Evan MacLean and colleagues write this week in PNAS about the evolution of self-control.
As a blogger, I have little interest in the subject.
But I read the paper closely, because presents an important analysis of something well beyond self-control: the relationship of brain size and cognition across species. The list of authors includes a huge array of experimental psychologists and animal behaviorists, representing the thirty-six species considered in the analysis – everything from scrub jays and pigeons to elephants and aye-ayes. In all these species, the experimenters assessed performance on tasks related to self-control. They found that the species’ performance was predicted by brain size. Bigger brained species tended to exhibit greater ability to control their immediate responses to stimuli in favor of a previously learned behavioral routine.
Self-control and cognition
It may not be intuitive that self-control is such an important aspect of cognition. Self-control includes the ability to inhibit responses to immediate stimuli, in favor of more useful or adaptive learned behaviors. Being able to pause and consider the best response to a situation is essential to higher cognitive abilities.
This study involved two kinds of experiments, in both of which animals learned to obtain a food reward in a certain way, but then were presented with an alternative scene in which a food reward was prominently visible to them but not immediately attainable. Animals who lack self-control immediately go for the new food stimulus, even though they can’t get it. By contrast, an animal who is not overly distracted by the new stimulus, and goes back to the learned pattern to obtain the reward, is said to exhibit self-control.
As the authors describe, self-control defined in this way is relevant to fitness and an important component of being able to rely on learning to modulate behavior:
We chose to measure self-control—the ability to inhibit a prepotent but ultimately counter-productive behavior—because it is a crucial and well-studied component of executive function and is involved in diverse decision-making processes (167–169). For example, animals require self-control when avoiding feeding or mating in view of a higher-ranking individual, sharing food with kin, or searching for food in a new area rather than a previously rewarding foraging site. In humans, self-control has been linked to health, economic, social, and academic achievement, and is known to be heritable (170–172). In song sparrows, a study using one of the tasks reported here found a correlation between self-control and song repertoire size, a predictor of fitness in this species (173). In primates, performance on a series of nonsocial self-control control tasks was related to variability in social systems (174), illustrating the potential link between these skills and socioecology. Thus, tasks that quantify self-control are ideal for comparison across taxa given its robust behavioral correlates, heritable basis, and potential impact on reproductive success.
So what did they find? Bigger brains correlate with greater self-control:
Our phylogenetic comparison of three dozen species supports the hypothesis that the major proximate mechanism underlying the evolution of self-control is increases in absolute brain volume. Our findings also implicate dietary breadth as an important ecological correlate, and potential selective pressure for the evolution of these skills. In contrast, residual brain volume was only weakly related, and social group size was unrelated, to variance in self- control. The weaker relationship with residual brain volume and lack of relationship with social group size is particularly surprising given the common use of relative brain volume as a proxy for cognition and historical emphasis on increases in social group size as a likely driver of primate cognitive evolution (85).
Absolute, not relative brain size
Biologists have been interested in the comparative study of brain size and cognition for more than a hundred years. Eugene Dubois, famous for finding the “Pithecanthropus erectus” skullcap and femur at Trinil, Indonesia, was the first to consider explicitly the relation of relative brain size and intelligence. From his point of view, the problem was the inverse of the one we generally face. Dubois did not know the relationship between body size and brain size across mammals. He knew that larger mammals had larger brains, and that the relationship is not linear. From that, he was interested in working out the relationship of brain size and body size between animals with equal intelligence.
If you blew up a Hyracotherium to the size of a horse, its brain would be much larger than a horse’s brain. The evolutionary history leading from small bodies to larger bodies in the horse lineage was accompanied by a relative reduction in the size of the brain – and that is true across every group of mammals. Blow up a capuchin monkey to human size, and its brain would be bigger than ours. Larger animals have absolutely bigger brains but relatively smaller brains. At least, if we expect that the brain should increase in a linear fashion.
One way of looking at this is that larger animals tweak greater efficiency out of brain tissue than smaller animals. Another way of looking at it is that the brain consists of two parts: a part with functions that depend on body mass, and a part with functions that are independent of body mass. Growing the body requires the first to increase, but not the second.
Dubois wanted to determine just how much the brain ought to expand as body size increases. He had a very practical example – Pithecanthropus. Its brain was considerably larger than the brains of living apes, but was it unusually so for its body size? Consider cats and lions as examples – closely related within the same family, but extremely different in body size. In Dubois’ way of thinking, cats and lions have similar cognitive abilities, similar intelligence. The differences in brain size between the two species should therefore be a simple reflection of body size. Absolute brain size would not be an indicator of differences in cognitive adaptations. Instead, we should pay attention to the brain size relative to what is expected for an animal of the same body size.
During the twentieth century, the most prominent name in the systematic study of brain size in different kinds of organisms was Harry Jerison. His groundbreaking studies of allometry culminated in the 1973 book, Evolution of the Brain and Intelligence.
Jerison proposed a model in which the cognitive specializations of some species could be explained by “extra neurons” – the quantity of brain tissue above that predicted for the body size of those species. To express the relative brain size of a species, Jerison used the term, “EQ”, or “encephalization quotient.” The EQ is a measure of the brain size of a species, relative to the size expected for species of the same body mass when comparing across many related species.
