7.1.4: Brains, Development, Behavior

Big Brains

Primates have notably large brains relative to their body size. On average, a primate's brain is about twice as large as you would expect for a mammal of the same size. The visual processing centers are especially enlarged, reflecting our dependence on sight. The neocortex, the region associated with higher-level thinking, sensory perception, and spatial awareness (and in humans, language and self-awareness), is also proportionally larger in primates than in other mammals. In many non-primates, the neocortex is relatively smooth, while in primates it is folded into many ridges and grooves, which dramatically increases its surface area.

Two competing hypotheses attempt to explain why primate brains got so big. One focuses on diet: finding and processing fruits requires more cognitive work than eating leaves, and fruit-eating primates do tend to have larger relative brain sizes (Clutton‐Brock and Harvey 1980). The other hypothesis focuses on social complexity: living in large, dynamic social groups requires keeping track of many relationships, alliances, and rivalries, which demands significant brainpower (Dunbar 1998). Current evidence suggests both pressures likely played a role.

Life in the Slow Lane

Animals with large brains tend to grow slowly, reproduce less frequently, and live long lives, and primates are no exception. Compared to other mammals of similar body size, primates take longer to mature, have fewer offspring per pregnancy (usually just one), wait longer between births, and live significantly longer. A few exceptions exist. Some small New World monkeys regularly give birth to twins, and certain lemurs can produce multiple offspring at once, but these are outliers in the primate world.

A few exceptions exist. Some small New World monkeys regularly give birth to twins, and certain lemurs can produce multiple offspring at once, but these are outliers in the primate world. Among primates, the orangutan takes the slow strategy to an extreme: females typically give birth only once every seven to nine years, making them the slowest-reproducing mammals on Earth.

The payoff for this slow pace is longevity and deep parental investment in each offspring. Some large-bodied apes are estimated to live up to 58 years in the wild, and among great apes, offspring may stay close to their mothers for a decade or more after birth (Robson et al. 2006).

Social Lives

Primates are intensely social animals. Research has shown that strong social bonds in primates are linked to lower stress, better health, and greater reproductive success (Silk et al. 2009). The long-term, stable social relationships primates form over their lifetimes lead to some rather complex and fascinating behaviors that we’ll learn about in more depth in the next chapter.

Behavioral Adaptations

Chapter 9 is dedicated to primate behavior, so this is just a brief summary of the 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.