When trying to place primate species into specific taxonomic groups, we use a variety of dental characteristics, locomotor adaptations, and behavioral adaptations. Differences in these characteristics across groups reflect constraints of evolutionary history as well as variation in adaptations.
Dental Characteristics
Teeth may not seem like the most exciting topic with which to start, but we can learn a tremendous amount of information about an organism from its teeth. First, teeth are vital to survival. Wild animals do not have the benefit of knives and forks, and so rely primarily on their teeth to process their food. Because of this, teeth of any species have evolved to reflect what that organism eats and so tell us directly about their diet. Second, variation in tooth size, shape, and number tells us a lot about an organism’s evolutionary history. Some taxa have more teeth than others or different forms of teeth than others. Furthermore, differences in teeth between males and females can tell us about competition over mates (see Chapter 6). Lastly, teeth preserve really well in the fossil record. Enamel is hard, and there is little meat on jaws so carnivores and scavengers often leave them behind. Because of this, very often we find a lot of fossil jaws and teeth, and so we need to be able to learn as much as we can from those pieces.
Figure \(\PageIndex{1}\): This open-mouthed Hamadryas baboon clearly demonstrates the diastema between his upper canine and front teeth. This space is taken up by his lower canine when he closes his mouth.
If you’ve ever seen the jaws of a shark, dinosaur, or crocodile, you were probably struck by how sharp their teeth were and by the sheer number of teeth they had. What you probably didn’t think about was that they also only have one type of tooth, referred to as homodont. In fact, one of the ways that mammals differ from other organisms is that we have multiple types of teeth (heterodont) that we use in different ways. We have incisors, which we use for slicing; we have premolars and molars, which we use for grinding up our food; and we have canines, which most primates (not humans) use as weapons against predators and each other. The sizes of canines vary across species and can often be sexually dimorphic, with male canines usually being larger than those of females. Non-human primates often hone, or sharpen, their canines by gnashing the teeth together to sharpen the sides. The upper canine sharpens on the first lower premolar and the lower canine sharpens on the front of the upper canine. As canines get larger, they require a space to fit in order for the jaws to close. This space between the teeth is called a diastema (Figure 5.4).
Figure \(\PageIndex{2}\): This drawing shows half of the human mandible. With the four types of teeth labeled, you can determine that the dental formula is 2:1:2:3.
As discussed before, primate taxa can vary in the numbers and forms of teeth they have. We determine the number of each type of tooth an organism has by its dental formula. The dental formula tells you how many incisors, canines, premolars, and molars are in each quadrant of the mouth (half of the top or bottom). For example, Figure 5.5 shows half of the lower teeth of a human. You can see that in half of the mandible, there are two incisors, one canine, two premolars, and three molars. This dental formula is written as 2:1:2:3. (The first number represents the number of incisors, followed by the number of canines, premolars, and molars). Some early fossil primates had a dental formula of 2:1:4:3, but among the living primates, none have more teeth than can be found in a 2:1:3:3 dental formula. Many have fewer teeth, however, and some have a different dental formula on the top than they do on the bottom.
To determine the dental formula, you need to be able to identify the different types of teeth. You can recognize incisors because they often look like spatulas with a flat, blade-like surface. Premolars and molars can be differentiated by the number of cusps that they have. Cusps are the little bumps (which in some species can be quite sharp) that you can feel with your tongue on the surface of your back teeth. Premolars are smaller than molars and, in primates, often have one or two cusps on them. Molars are bigger, with a larger chewing surface, and so have more cusps. Depending on the species of primate and whether you’re looking at upper or lower teeth, molars can have between three and five cusps. There is even one extinct primate (Oreopithecus) who had six cusps on its molars. Molar cusps can also vary between taxa in how they are arranged, as you will learn more about later in this chapter. Canines are often easy to distinguish because they are usually much longer and more conical than the other teeth. This is not always the case, however, as you will see when you read about the teeth of lemurs and lorises.
Teeth also tell us directly about an organism’s diet. Primates are known to eat a wide range of plant parts, insects, gums, and, rarely, meat. While all primates eat a variety of foods, what differs among primates are the proportions of each of these food items in the diet. That is, two primates living in the same forest may be eating the same foods but in vastly different proportions, and so we would categorize them as different dietary types. The most common dietary types among primates are those whose diets consist primarily of fruit (frugivores), those who eat mostly insects (insectivores), and those who eat primarily leaves (folivores). Fewer primates are gummivores, who specialize in eating gums and saps, so we will not discuss the adaptations for this dietary type in great detail.
Frugivores
Plants want animals to eat their fruits because, in doing so, animals eat the seeds of the fruit and then disperse them far away from the parent plant. Because plants want animals to eat the fruit, plants often “advertise” fruits by making them colorful and easy to spot, full of easy-to-digest sugars that make them taste good—and, often, easy to chew and digest (not being too fibrous or tough). For these reasons, frugivores often do not need a lot of specialized traits to consume a diet rich in fruits (Figure 5.6a). Their molars usually have a broad chewing surface with low, rounded cusps (referred to as bunodont molars). Frugivores also often have large incisors for slicing through the outer coatings on fruit. Primates that eat fruit tend to have stomachs, colons, and small intestines that are intermediate in terms of size and complexity between insectivores and folivores (Chivers and Hladik 1980). They are also usually of intermediate body size between the other two dietary types. Because fruit does not contain protein, frugivores must supplement their diet with protein from insects and/or leaves. Some frugivorous primates get protein by eating seeds and so have evolved to have thicker enamel on their teeth to protect them from excessive wear.
Table 5.2.1: Frugivores are characterized by large incisors, bunodont molars, and digestive tracts that are intermediate in complexity between the other two dietary types.
