IN SEARCH OF BEHAVIORAL INDIVIDUALITY David P. Barash

University of Washington

Living things are not created identical: In sexually reproducing species, individuals--except monozygotic twins--are different. Although widely acknowledged, behavioral individuality has received relatively little empirical or theoretical attention. Yet it seems likely that research focusing on individual differences will yield important insights for evolutionarily minded students of behavioral biology, including those interested in better understanding Homo sapiens. KEYWORDS: Evolutionarily stable strategies; Individuality; Individual differences; Marmots; Sociobiology; Variance.

We need a general theory of individual differences. Perhaps this is an oxymoron, since to recognize individual differences is to celebrate particularity, whereas any general theory must, by definition, submerge the individual case in a wider sea of pattern. Indeed, attention to individual differences runs the risk of being itself "unscientific," insofar as science aims at generalizing, at raising our heads above the individual trees to recognize (and generalize about) the forest. Yet the need persists. One of the major unspoken secrets in the study of animal behavior and ecology (and physiology, psychology, psychiatry, anthropology, and sociology as well) is that individuals are nearly always different from each other, and that--to an extent rarely admitted, and virtually never p u r s u e d - - o u r generalizations tend to hush up these differences (Clark and Ehlinger 1987). It can be argued, of course, that this is what general Received December 5, 1996; accepted January 9, 1997.

Address all correspondence to David P. Barash, Department of Psychology, Box 351525, University of Washington, Seattle, WA 98195. E-mail: [email protected] Copyright 9 1997 by Walter de Gruyter, Inc., New York Human Nature, Vol. 8, No. 2, pp. 153-169.

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izations are, statements that apply to a larger class of phenomena, and which must, by definition, do violence to individuality. But insofar as science seeks to explain observed phenomena, it should also be able to explain even the particularity of such phenomena, no less than the generality of which they are a part. In fact, generalities lose potency if they occur at the cost of artificially leveling otherwise significant features of a species' evolutionary landscape. Whereas every pet owner knows, for example, that individual dogs, cats, or horses are not interchangeable, biological science often proceeds as though they were. The problem is precisely the opposite of missing the forest for the trees: missing the trees because of our eagerness to describe the forest. It is undeniable, however, that the "trees" which make up the forest of life are different from each other. No geneticist will dispute that in every sexually reproducing species, the individuals possess distinct genotypes (monozygotic twins excepted). In the best-studied species, Homo sapiens, we know that individuals differ even in apparently nonadaptive traits such as fingerprints, as well as in physical appearance and personality. Field biologists can often distinguish individuals among their study animals, by distinct physical and/or behavioral traits. It also seems likely that such individuality is at least as apparent to the animals themselves. Medical science is unusual in that it has long acknowledged the importance of individuality among its subjects (Bearn 1993). Thus, in their training, physicians are repeatedly urged to treat the patient, not the disease. Although there are "typical" syndromes and basic commonalities among organ systems as well as their ailments, good physicians know that individual Homo sapiens may, for example, develop tuberculosis without fever, or idiosyncratic unresponsiveness or hyperresponsiveness to certain drugs. This is why The New England Journal of Medicine as well as the various specialty journals devote considerable space and attention to individual "case reports," something rarely found in other sciences. Maybe our own species is unique in being composed of unique individuals. More likely, a journal about lobsters, written and edited by lobsters for lobsters, would be filled with crucial details demonstrating something that its readership never seriously doubted: that having seen one lobster, you definitely have not seen them all.

THE PARADOX OF INDIVIDUALITY

One of the frustrating things about reading older classics of natural history is the extent to which earlier observers such as Ernest Thompson Seton or C. Hart Merriam reported on, for example, prey-catching behavior of "the" lynx, or vocalizations of "the" golden eagle, without

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specifying whether the subjects in question were male or female, juvenile or adult, etc. Modern biologists demand this information because we recognize that neither "the" lynx nor "the" golden eagle exists; rather, there is only this lynx and that golden eagle. In fact, one of the triumphs of modern biology has been precisely this overcoming of a tendency to think in something like Platonic ideals (Mayr 1982). Taxonomists, for example, no longer concern themselves with the "type species" for a genus, implicitly recognizing the crucial role of variation. Yet there is a paradox here, because even as we yearn for more detailed identifying information about the lynxes or golden eagles under study, we do so in order to combine them into yet another conceptual group, smaller than the species but larger than the individual, substituting for "the" lynx or golden eagle, "the" adult male lynx, or "the" juvenile female golden eagle. Modern evolutionary theory, for example, while denying a place for "the" _ _ (fill in your own species of choice), is replete with theories of what adult males or juvenile females, etc., should be like . . . as, indeed, they often are. Most textbooks on animal behavior--including my own--contain extensive material, both descriptive and theoretical, concerning the behavior of such larger categories, but not a single entry for behavioral individuality (e.g., Alcock 1993; Barash 1982; Trivers 1985; Wilson 1975). Generalization is gained, but the individual gets lost. Whether comparative psychologists looking for laws of learning, ethologists seeking to identify species-typical behaviors, or sociobiologists concerned with the adaptive value of behavior, researchers in behavioral biology have generally seen deviations from the norm as aberrant, either genetically or experientially: as distractions from an almost Platonic ideal. All are inclined to look beyond the individual peculiarities of particular subjects. And for good reason. Few of us would credit as "science" a lengthy rendition of seemingly disconnected anecdotal accounts of individual cases. Furthermore, such detailed descriptions quickly become downright boring to anyone not intimately concerned with the individuals in question. Accordingly, most of us try to reveal underlying processes, to identify and enunciate principles, to go inductively from the specific to the general. Yet, especially when it comes to living things, each specific case tends to be distinct, often rendering the whole resistant to easy generalization. This is why the biological and social sciences are so involved with statistics. Chemists can concern themselves with "the" sulfuric acid molecule, or physicists with "the" neutron, confident that having seen one, they have pretty much seen them all. But students of the life sciences are always confronted with diversity. As a result, we lean heavily on complex mathematical techniques that tell us whether it is safe

