Share This Page
Personality, Behavior, and the Brain
Primatologists will assure you that chimpanzees have personalities in the same sense that you do. Get to know a chimp’s personality, they will say, and you can predict how it will act in a new situation. This makes many other researchers in the ﬁeld of animal behavior cringe: How can scientists bandy about terms like “personality”? It smacks of anthropomorphism, doesn’t it? Just the sort of talk animal behaviorists (“ethologists”) should avoid at all costs?
Perhaps we can defuse this argument a bit by clarifying our terms. I deﬁne personality differences as consistent, long-term behavioral differences among individuals. If you take risks constantly throughout life, for example, you have a bold personality. If you have had lifelong trouble interacting in social situations, avoid conﬂict, and are withdrawn, it is fair to say that you are shy. Behavioral differences like this are what most psychologists mean by personality variation, and it is found in animal as well as human behavior. In fact, psychologists and biologists alike are turning to animals to help understand personality in the broadest sense because personality differences in nonhumans often have an evident biological basis, including direct links to the brain. For instance, personality differences in silver foxes can be traced to variations in production of dopamine in the left and right halves of the brain’s striatum.
Many animals not nearly as closely related to humans as are chimps (or even silver foxes) display what might arguably be called personality types. For example, if you test individual rats, hyenas, octopuses, sunﬁsh, or even guppies in the absence and then the presence of danger, you observe distinct behavioral types. Shy sunﬁsh avoid risky situations, such as those associated with exploring ﬁsh traps; bold sunﬁsh do just the opposite. This one basic difference translates into bold and shy ﬁsh behaving differently enough in ponds that they eat different foods and are infected by different parasites. Study a large group of hyenas long enough and you will see consistent differences among individuals in terms of assertiveness, excitability, and sociability. Watch a group of guppies and you will see risk takers and inhibited males. The same sort of differences have been found even in invertebrates: Controlled experiments have uncovered anxious/calm and bold/inhibited personalities in Octopus rubescens. Some octopuses are willing to take risks and inspect danger, but others are not.
Nor are personality differences limited to the bold-shy continuum. Animals can be cooperators or cheaters, ﬁghters or non-ﬁghters, producers or scroungers. For example, in many species producers ﬁnd and procure food, while scroungers are parasites, surviving on the food the producers have uncovered. Individuals are very consistent in terms of where they fall on the producer/scrounger continuum, usually settling at one extreme or the other.
In most cases, individuals in a single population of a single species are the focus of personality research in animals. On occasion, however, individuals in one species are compared to individuals in another in order to provide additional perspective on behavioral traits. We shall see, for example, that consistent differences in the food hoarding personality can be seen across an array of bird species and that such differences can be tied directly to brain structure and function.
In this photo essay, we look brieﬂy at half-a-dozen personality variations and the species in which they have been investigated. In each case, something is known about the biological basis of the variation in question and how the brain may be involved. Together, they provide intriguing suggestions that may inform the search to understand the genetics and neurobiology of human personality.
Foragers and Homebodies
The life of a honeybee is regimented, with a bee’s role tied tightly to its age. Individuals typically spend the early part of their lives working in the hive and the later part foraging outside the hive.
Seeking the possible biological underpinnings of this developmental shift to the role of forager, Daniel Toma and his colleagues at the University of Illinois focused on a gene called per that had already been identiﬁed as important for developmental changes in the fruit ﬂy, Drosophila melanogater. Speciﬁcally, they measured brain messenger RNA (mRNA) levels of per in three sets of laboratory-raised bees (4 to 6, 7 to 9, and 20 to 22 days old) and in bees from a single, free-living hive. As compared to younger bees, who typically remained at the nest, older bees who foraged for food and brought it back to their colony had signiﬁcantly more per mRNA in their brains.
The researchers wanted to know, however, whether increases in per mRNA were strictly associated with age, or whether per mRNA and foraging might be linked even if age could be removed as a factor. It turns out that precocious foragers are sometimes found in bee hives. These individuals begin searching for food outside the nest at about seven days old, displaying the long-term behavioral trait we might call food gatherer. Precocious foragers had the same per mRNA levels in their brains as typical (older) foragers, suggesting a fundamental link between per mRNA and foraging in general and in particular between per mRNA and the food-gatherer personality.
The arrow of causality here is still somewhat obscured, as the research was not designed to determine whether increased per mRNA leads to foraging or foraging produces high per mRNA levels. To do that would require manipulating per mRNA levels in bee brains, a project for future scientists.
