting this heavy metal can wreak havoc on one’s nervous, digestive, and immune systems and does irreparable damage to the lungs and kidneys. While this practice has mostly gone out of style among large-scale gold mining companies, the types of small-scale, undercover operations going on in Sulawesi are not affected by the moral backlash of using mercury to extract gold. Thus, they continue to do it, essentially in secret, without worrying about potential repercussions to their business.
Bushmeat is another industry that adversely affects the primate residents of Sulawesi. Crested black macaques and Gorontalo macaques are particularly prized by local populations for their meat. The macaques are cooked and served at special celebrations, like weddings and Christmas dinners. Originally, locals preferred hunting crested black macaques only. Gorontalo macaques tend to live in more mountainous regions where it is more difficult for humans to hunt. As the numbers of crested black macaques decreases in the wild, the pressure mounts on Gorontalo macaque populations. It is more than likely that Heck’s macaques will become their next target if nothing is done to halt these trends. Surveys have already found Heck’s macaque meat for sale at markets where once there was none. Despite hunting and selling of their meat being illegal, it is still quite common to see Sulawesi macaques displayed at local meat markets.
Heck’s macaques are also known to raid crops, making them targets of local farmers. Besides protecting their crops, farmers may have the added incentive to kill trespassing macaques in order to sell their meat for profit.
A key obstacle for Heck’s macaque conservation is our lack of knowledge about them. Knowing the behavior and ecology of a species gives conservationists significant insight into how they can best help it to thrive in the wild. Without this information, efforts ultimately lack the nuance that any bona fide success requires.
The Heck’s macaque is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival. They are found in multiple protected areas as well, including the Bogani Nani Wartabone National Park. This park is one of the few places in Sulawesi where the Wildlife Conservation Society—a nongovernmental organization—maintains a strong presence.
That being said, almost all of these areas are fragments too small to allow the macaques to thrive and too separate to promote healthy gene flow between groups. Another factor is enforcement: protections in Sulawesi have been too rarely enforced over the years, preventing this legislation from having a genuinely positive impact on the well-being of Heck’s macaques.
So far, Heck’s macaques have mostly gone under the radar of conservationists. The IUCN lists a number of basic measures paramount to successfully conserving them in the wild. Conducting more population surveys is at the top of their list—but that’s just the beginning. The organization also calls for the initiation of a multi-stakeholder effort between governmental and non-governmental agencies in order to better enforce protections and reduce supply and demand for Sulawesi macaques’ meat overall. Ending the illegal sale of macaque meat in Sulawesi won’t be good enough. But the more that locals begin see how these macaques are worth more alive than dead, the more they will want to participate firsthand in their protection; and that’s when true conservation can really happen!
Lynn Clayton, who founded the Nantu Wildlife Reserve within Heck’s macaques natural range, has already begun to initiate such efforts. But expanding them to other locations is paramount for the conservation of all Sulawesi macaques. The longer these trends continue, the closer we get to losing this unique opportunity that the island of Sulawesi—with its seven macaque species—provides to science.
Written by Zachary Lussier, August 2022