6.1: Ecology

The more than 600 species and subspecies of living primates are highly diverse in their dietary preferences and the habitats they occupy. These aspects of primate ecology have significant impacts on every part of a primate’s life, including their morphology, physiology, and body size as well as their interactions with other individuals inside and outside their social group. They even play a role in determining whether a primate lives in a group or is solitary and lives alone. A primate’s habitat determines the food to which they have access and the community of other species with whom they interact, including predators.

Primate Diets

Diet may be the most important variable influencing variation in primate morphology, behavior, and ecology, and primate diets are highly varied. Some primatologists separate foraging, the act of finding and handling food, from feeding, the act of consuming food, while others combine these into one category. Most primates are omnivores who ingest a variety of foods in order to obtain appropriate levels of protein, carbohydrates, fats, and fluids, but one type of food often makes up the majority of each species’ diet. Because you learned about the dental and digestive adaptations experienced by frugivores (who feed primarily on fruit), folivores (whose diet consists mostly of leaves), and insectivores (who eat mainly insects) in Chapter 5, we will not discuss them here. Instead, we will focus on the relationship between diet and body size and the variation in food abundance (how much is available in a given area) and distribution (how it is spread out).

Body Size and Diet

Figure \(\PageIndex{1}\): A spectral tarsier eating a grasshopper.

Figure \(\PageIndex{2}\): A mountain gorilla eating leaves.

As you learned in Chapter 5, insects are a high-quality food. Full of easily digestible protein and high in calories, insects are an excellent source of nutrients, meeting most of a primate’s dietary needs. Although all primates will eat insects if they come upon them, those species that rely most heavily on insects tend to be the smallest. If insects are such a high-quality food, why aren’t all primates insectivores? The answer is that larger primates simply cannot capture and consume enough insects every day to survive. This is because the basal metabolic rate (BMR), or the rate at which energy is used to maintain the body while at rest, increases more slowly than body size. The result? Heavier animals must consume absolutely more food, but they have a slower metabolism so they need fewer calories per unit of body weight. Because of their small size (less than 150 g), tarsiers do not need to consume large amounts of food each day, but their high metabolic rate means they convert food into energy very quickly. This is only possible by consuming food that is easily digestible, like insects. It does not matter to a tarsier that a grasshopper only weighs 300 mg, because the tarsier itself is so small that one grasshopper is a good-size meal (Figure 6.6a). However, an adult male gorilla, who may weigh up to 200 kilograms, cannot possibly consume enough insects to meet its caloric needs. And it does not need to. Because of their large body size, gorillas have a much lower metabolic rate than tarsiers, so they can consume low-quality food, like leaves, and take their time digesting it, so long as they get enough (Figure 6.6b). Fortunately for gorillas, leaves are plentiful, as we will see in the next section. Most medium-size primates are highly frugivorous. Whether they supplement their high-fruit diet with insects or leaves also depends on their size. Smaller frugivores tend to supplement with insects, while larger frugivores tend to supplement with leaves.

Food Abundance and Distribution

Figure \(\PageIndex{3}\): Food is abundant when there is a lot of it in a given area.

Figure \(\PageIndex{4}\): Food is scarce when there is not much of it in a given area.

Nutrients (see Chapter 5) and food quality are not the only dietary considerations primates must make. They must also ensure that they consume more calories than they burn. The abundance and distribution of food affect energy expenditure and calorie intake because they determine how far animals must travel in search of food and how much they must compete to obtain it. Abundance refers to how much food is available in a given area while distribution refers to how food is spread out. Food abundance is either plentiful (Figure 6.7a) or scarce (Figure 6.7b). Food is distributed in one of three ways: uniformly (Figure 6.8a), in clumps (Figure 6.8b), or randomly (Figure 6.8c). In general, higher-quality foods, like fruit and insects, are less abundant and have patchier distributions than lower-quality foods, like leaves. In a rainforest, like the Amazon, every tree has leaves, so they are abundant and uniformly distributed (Figure 6.9a). Folivores do not have to travel very far to find food so they do not burn many calories searching for food. In comparison to leaves, fruit is scarce and clumped (Figure 6.9b). Because not every tree contains fruit, it is less plentiful than leaves. In addition, in a rainforest, a single tree with fruit may be surrounded by many trees without fruit. To a frugivore, this one fruit tree is a clump of fruit. Frugivores who do not eat leaves must travel farther distances in search of food because they can only feed in some trees (i.e., those producing fruit). Frugivores burn more calories searching for food than folivores, so it is a good thing that fruit is such a high-quality food. Lastly, insects are scarce, and due to their mobile nature, most are randomly distributed (Figure 6.9c). This combination makes it impossible for larger primates to rely on insects for a significant part of their diet.

