The Order Primates is distinguished from other groups of mammals in having a suite of characteristics. This means that there is no individual trait that you can use to instantly identify an animal as a primate; instead, you have to look for animals that possess a collection of traits. What this also means is that each individual trait we discuss may be found in nonprimates, but if you see an animal that has most or all of these traits, there is a good chance it is a primate.
Primates are most distinguishable from other organisms in traits related to our vision. Our Order relies on vision as a primary sense, which is reflected in many areas of our anatomy and behavior. All primates have eyes that face forward with convergent (overlapping) visual fields. So if you cover one eye with your hand, you can still see most of the room with your other one. This also means that we cannot see on the sides or behind us as well as some other animals can. In order to protect the sides of the eyes from the muscles we use for chewing, all primates have at least a postorbital bar, a bony ring around the outside of the eye (Figure 5.2). Primate taxa with more convergent eyes need extra protection, so animals with greater orbital convergence will have a postorbital plate or postorbital closure in addition to the bar (Figure 5.2).The postorbital bar is a derived trait of primates, appearing in our earliest ancestors.
Another distinctive trait of our Order is that many primates have trichromatic color vision, the ability to distinguish reds and yellows in addition to blues and greens. Birds, fish, and reptiles are tetrachromatic (they can see reds, yellows, blues, greens, and even ultraviolet), but most mammals, including some primates, are only dichromatic (they see only in blues and greens). It is thought that the nocturnal ancestors of mammals benefited from seeing better at night rather than in color, and so dichromacy is the ancestral condition for mammals. Trichromatic primates are known to use their color vision for all sorts of purposes: finding young leaves and ripe fruits, identifying other species, and evaluating signals of health and fertility.
The primate visual system uses a lot of energy, so primates have compensated by cutting back on other sensory systems, particularly our sense of smell. Compared to other mammals, primates have reduced snouts, another derived trait that appears even in the earliest primate ancestors. There is variation across primate taxa in how much snouts are reduced. Those with a better sense of smell usually have poorer vision than those with a relatively dull sense of smell. The reason for this is that all organisms have a limited amount of energy to spend on running our bodies, so we make evolutionary trade-offs, as energy spent on one trait cuts back on energy spent on another. So primates with better vision are spending more energy on vision and thus have a poorer smell (and shorter snout), and those who spend less energy on vision will have a better sense of smell (and a longer snout).
Primates also differ from other mammals in the size and complexity of our brains. On average, primates have brains that are twice as big for their body size when compared to other mammals. Not unexpectedly, the visual centers of the brain are larger in primates and the wiring is different from that in other animals, reflecting our reliance on this sense. The neocortex, which is used for higher functions like consciousness and language in humans, as well as sensory perception and spatial awareness, is also larger in primates relative to other animals. In nonprimates this part of the brain is often smooth, but in primates it is made up of many folds, which increase the surface area. It has been proposed that the more complex neocortex of primates is related to diet, with fruit-eating primates having larger relative brain sizes than leaf-eating primates, due to the more challenging cognitive demands required to find and process fruits (Clutton-Brock and Harvey 1980). An alternative hypothesis argues that larger brain size is necessary for navigating the complexities of primate social life, with larger brains occurring in species who live in bigger, more complex groups relative to those living in pairs or solitarily (Dunbar 1998). There seems to be support for both hypotheses, as large brains are a benefit under both sets of selective pressures.
Animals with large brains usually have extended life history patterns, and primates are no exception. Life history refers to the pace at which an organism grows, reproduces, and ages. Some animals grow very quickly and reproduce many offspring in a short time frame but do not live very long. Other animals grow slowly, reproduce few offspring, reproduce infrequently, and live a long time. Primates are all in the “slow lane” of life history patterns. Compared to animals of similar body size, primates grow and develop more slowly, have fewer offspring per pregnancy, reproduce less often, and live longer. Primates also invest heavily in each offspring. With a few exceptions, most primates only have one offspring at a time. A group of small-bodied monkeys in South America regularly give birth to twins, and some lemurs can give birth to multiple offspring at a time, but these primates are the exception rather than the rule. Primates also reproduce relatively infrequently. The fastest-reproducing primates will produce offspring about every six months, while the slowest, the orangutan, reproduces only once every seven to nine years. This very slow reproductive rate makes the orangutan the slowest-reproducing animal on the planet! Primates are also characterized by having long lifespans. The group that includes humans and large-bodied apes has the most extended life history patterns among all primates, with some large-bodied apes estimated to live up to 58 years in the wild (Robson et al. 2006).
Figure 5.3: These drawings of the hands and feet of different primates show the opposable thumbs and big toes, pentadactyly, flattened nails, and tactile pads characteristic of our Order. Credit: PrimateFeet by Richard Lydekker, original from The Royal Natural History 1:15 (1893), is in the public domain.
Primates also differ from other animals in our hands and feet. The Order Primates is a largely arboreal taxonomic group, meaning that most primates spend a significant amount of their time in trees. As a result, the hands and feet of primates have evolved to move in a three-dimensional environment. Primates have the generalized trait of pentadactyly— possessing five digits (fingers and toes) on each limb. Many nonprimates, like dogs and horses, have fewer digits because they are specialized for high-speed, terrestrial (on the ground) running. Pentadactyly is also an ancestral trait, one that dates back to the earliest four-footed animals. Primates today have opposable thumbs and, with the exception of humans, opposable big toes (Figure 5.3). Opposable thumbs and toes are a derived trait that appeared in the earliest primate fossils about 55 million years ago. Having thumbs and big toes that go in a different direction from the rest of the fingers and toes allow primates to be excellent climbers in trees as well as to manipulate objects. Our ability to manipulate objects is further enhanced by the flattened nails on the backs of our fingers and toes that we possess in the place of the claws and hooves that many other mammals have. On the other side of our digits, we have sensitive tactile pads that allow us to have a fine sense of touch. Primates use this fine sense of touch for handling food and, in many species, grooming themselves and others. In primates, grooming is an important social currency, through which individuals forge and maintain social bonds.
Lastly, primates are very social animals. All primates, even those that search for food alone, establish strong social networks within species. Unlike many animals, primates do not migrate: they stay in a relatively stable area for their whole life, often interacting with the same individuals for their long lives. The long-term relationships that primates form with others of their species lead to complex and fascinating social behaviors (see Chapter 6). Finally, nonhuman primates show a clear preference for tropical regions of the world. Most primates are found between the Tropic of Cancer and the Tropic of Capricorn, with only a few taxa living outside these regions. Figure 5.4 shows a summary of primate traits.