How an animal copes with sources of stress—everyday or extreme—can have a signiﬁcant impact on its health. Based on many reports on research into animal stress and coping, Jaap Koolhaas and his colleagues at the University of Groningen in Holland identiﬁed two coping styles: proactive and reactive. Proactive personalities tend to be aggressive, territorial, and bold; reactive personalities are less aggressive, less territorial, and shy.
In one series of studies, Frans Sluyter and his colleagues at the University of Paris studied genetic strains of rats that were selected for either proactive or reactive personalities. When tested in an intruder experiment in which another male was placed in an individual’s home cage, proactive male rats were much more territorial and aggressive than reactive rats. The rats were also tested in a “defensive burying” experiment. Here, a small electric prod is placed in a male’s cage. If the rat investigates and touches the prod, it receives a mild shock. Once shocked, the rat has essentially two ways to avoid future shock: It can either bury the prod under its bedding (an active response) or signiﬁcantly curtail its own movements. Proactive rats were far more likely to bury the prod than were reactive rats. This defensive burying seemed to be associated with high blood concentrations of the neurotransmitter noradrenaline and low concentrations of the neurotransmitters adrenaline and corticosterone. Reactive rats, on the other hand, typically have low concentrations of noradrenaline and high concentrations of corticosterone.
Speaking more generally, there is a consistent link between personality and the neuroendocrine system. Proactive rats showed high reactivity of the sympathetic nervous system, low reactivity of the parasympathetic nervous system, and low activity of the hypothalamic-pituitary-adrenal axis. Related observations have been made in other species, including domesticated pigs and cows.
Proactive and reactive personalities in a wide range of species correlate with different susceptibilities to medical problems. Because of their strong sympathetic nervous system activity, for example, proactive animals are more susceptible to hypertension, atherosclerosis, and autoimmune disease (an animal analogue to multiple sclerosis). As we learn more about the medical implications of animal personality, it becomes obvious that this research has practical implications for humans.
Hiding and Finding 6,000 Seeds
Although most research on animal personality is undertaken in a single species, our next example compares different species to get at the possible basis in the brain of variations in hoarding behavior. A case in point is the bird family Corvidae, which includes species such as crows, magpies, and jays. Here the variations in behavior are from species to species, and they are quite remarkable. Some species store no food at all; others rely on the food they have stored for their survival over winter. Could this difference in behavior be related to structural differences in the brain?
Oxford University researchers Susan Healey and John Krebs examined seven species of corvid birds for their food-storing abilities and the size of their hippocampus, a brain area associated with memory. The hippocampal region of the bird brain is known to be associated with food retrieval, and increases in size in this region have been hypothesized to be a result of natural selection operating on food-storing abilities.
The scientists examined hippocampal size in two species that rarely, if ever, store food, jackdaws and alpine choughs; four species in which food storing plays some role (rooks, European crows, European magpies, and Asian red-billed blue magpies); and one species, European jays, in which food storing plays such an important role that a jay must remember the location of 6,000 to 11,000 seeds for as long as nine months. They found that the more food-storing behavior in a species, the greater the hippocampal volume, strongly supporting a brain-based, biological explanation for the differences in hoarder personality types across bird species. Similar results were found in a cross-species study of passerine (perching) birds, such as warblers and sparrows. The volume of the dorsomedial cortex region of the hippocampus (relative to other brain areas) was greater in hoarding passerine birds than in those species that do not store food.
Mating: The Independents vs. the Satellites
While studying mating strategies of the male ruff bird (Philomachus pugnax), David Lank and his colleagues at Simon Fraser University in Canada discovered two distinct personality types. Independent males consistently stake a claim on mating areas known as “leks” and actively guard their own parts of the lek from other independent males. Satellite males, by contrast, do not have their own spots but temporarily share those of independent males, forming an uneasy alliance with them. Nor is mating strategy the only way independent and satellite males differ; satellites are generally smaller and have lighter plumage.
Exploring the biological basis of these two personality types, Lank and his colleagues reared ruff chicks collected from eggs at 43 nests in Finland. They observed behavior at the nest to identify the mother of the chicks and used DNA testing to identify the father. The hatched chicks were raised under controlled conditions and, as adults, were observed to determine whether they became independents or satellites. Independent males had sired 27 sons, and of these 25 developed into independent males themselves. Fourteen of the 30 sons of satellite males matured into satellites, not as high a ﬁgure as for independents, but still fairly high for a complex behavioral trait. It appears that males, at least in part, inherit their mating strategy from their fathers.