Figure \(\PageIndex{5}\): Food has a uniform distribution when it is spread out evenly in the environment.

Figure \(\PageIndex{6}\): Food has a clumped distribution when it is found in patches.

Figure \(\PageIndex{7}\): Food is randomly distributed when it has neither uniform nor clumped distribution.

Figure \(\PageIndex{8}\): For folivores living in a rainforest like the Amazon, food is abundant and everywhere. In this drawing, every tree has edible leaves.

Figure \(\PageIndex{9}\): For frugivores, only trees producing fruit contain food so food is scarce and found in clumps. In this drawing, only the four fruit trees contain food.

Figure \(\PageIndex{10}\): Insects are generally scarce and randomly distributed because they are highly mobile. In this drawing, only the insects on the trees are edible.

It is important to remember that species preferences for specific types of food may cause it to exist in abundance or distribution that is different than the general patterns we’ve discussed here. For example, for folivores who prefer young (i.e., immature) leaves, their food supply is patchier and less abundant than it appears to a researcher looking at the lush green carpet of the Amazon forest because only some leaves are immature at any point in time. During fruiting season, when many trees are producing fruit, fruit may be temporarily abundant and less clumped. Similarly, some insects, like termites in a termite mound (Figure 6.10), are found in clumps, similar to the way a single fruit tree is a “clump” of fruit surrounded by trees with no fruit.

Competition for Food

Figure \(\PageIndex{11}\): Individual insects are usually scarce and randomly distributed in the environment. A notable exception are termite mounds, like this one in Tanzania, where, inside the mound, insects are abundant and clumped.

When a resource that is important for survival or reproduction is scarce, individuals will compete to obtain that resource. This is a central tenet of Charles Darwin’s theory of evolution by natural selection (see Chapter 2). Because female primates (like all mammals) devote a lot of energy to offspring production and care (discussed in detail in the “Parental Investment” section of this chapter), especially while pregnant and nursing, they compete for access to food, so long as the food is worth competing for. Competition between primates takes two forms: Individuals engage in direct competition (e.g., fighting) over resources that are large and worth defending (fruit is a good example of a food resource over which primates will fight) or individuals engage in indirect competition (e.g., eating food before another individual gets to it), which occurs when a resource is small or not worth defending. Primates often engage in indirect competition for insects, like grasshoppers, that are eaten quickly (Figure 6.6a). Primates may engage in direct and/or indirect competition with members of their own group or with members of other groups.

Effects of Food Abundance and Distribution on Interactions Between and Within Groups

The amount, or abundance, of food determines the nature of competition between different groups (Isbell 1991). Between-group competition is seen in terms of changes to home range size and nature of interactions between groups. A group’s home range is the area over which the group moves in search of food. Groups that defend the boundary of their home range are said to occupy a territory. Consider a patch of forest covered in leaves (e.g., Figure 6.9a). If you are a folivore, every tree is a dinner table. When your group size increases, your home range does not expand because there is more than enough food for everyone, and it is a waste of energy to travel farther than you need to. And as long as your group does not expand its home range, you will not encroach on the trees of neighboring groups. This keeps competitive interactions between groups of folivores to a minimum. Now imagine you are a frugivore. Unlike leaves, not every tree has fruit on it. Maybe only two or three trees are in fruit at any given time, so fruit is scarce (Figure 6.9b). Thus, if your group size increases, it is likely that the few fruit trees available in your current home range will not be able to feed everyone. If that’s the case, then your group will need to expand its home range in search of additional fruit trees, which are in the home ranges of neighboring groups. Home-range expansion is also often accompanied by fighting between groups as members attempt to keep intruders away from valuable, scarce food resources.