The extra neuron hypothesis was centered around the problem of absolute versus relative brain size. If more brain matter was associated with more intelligence, we have a problem explaining why elephants and whales are not smarter than people, with our absolutely puny brains. A good hypothesis is that the relative brain size, and not the absolute brain size is important to cognitive adaptations. Humans have brains that are absolutely smaller than those of whales, but we have brains much bigger than other mammals that have our medium-sized body mass. Humans have a very high EQ.
Here’s a hitch: Whales and elephants are very smart animals. Sure, people are smarter, but comparing every species to the single exceptional example doesn’t tell us about the general trend.
MacLean and colleagues looked at one particular aspect of cognition, self-control, in a way that controls for phylogenetic relationship. Animals like elephants and scrub jays who are smarter than their immediate relatives, tend to have bigger brains. And across species, it is the absolute size of the brain that predicts their that hypothesis to be false, at least for self-control. The absolute brain size predicts the performance of different species on the tasks examined here. Relative brain size doesn’t.
Given a bunch of species with lion-sized brains, and a bunch with cat-sized brains, this study should lead us to expect that the lion-sized brains have greater self-control. Lions and cats aren’t cognitively equivalent; lions are smarter than cats.
Group size isn’t important
Considering the history of study of brain size and cognition, I find one aspect of the study more surprising than the rest. These tasks related to self-control do not correlate with group size, at least not in the primate species studied here.
A long literature has been developed on the idea that group size has driven the evolution of primate cognition. This literature mostly depends on the observation that primates with larger group sizes have relatively larger neocortex sizes. That “social brain hypothesis” intuitively makes sense: many of the determinants of fitness in social mammals involve predicting or adjusting to the social behaviors of other individuals. That is why it has gotten the nickname “Machiavellian intelligence” – the idea is that natural selection has crafted our brains to manipulate other individuals.
Self-control seems like an important skill for social life, providing a way for learned social behaviors to emerge in the buzzing, blooming world of social stimuli. Yet across primates, social group size had no predictive value for self-control. The authors suggest that social group size may still be a factor selecting for cognitive abilities in primates, just not the ones they tested for:
With the exception of dietary breadth we found no significant relationships between several socioecological variables and measures of self-control. These findings are especially surprising given that both the percentage of fruit in the diet and social group size correlate positively with neocortex ratio in anthropoid primates (86, 142). Our findings suggest that the effect of social and ecological complexity may be limited to influencing more specialized, and potentially domain-specific forms of cognition (188–196). For example, among lemurs, sensitivity to cues of visual attention used to outcompete others for food covaries positively with social group size, whereas a nonsocial measure of self-control does not (146).
This is a claim that disparate cognitive abilities have been selected for their effects in very contextually narrow environments; self-control in foraging tasks being very narrowly targeted toward foraging; visual attention to other individuals being narrowly targeted toward social competition. The implication is that selection can fine-tune different cognitive functions independently of each other; otherwise known as the “modularity” hypothesis for cognition.
Instead of group size, MacLean and colleagues found that primates with broader diets have more self-control:
Within primates we also discovered that dietary breadth is strongly related to levels of self-control. One plausible ultimate explanation is that individuals with the most cognitive flexibility may be most likely to explore and exploit new dietary resources or methods of food acquisition, which would be especially important in times of scarcity. If these behaviors conferred fitness benefits, selection for these traits in particular lineages may have been an important factor in the evolution of species differences in self-control.
Are primates smart because they learned to forage for specialized foods? That would be consistent with what we know about tool use. It’s also consonant with the experimental evidence on other aspects of cognition. It’s a provocative idea. Although group size is related to neocortex size across primates, there are many notable exceptions who have small groups and large neocortices, or vice-versa.
For example, the apes are very smart, have high diet breadth, and a huge diversity of group sizes in their natural habitats – chimpanzees have large groups, gorillas small groups, and orangutans are often solitary. It may not be surprising to see that diet breadth has a much more stable association with cognitive adaptations, with group size being a flexible response to different habitat types, levels of predation, and other factors.
Can we draw any conclusions about human evolution from this study? It’s hard to generalize – just as we cannot conclude much about cognitive evolution from comparing whales and elephants only to humans, we can’t predict much about humans by looking only at the broadest phylogenetic pattern. Yet, the importance of foraging and diet breadth to the evolution of primate cognition is provocative. The study seems to weigh in favor of tool use and foraging as primary drivers of human brain evolution, instead of social dynamics.
But that contrast is surely misleading. Human tool use and foraging depend on social cooperation and social learning. Our foraging strategy is principally a social strategy. Broad comparisons across primates are unlikely to tell us much about the uniquely human aspects of our evolutionary history. Instead, we’ll have to depend on archaeology.
This kind of study is the future of the study of animal intelligence and cognitive evolution. MacLean and colleagues carried out similar experiments across a broad group of species, allowing them to compare those species in a phylogenetic framework. And they showed very clear differences between the predictive power of relative and absolute brain size. This wasn’t a mishmash of results, the results were very clear because of the power of the analysis.
MacLean EL, Hare B, Nunn CL and many others. 2014. The evolution of self-control. Proceedings of the National Academy of Sciences, USA (in press) doi:10.1073/pnas.1323533111