The Measurable Brain

In this chapter, in order to provide an understanding of the data on which the comparative analyses are based, I describe the methods that have been most commonly used to collect data on brain size. The key to the success of a comparative evolutionary analysis is sample size, the larger the better. To answer the question ‘why do some animals have big brains?’, brains have been measured directly in a variety of ways and brain size has also been inferred (in around half of the analyses) from skull volume. I describe the most commonly used methods briefly, along with the pros and cons of their use and interpretation.

The Social Brain

The first discussion of a relationship between sociality and intelligence came in the middle of the twentieth century, especially by Humphrey who suggested that living socially demanded intellectual abilities above and beyond those required by an animal’s ecology. This led to the Social Intelligence Hypothesis, and then the Machiavellian Intelligence Hypothesis, both proposing that sociality was the main driver of the superior intellect of primates, especially humans. Two key challenges for this hypothesis are that sociality is difficult to quantify and cognition is not well tested by problem solving. More importantly, as data from more species have been examined, the analyses increasingly fail to show that sociality explains variation in brain size, even in primates. I conclude that appealing as this hypothesis is, it does not do a very compelling job of explaining variation in brain size.

The Sexual Brain

Morphological and behavioural differences between the sexes are ubiquitous across the animal kingdom. There is also good evidence for differences in some brain regions between males and females, in humans, some rodents, and many songbirds. I look at the data for sex differences in cognition, of which there are some that show differences in spatial cognition and in hippocampal structure, at least some of which may be explained by variation in hormone levels. The thesis of The Mating Mind by Geoffrey Miller considerably increased interest in using sexual selection to explain variation in brain size. From female mate choice, male–male competition, sperm competition, mating strategy, to parental care, there are some data that appear to support selection acting on one species rather than the other in sexually a selected manner but I conclude that the data are not generally supportive of the Sexual Brain Hypothesis.

The Innovative Brain

Not least because humans are inveterate inventors, innovation is considered to be a reason that other animals have big brains. The Innovative Brain Hypothesis really came into being when Lefebvre et al. suggested that innovation could be a way to assess cognitive abilities in wild animals, which tend not to be amenable to cognitive testing in the traditional, laboratory context. The collection of a data set of foraging innovations in birds (and a similar data set later collected for primates) has provided opportunities to test the main hypothesis in this chapter. Innovations were later divided into technical and non-technical innovations, with the former perhaps leading to more correlations with brain size than the latter. I conclude that this is a very appealing hypothesis but that the evidence is not especially persuasive.

The Ecological Brain

In this chapter, I examine the evidence for a role for the preeminent selection pressure, ecology, in shaping animal brains and in causing changes in brain size within and among species. I describe what ‘ecology’ has meant in comparative analyses, e.g. foraging, range size, and life history variables. I provide evidence for a clear association between ecology and the size of sensory-motor brain regions and go on to use the relationship between space and the hippocampus to show the generality of this relationship beyond food storing. I discuss the strength of the data showing that migration, foraging, and domestication have caused changes in brain size. I conclude that while there is evidence of domestication, in particular, having changed whole brain size, it is at the level of brain regions that there is overwhelming evidence for an effect of ecology on brain size.

Introduction

This brief introductory chapter begins with the key question to be addressed in the book: why does brain size vary among animal species? It contains a short outline of the book’s contents and establishes the rationale for the examination of the evidence that has been gathered using the comparative method over the past five decades. I explain that the book will be both a review and a critique of the work that has attempted to explain which natural selection pressures led to changes in brain size. This is a focus that, to a large extent, excludes work that addresses mechanistic explanations for brain size.

The Intelligent Brain

In this chapter I address the question as to why brain size and its variation is interesting. Because for many the answer is that a bigger brain endows greater intelligence, in this chapter I first provide a definition of intelligence, or cognition. I then describe a variety of factors that need to be considered in order to attempt to make comparisons, within or among species, of cognitive abilities. These include motivation, environmental factors, and methodologies used to train and test animals (typically) in the laboratory. For the remainder of the chapter I use the demonstration of the relationships between food storing, spatial cognition, and the hippocampus to illustrate these issues.

The Technical Brain

In this chapter I discuss the data for tool use having driven increases in brain size. Because humans habitually make and use tools and because hominid brain size appears to have increased around the time that we see tools in the fossil record, tool use has been suggested to be key to increasing brain size. As an increasing number of animals are being shown to use tools, and sometimes to make them, there is an opportunity to use the comparative method to examine whether tool making really has led to brain size increases. I discuss issues with attributes of tasks used to test physical cognition and propose that nest building is a plausible model behaviour with which to look at all aspects of physical cognition, including its neural bases. I conclude that the data are far too few to give much support to the Technical Brain Hypothesis.

Concluding Remarks

In this chapter, I conclude that ecology has been shown to explain variation in the size of brain regions in multiple species, which is not the case for any of the other hypotheses. I go on to suggest the steps that need to be taken to collect the requisite data: collecting data on size of brain regions with identifiable function, choosing the appropriate cognitive test and collecting data from appropriate species, better quantification of ecological factors along with data collected from individuals that differ in age, sex, and geographical location, and demonstration that better cognition confers fitness benefits. Each of these is in both principle and practice feasible, if challenging to assemble for one hypothesis/taxonomic group. With these data, we may eventually be able to shed light on what has caused human brains to become relatively large.