Large incisors
Bunodont molars
Intermediate complexity of digestive tract
Insectivores
Insects can be difficult to find and catch but are not typically difficult to chew. As a result, insectivorous primates usually have small molars with pointed cusps that allow them to puncture the exoskeleton of the insects (Figure 5.6b). Once the outer shells of the insects are punctured, insects are not difficult to digest, so insectivores have simple stomachs and colons and a long small intestine. Nutritionally, insects provide a lot of protein and fat but are not plentiful enough in the environment to support large-bodied animals, so insectivores are usually the smallest of the primates.
Table 5.2.2: Insectivores need sharp, pointed molar cusps in order to break through the exoskeletons of insects. Insects are easy to digest, so these primates have simple digestive tracts.
Sharp, pointed molars
Simple digestive tract
Folivores
Unlike with fruits, plants do not want animals to eat their leaves. Leaves are the way plants get their energy from the sun, therefore, plants evolved to make their leaves very difficult for animals to eat. Leaves often have toxins in them, taste bitter, are very fibrous and difficult to chew, and are made of large cellulose molecules that are difficult to break down into usable sugars. Because of these defenses, animals who eat leaves need a lot of specialized traits (Figure 5.6c). Folivorous primates have broad molars with high, sharp cusps connected by shearing crests. These molar traits allow folivores to physically break down fibrous leaves when chewing. Folivores then have to chemically break down cellulose molecules into usable energy, so these animals need specialized digestive systems. Some folivores have complex stomachs with multiple compartments, but all leaf eaters have large, long intestines and special gut bacteria that can break up cellulose. Folivores are usually the largest bodied of all primates, and they spend a large portion of their day digesting their food, so they are often less active than frugivores or insectivores.
Table 5.2.3: In order to derive energy from leaves, folivores have smaller incisors, high, sharp molar cusps connected by shearing crests and complex digestive tracts filled with specialized bacteria.
Smaller incisors
High, sharp molar cusps with shearing crests
Complex digestive tract
Behavioral Adaptations
Chapter 6 is entirely dedicated to primate behavior, so only broad differences related to taxonomic classification will be discussed here. These differences include variations in activity patterns, social grouping, and habitat use. Primate groups often differ in activity patterns—that is, whether they are active during the day (diurnal), at night (nocturnal), or through the 24-hour period (cathemeral). We also see variations among primate groups in social groupings: some taxa are primarily solitary, others live in pairs, and still others live in groups of varying sizes and compositions. Lastly, some taxa are primarily arboreal while others are more terrestrial.
Locomotor Adaptations
Figure \(\PageIndex{3}\): An example of a vertical clinger and leaper. Note the longer legs than arms, long lower back and long fingers and toes. This vertical clinger and leaper doesn’t have a tail, but most have long tails as well.
Finally, primate groups vary in their adaptations for different forms of locomotion, or how they move around. Living primates are known to move by vertical clinging and leaping, quadrupedalism, brachiation, and bipedalism. Vertical clinging and leaping is when an animal grasps a vertical branch with its body upright, pushes off with long hind legs and then lands on another vertical support branch (Figure 5.7). Animals who move in this way usually have longer legs than arms, long fingers and toes, and smaller bodies. Vertical clinger leapers also tend to have elongated ankle bones, which serve as a lever to help them push off with their legs and leap to another branch.
Figure \(\PageIndex{4}\): Here are examples of a typical quadrupedal primate. Note that the arms and legs are about the same length and the back is long and flexible. This is a terrestrial quadruped so the arms and legs are relatively long and the tail is shorter.
Quadrupedalism is the most common form of locomotion among primates (Figure 5.8). The term quadrupedal means to walk on all fours. Animals that move in this way usually have legs and arms that are about the same length and typically have a tail for balance. Arboreal quadrupeds usually have shorter arms and legs and longer tails, while terrestrial quadrupeds have longer arms and legs and, often, shorter tails. These differences relate to the lower center of gravity needed by arboreal quadrupeds for balance in trees and the longer tail required for better balance when moving along the tops of branches. Terrestrial quadrupeds have longer limbs to help them cover more distance more efficiently. You will learn more specific anatomical features of quadrupedalism later in the chapter.
Figure \(\PageIndex{5}\): These are examples of a typical brachiator. Note the longer arms than legs, short back, and lack of a tail. You will read about more details of their anatomy later in the chapter.Figure \(\PageIndex{6}\): Spider monkeys, like the one shown here, are considered semi-brachiators who can swing below branches but use their tails as a third limb. On the ground they move via quadrupedal locomotion.
The third form of locomotion seen in primates is brachiation, the way of moving you used if you played on “monkey bars” as a child. Brachiation involves swinging below branches by the hands (Figure 5.9). To be an efficient brachiator, a primate needs to have longer arms than legs, flexible shoulders and wrists, a short lower back, and no tail. You will learn more about the specifics of these traits when you learn about apes later in this chapter. Some primates move via . These taxa also swing below branches but do not have all of the same specializations as brachiators. They have flexible shoulders, but their arms and legs are about the same length, useful because they are quadrupedal when on the ground. Semi-brachiators also use long prehensile tails as a third limb when swinging (Figure 5.10). The underside of the tail has a tactile pad, resembling your fingerprints, for better grip.
Lastly, humans move around on two feet, called bipedalism. Some primates will occasionally travel on two feet but do so awkwardly and never for long distances. Among mammals, only humans have evolved to walk with a striding gait on two legs as a primary form of locomotion. To move bipedally, humans need many specialized adaptations that will be discussed in detail in later chapters.
Figure \(\PageIndex{5}\): These are examples of a typical brachiator. Note the longer arms than legs, short back, and lack of a tail. You will read about more details of their anatomy later in the chapter.