While food abundance determines interactions between groups, food distribution determines the interactions between individuals within a group (Isbell 1991). Competition within a group is marked by changes in day-range length and the presence of dominance hierarchies. Day-range length measures the distance a group must travel in a single day in search of food. A dominance hierarchy reflects the place of each individual in the group in comparison to others. An individual’s place in the hierarchy, or “rank,” determines their priority of access to resources. If food is evenly distributed (as with leaves; Figure 6.9a), individuals can spread out while feeding so that their day-range length does not increase when their group size increases. Likewise, because leaves are “everywhere,” there is little benefit to females engaging in interactions that determine “priority of access” to resources, so dominance hierarchies are uncommon among folivores. However, if food is clumped (as with fruit; Figure 6.9b), individuals in groups must feed in more cohesive units (i.e., all in one fruit tree). When group size increases, the group must travel farther each day in order to visit enough fruit trees to feed all group members. Likewise, when food is clumped, individuals have the opportunity to monopolize it and keep others from feeding. Under such circumstances, females benefit from competing with one another for “priority of access” to the resource, and dominance hierarchies result.

The fact that food abundance and food distribution vary independently helps us understand the complex nature of between-group and within-group interactions (Isbell 1991). For example, both olive baboons and patas monkeys feed on scarce resources, and both species engage in competition with other groups and expand their home-range size when food is in short supply. But olive baboons’ food is clumped while patas monkeys’ is dispersed, so the interactions within groups are very different. Female baboons have a strong dominance hierarchy, and the distance they travel each day increases with group size. Patas monkeys have a weak dominance hierarchy, and when group size increases, individuals spread out while feeding and daily travel distance does not increase.

Community Ecology

Figure \(\PageIndex{12}\): A patas monkey.

Figure \(\PageIndex{13}\): A vervet.

In addition to interactions with other members of their own group and other groups of conspecifics (members of the same species), primates are members of broader ecological communities composed of other species, including other primates, predators, and even humans. When two species (or populations) occupy the same geographic area, they are sympatric. The patas monkeys and vervets that I studied in Kenya, along with olive baboons and Senegal bush babies, are sympatric and form a primate community (Figure 6.11a–d). However, vervets (Figure 6.11b) and muriquis of Brazil (Figure 6.12) are allopatric, meaning their geographic ranges do not overlap. Some habitats support highly diverse primate communities consisting of 10 or more species (Figure 6.13). How can so many species of primate occupy the same area and avoid competition? Sympatric species do sometimes compete with each other.

Figure \(\PageIndex{14}\): A Senegal bush baby.

Figure \(\PageIndex{15}\): An olive baboon.

Observations of one species displacing another at a food site is a sign of competition between the two species. When this happens, usually it is the large-bodied species that supplants the small-bodied species. The exception is when the small-bodied species significantly outnumbers the larger-bodied one. The competitive exclusion principle states that two species that compete for the exact same resources cannot coexist. This means that two species cannot occupy the same niche—cannot seek to meet their needs for food and shelter in the exact same way. Because tropical rainforests are highly variable, with many habitats and many sources of food and shelter, there are many different niches for multiple species

Figure \(\PageIndex{16}\): A muriqui mother an infant.

to exploit, and large primate communities result (Figure 6.13). In non-rainforest habitats, like Kenya’s open woodland, which is home to four species (Figure 6.11a–d), there are fewer niches for multiple species to occupy. Regardless of habitat type, sympatric species avoid competition through niche partitioning (using the environment differently). Niche partitioning includes differences in diet, ranging behavior, and habitat use. In Laikipia, Kenya, bush babies reduce competition with vervets by feeding more heavily on insects. They further reduce competition by being nocturnal while vervets are diurnal. Even though bush babies (Figure 6.11d) and vervets (Figure 6.11b) do sometimes eat the same food, since they eat at different times of day they rarely, if ever, interact.

