Chapter 8: Primate Life Histories and the Evolution of Intelligence
Chapter Review
Why Are Primates So Smart?
Behavioral complexity is the hallmark of the primate order, and this has been attributed in part to the relatively large brains all primates possess. Defining intelligence, however, is a highly problematic issue. An operational definition used here attributes the primary component of intelligence to flexible problem solving and the ability to cope with novel situations.
Arriving at a consensus for the driving factors favoring intelligence in primates is even more difficult. Many theoretical positions have been advanced as possible selective mechanisms for the trends toward increased intelligence in primate evolution, a few of which are highlighted later in this summary. Some hypotheses emphasize complex foraging strategies and ecological pressures as the primary forces driving an evolutionary increase in cognitive abilities, and others suggest that increased social complexity favored the evolution of primate intelligence.
Life History Theory
Before we can examine these theories, however, a discussion of the basics of life history theory is warranted. Life history theory directly addresses the kinds of evolutionary “bargains” organisms are engaged in to achieve a selective advantage in their environments. So, since a large brain correlates with greater intelligence, we first have to examine how larger brains can evolve. Primates would not have been able to evolve larger brains if there was no selective advantage to doing so, yet a large brain is also a massive consumer of bodily resources. Additionally, there is also a correlation between having a larger brain and living a longer life. How do all these associations occur or develop? For starters, many animals maximize their reproductive potential by maturing fast and having a number of offspring at a relatively young age. Animals that mature faster are also typically smaller, have smaller brains, experience high mortality, and have generally short life spans. Other animals, like elephants, do not conceive for the first time until they are 10 years old or older. Elephants also have long pregnancies and typically have one offspring at a time. For larger animals with larger brains—many primates included—spending long periods of time maturing and investing in just a few offspring have better evolutionary payoffs than having more offspring more quickly. Populations of large-brained animals who live long lives, however, contain old individuals.
Senescence, or aging, occurs in all animals. Systems and processes slow down, with physiological deterioration eventually culminating in death. While death itself is the irreparable breakdown of organic systems, the inevitability of death is not totally uncontested. All organisms heal wounds; some even regenerate whole limbs (frogs) or entire bodies (starfish). Some asexually reproducing organisms do not undergo a period of senescence at all. Why, then, has natural selection not produced an organism that can live indefinitely? Why do organisms grow old?
It is important to keep in mind that the effects of natural selection weaken with age. That is, selection that eliminates young individuals has much more of an impact on future generations than does selection that removes old individuals. This has to do with the reproductive potential of each individual: older individuals have much less reproductive potential left, and their deaths make little mark in a strict Darwinian sense. That being the case, one hypothesis for senescence suggests that the effects of genes favoring youthful fertility also act to decrease longevity. Thus, genes producing more of these prolific youths would become more common in the population.
In other words, individual life history traits, such as being larger and going through an aging process, tend to cluster together—or correlate—in ways that make sense to maximize reproductive potential. You can either mature faster, have several offspring at an earlier age, and die young, or mature more slowly, have a few offspring that you invest a great deal of time in, and die older.
Primates then, tend to be at the slower maturation end of a life history continuum, especially relative to other mammals. Yet within the primate order there is a great deal of diversity in life history traits. For example, lemurs and other prosimians mature more slowly in comparison to other mammals, but in comparison to monkeys and apes they mature faster. Selection pressures have influenced primate development, maturation, and reproduction for the entire history of the order, with important shifts occurring when relatively smarter monkeys and then apes evolved.
Selection, Large Brains, and Intelligence
Now that we know a little bit more about how large brains articulate with other aspects of life history, how does having a large brain translate into intelligent behavior? Again, researchers have explained why and what primates know and how they express their intelligence in a number of ways.
Foraging Hypotheses
Several researchers—notably Katharine Milton (University of California at Berkeley), Katherine Gibson (University of Texas), Sue Parker (Sonoma State University), Simon Reader (Utrecht University), and Kevin Laland (University of St. Andrews)—have attributed the enhanced cognitive capabilities of primates to the rigors of foraging. Milton describes how monkeys that feed mainly on fruit need to evaluate the ripeness, nutritional content, and toxicity of their finds. Because this resource is only available during specific seasons in certain geographic areas, frugivores need to have a better memory, capacity for learning, and ability to plan a course of action to find food. Selection for memory and efficient foraging techniques would definitely prove to be an advantageous attribute.
Another foraging hypothesis suggested by Gibson and Parker states that the extraction of embedded resources provided the impetus for the evolution of primate intelligence. Extracted resources—foods that are hidden from sight and often difficult and time-consuming to exploit (for example, roots and underground tubers)—usually require delicate manipulation and processing to recover successfully. In addition, many of these resources provide essential nutrients during the dry season. The ability to augment dietary intake with nutritious, hard-to-reach resources may also have played a part in the increasing cognitive capacity of the primate order. A third model developed by Reader and Laland states that natural selection favored behavioral flexibility—the ability of a primate to solve a variety of new or novel problems presented to it. Primates that benefited from their problem-solving abilities were better able to deal with ecological and social changes.
Social Intelligence Hypotheses
An alternative view considers group living to be the primary force behind the evolution of behavioral complexity and intelligence. One of the proponents of this idea—Robin Dunbar—considers the relationship between group size and relative size of the neocortex of the brain, under the assumption that neocortical increases should be observed in groups with more members. As group size increases, so does the complexity of social dynamics among its members; there are more individuals (and their past actions) to remember and more competition for resources.
The idea that social manipulation, including deception, has been instrumental to the development of cognitive complexity is a theory proposed by Andrew Whiten and Richard Byrne. Deception and alliance formations are but two of a vast array of techniques used by members of the primate order to manipulate other individuals in their social group. Furthermore, the core concept behind social manipulation and the initial definition of intelligence—flexible problem-solving behavior in novel situations—may be quite similar: both require a conscious manipulation of elements and a rudimentary knowledge of the relationship between different elements.