Chapter 9: Primate Life History and the Evolution of Intelligence
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 we all possess (Fig. 9-1). 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.

Figure 9.1 A chimpanzee extracting termites.

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: the why for bigger primate brains.
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. Yet, 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 who 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 pay-offs than having more offspring more quickly. Long large-brained lives, however, also result in populations with 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 in 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, though, 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 though, life history traits, like being larger and going through an aging process, tend to cluster together—or correlate—in ways