Table 6.1.1: Examples of primate communities.

Site	Primate Community
Krau Game Reserve, Malaysia	white-handed gibbon, siamang, dusky leaf monkey, mitered leaf monkey, long-tailed macaque, pigtail macaque, slow loris
Manu National Park, Peru	black spider monkey, red howler monkey, brown capuchin, white-fronted capuchin, South American squirrel monkey, owl monkey, dusky titi monkey, common woolly monkey, monk saki monkey, Goeldi’s marmoset, emperor tamarin, saddleback tamarin, pygmy marmoset
Beza Mahafaly Reserve, Madagascar	ring-tailed lemur, Verreaux’s sifaka, white-footed sportive lemur, reddish-gray mouse lemur
Kibale National Park, Uganda	Ugandan mangabey, L’Hoest’s monkey, Ugandan red colobus monkey, vervet monkey, olive baboon, blue monkey, grey-cheeked mangabey, potto, galago, black and white colobus monkey, chimpanzee

Predation

Figure \(\PageIndex{17}\): Opportunistic hunting of a lizard by a lion-tailed macaque

Figure \(\PageIndex{18}\): Adult male chimpanzee in Gombe Stream National Park, Tanzania with dead bushbuck.

An important aspect of primate communities is the predators that also occupy them. As discussed in the “Primate Diets” section, all primates incorporate some insects into their diet, and so they may be themselves considered predators in this respect. In this section, we will limit our discussion to predation of and by vertebrates (animals with an internal spinal column or backbone). Many primates incorporate some vertebrate prey into their diet. Often, predation by primates is opportunistic, occurring because the prey happen to be in the right place at the right time. I’ve observed vervets opportunistically killing lizards by smashing them against a rock or tree trunk and eating them (much as this lion-tailed macaque has done; Figure 6.14a). In some parts of their range (including Gombe Stream National Park and Mahale Mountain National Park, both in Tanzania), chimpanzees are described as opportunistic hunters, with the vast majority of hunts occurring after a chance encounter with prey (Figure 6.14b; Boesch and Boesch 1989). Other primates are more deliberate predators, and some even work together to increase their chances of success. Cooperative hunting has been observed in white-faced capuchins and some chimpanzee populations. White-faced capuchins hunt more often during the dry season, when other food is scarce, and sometimes work together to chase, surround, and capture small mammals like young squirrels or coatis (Fedigan 1990). The chimpanzees of Taï National Park in Côte d’Ivoire take deliberate, cooperative hunting to the next level. Unlike their Tanzanian counterparts, they form hunting parties to search for red colobus monkeys and, once located, anticipate the prey’s movement and coordinate with other hunters to drive, isolate, and capture prey (Boesch 2002). Since hunting-party size correlates with hunting success, it is not surprising that sharing the spoils of a successful hunt is more common in Taï chimpanzees who rely on others for success (Boesch and Boesch 1989).

Figure \(\PageIndex{19}\): African leopard.

All primates are susceptible to predation by mammalian carnivores (animals whose diet consists primarily of animal tissue) (Figures 6.15a–d), birds of prey (Figures 6.15e-f), and/or reptiles (Figure 6.15g), although the specific predators differ based on geography and primate body size. Smaller primates fall prey to a wider range of predators than larger primates, and some habitats contain a greater diversity of predators. Primates use a variety of anti-predator tactics to avoid and/or escape predation. Perhaps the best way to avoid predation is to avoid being detected by predators in the first place, and some primates use crypsis to great effect.

Figure \(\PageIndex{20}\): Jaguar of Latin America.

Nocturnal primates are often small and solitary or live in very small groups. If you are already hard to see because you are active at night, moving quietly in small groups is a good strategy to avoid detection by predators. The slow loris of Southeast Asia exemplifies this strategy (Figure 6.16). Nocturnal and solitary, the slow loris moves slowly (as its name suggests) and quietly as its primary strategy to avoid predation (Wiens and Zitzmann 2003). If detected, however, the slow loris will attempt to escape by releasing its grip and falling off the branch or biting in defense.