Through genetic analysis, Lank and his colleagues determined that male mating strategy was not inherited via the sex chromosome. Since the satellite and independent personality types in males are not sex-linked, then in principle it should be possible to produce these personality traits experimentally in females (where they are normally absent). So the researchers gave testosterone implants to females whose pedigree they knew and observed their behavior to determine whether this would bring out the independent and satellite personality types. Two days after the implants, female ruffs were displaying typical male behavior, and a week later they were forming leks. More importantly, females were exhibiting the same personality trait (satellite or independent) as their male relatives, further strengthening the claim that this personality trait is genetically controlled. How such genetic control is related to brain structure and function remains a fascinating, but unexplored, area of research.
Silver Foxes: Domestication and Dopamine
Over some four decades, Lyudmila Trut and her colleagues at the Russian Academy of Sciences have experimented with the effects of domestication on silver foxes, work initiated in the 1940s by Russian geneticist Dmirtri Belyaev. Belyaev believed that evolutionary changes in behavior caused concurrent changes in an animal’s physical form, immunology, endocrinology, and brain structure. For example, controlled breeding of the silver foxes selected for tameness produced not only increasingly tame offspring but individuals with ﬂoppy ears and shorter tails, different skull dimensions, different color patterns, and reduced levels of the brain chemicals known as basal corticosteroids. What is more, the tame foxes reproduce earlier and produce more offspring per litter than their wilder counterparts.
This study, known as the Farm-Fox experiment, has also examined the concept of personality as it relates to brain neurotransmitter systems. Using different lines of domesticated foxes selected for either tame or aggressive personalities, Trut and her colleagues discovered that both groups show differences from wild foxes in their brain levels of the neurotransmitter dopamine. Comparison to wild foxes, a higher level of dopamine was found in the right half of the striatum of domesticated foxes selectively bred for aggressive personalities, and in domesticated foxes bred for tame personalities, the increased dopamine was uncovered in both the left and right halves of the striatum. It appears that artiﬁcial selection for personality type in the silver fox has produced signiﬁcant differences in the brain chemistry that underlies personality.
Trut and her colleagues suggest that the increased dopamine levels in the right halves of the striatum of both aggressive and tame domesticated foxes may have occurred because both types of fox have been bred for stronger emotional responses than occur naturally in wild foxes. The increase in dopamine speciﬁcally in the left half of the striatum of tame foxes and not found in the aggressive line may result from a deterioration of the pituitary-adrenal system, which plays a fundamental role in aggressive behavior.
Our last example of a personality trait is dominance, and our animal example is the chimpanzee. Based on personality measures developed for studying humans, Alexander Weiss and his collaborators at the University of Arizona have characterized a dominant chimp as independent, decisive, intelligent, persistent, bullying, and stingy. This is a personality proﬁle with both costs and beneﬁts. Dominant chimps are more likely than subordinate chimps to engage in aggressive interactions, which may prove dangerous, but they are also more likely to emerge victorious from such aggressive encounters, reaping rewards associated with victory, such as food and mating opportunities.
What makes work on chimpanzee personality both complex and intriguing is that genetics and culture are both signiﬁcant. In the case of dominance, Weiss and his colleagues determined that about two-thirds of the variation in this trait from individual to individual could be attributed to genetic causes, so biology plays a strong role in this instance. Scientists do not know, as yet, how genetic endowment translates into differences in brain function that presumably underlie differences in dominant and subordinate behavior.
Other traits of chimps appear to be shaped largely by culture. Researchers in seven extended studies of wild chimp populations have compiled a list of 65 behaviors that qualify as cultural variants and that are probably spread by imitation in chimpanzees. These variants, including some that might well be considered aspects of personality, ranged from using leaves as sponges, to picking out bone marrow, to dancing at the start of a rainstorm. Of the 65 behaviors, two-ﬁfths were present at some sites but completely absent at others, reinforcing a cultural explanation. We are far from understanding how genetic and cultural factors interact in shaping chimpanzee personality, but the very complexity of causation makes the ﬁeld an intriguing complement to exploration of human personality.
Understanding Human Personality
Personality pervades human social interaction. It affects who we choose as friends or coworkers and even which family members we like. Some of us, for example, prefer bold risk-takers as associates, while others prefer shy compatriots. But the situation is far more complicated, because who we choose as associates is inﬂuenced not only by their personalities but by ours. Scientists, however, are beginning to understand the genetics and biology of human personality.
Recent research has demonstrated how some human personality variation derives largely from brain structure and function. Studies of humans are based on sampling only one species, of course—homo sapiens. Fortunately, as we have seen, some of the same personality variables that are a staple of human personality research also can be measured in nonhuman species, providing additional information that can illuminate the human research.
Studying personality remains a daunting task, but with work on humans and animals in high gear, fundamental understanding of the evolutionary, neuroendocrinological, and psychological underpinnings of personality edges ever closer.