Figure \(\PageIndex{21}\): Tiger of Asia.

Figure \(\PageIndex{22}\): Fossa of Madagascar.

Interestingly, the slow loris is the only venomous primate. The venom is formed when the slow loris combines oil from a gland on its arm with its saliva (Nekaris et al. 2013). It can either apply the venom to its head for protection or store it in the mouth to deliver through a bite. Slow loris bites are painful and take a long time to heal. In extreme cases, individuals who are bitten may go into shock and die. It is not as easy for diurnal primates to avoid detection by predators, and most (but not all) diurnal primates, like Hanuman langurs, have larger body sizes and live in groups (Figure 6.17). Indeed, anti-predator behavior, including vigilance, alarm calling, and mobbing, may be one of the primary benefits primates get from living in groups; we will discuss these behaviors in a later section, entitled “Why do Primates Live in Groups?”

Figure \(\PageIndex{23}\): Martial eagle of Africa.

Figure \(\PageIndex{24}\): Harpy eagle of Latin America.

Figure \(\PageIndex{25}\): South African python.

Figure \(\PageIndex{26}\): The slow loris is a solitary nocturnal primate.

Figure \(\PageIndex{27}\): Hanuman langurs are group-living diurnal primates.

SPECIAL TOPIC: PRIMATE CONSERVATION

Figure \(\PageIndex{28}\): Deforestation of Bornean rainforest for conversion to palm oil plantations.

Figure \(\PageIndex{29}\): Men in Madagascar hunt and kill a white-fronted brown lemur for bushmeat.

There are over 600 species and subspecies of primates on the planet today, and almost half of them live under the threat of extinction. While there are many threats to primates, habitat destruction and hunting are the leading causes of population decline (Figure 6.18a–b). Primate populations have withstood small-scale forest clearing and low levels of hunting by local human groups for hundreds of years. However, the recent, intense pressure of expanding human populations on many primate habitats is resulting in rapid population declines for many species. The majority of primates live in tropical habitats, and the loss of tropical forest, whether due to logging or farming, is the single greatest factor contributing to the decline of primate populations across the planet. Between 1973 and 2010, almost 100,000 km2 of orangutan habitat was cleared for palm oil plantations in Borneo (Figure 6.18a). During this same time, the orangutan population decreased from almost 300,000 to 100,000, an average loss of more than 5,000 orangutans every year. As of 2017, that number may be as low as 60,000 (Schwitzer et al. 2017). If this rate of loss is not curtailed, the Bornean orangutan will go extinct in less than 15 years. Hunting, whether for bushmeat (Figure 6.18b), trophies, or the pet trade, has had devastating effects on many primate populations. Even though Grauer’s gorillas are legally protected, they are highly prized for bushmeat because they are relatively easy to track and shoot, and their large body size yields significant amounts of meat. Survey work has revealed that the Grauer gorilla population has declined significantly since the 1990s, due almost entirely to illegal hunting. The gorilla population in Kahuzi-Biega National Park, in Democratic Republic of Congo (DRC), is estimated to have declined 87% since 1994 (Schwitzer et al. 2017).

As consumers and concerned citizens, all of us are learning how to use our wallets to combat habitat and species loss. We do not buy palm oil or products made with palm oil in an effort to save orangutans. We donate to conservation organizations doing important on-the-ground work in Democratic Republic of Congo and other conservation hot-spots. We educate ourselves as well as our friends, families, and communities about the plight of endangered primates. Primatologists, too, contribute to conservation efforts. No longer is primatology research restricted to the “ivory tower” of academia. Current and future primatologists have the opportunity to affect real change in primate conservation (Chapman and Peres 2001). Whether understanding the mechanisms that determine species abundance, predicting the effects of human activity on species survival, documenting patterns of environmental change, understanding the effects of species removal in broader contexts, or evaluating different approaches to conservation, information gained from primate studies offers some of the best hope we have for a future that continues to include our closest living relatives. You can learn more about primate conservation in Appendix B.