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to generalize, and if so, how far we can go, and with how much confidence. Just as Galileo, in the course of his enforced recantation, is said to have muttered of the Earth, "Nonetheless, it moves," many a student of the life sciences, considering the homogenization of disparate data points so neatly massaged into a satisfying generalization, is likely to have muttered, of the various individuals thereby erased, "Nonetheless, they are different."

PROXIMATE CAUSES

A general theory of individual differences would presumably be concerned with both proximate (immediate) and ultimate (evolutionary) levels of causation. On the proximate level, several factors appear likely. Genetic differences among individuals seem obvious sources of individual variability, such differences being produced by mutation as well as (in sexually reproducing species) meiosis and sexual recombination. For example, individual variability in human neuroticism has been attributed, at least in part, to variation in alleles of a gene that encodes a transporter for the neurotransmitter serotonin (Lesch et al. 1996). Of course, since genotypes produce phenotypes only through the interposition of environments, proximate causes of individual differences must include not only the genetic differences among individuals, but also the different environments experienced by each individual, with "environment" defined broadly to include past behavioral events, nutrition, etc. Age-related effects would thus also be expected, insofar as the passage of time provides an opportunity for genetic differences to be more thoroughly expressed. Individual differences should probably be distinguished from differences based on different biological and social roles. Thus, adult male hoary marmots (Marmota caligata) are typically either socially dominant within their colony or they are satellites, clearly subordinate to a dominant individual. In a sense, the differences between dominant and subordinate males reflects important aspects of their behavioral individuality; when and if a satellite male assumes the role of dominant male, his behavior becomes that of such males generally. Reproductive females, for their part, are more aggressive than nonreproductive females and spend more time near their burrows; these roles also switch when their reproductive roles reverse. Although the social and biological role is crucial to the specific behaviors assumed, it seems most useful to control for such "role effects," and restrict the concept of behavioral individuality to distinctions between individuals that are socially and

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biologically as similar as possible in all other respects, notably age, sex, social status, physical health, residence situation, and reproductive state. Even in cases of genetically identical individuals, idiosyncratic differences in personal experiences can nonetheless be expected to generate a phenotypic gap between individuals. Such experiences may begin quite early in life: intrauterine positioning, for example, can influence phenotypic variation among rodents (vom Saal 1981). Despite extensive and intensive studies of behavioral ontogeny, we still know remarkably little about the ontogeny of individual differences although, in a sense, the study of proximate causes of individual differences becomes the study of development: the production of phenotypes. ULTIMATE CAUSES At the ultimate, or evolutionary level, individual differences are if anything even more problematic. One deceptively simple explanation is that the adaptive significance of individual differences is directly equivalent to the adaptive significance of sexual reproduction itself, a subject that has received substantial attention from evolutionary biologists but which remains oddly resistant to straightforward explanation (Michod and Levin 1987). It is increasingly acknowledged, however, that sexual reproduction enhances the fitness of its practitioners by generating an array of adapted offspring when environments change, and/or by keeping ahead of parasites and other disease-causing organisms, etc. (Hamilton et al. 1990). In any event, a common thread amid these diverse theoretical efforts has been that the adaptive significance of sex relates to the production of genotypic diversity. But in order for genotypic diversity to convey a fitness benefit to parents whose descendants display such diversity, it must be reflected in phenotypic diversity: in other words, there must be individual differences. It is therefore ironic that many biologists who are quite familiar with the theoretical dilemma associated with sexual reproduction, and with the received wisdom as to its adaptive significance, nonetheless tend to disregard the existence of substantial individual differences among their research subjects, or scratch their heads when asked to explain its prevalence. Before biologists fully appreciated--and were confounded by--the genetics of sexual reproduction, Charles Darwin recognized individual differences as essential to the process of natural selection itself. Thus, Darwinian natural selection produces evolutionary change only if competition takes place among individuals, and only if this competition in

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some way reveals the individual, genotypic distinctiveness of the individuals concerned. Individuality therefore occupies a fundamental place in our understanding of basic evolutionary biology, even though it has rarely been acknowledged, or investigated per se. There are other possible ultimate explanations for the existence of individuality. In some cases, at least, it might be neutral or nonadaptive, a by-product of selection that maintains genotypic differences for other reasons. It might be the unavoidable result of genetic "noise" which simply has not been selected against. It may even be maladaptive, although it stretches credulity that so fundamental a characteristic of living things should carry a pervasive evolutionary cost. On the other hand, individual differences--at least in certain cases--may warrant study as an important obstacle to optimality, insofar as a "best" behavior exists, but the vagaries of genotype and experience necessarily cause idiosyncratic departures from it. It is also possible that individual differences are directly selected, if individuals benefit by being distinct from others. For example, when predators develop a "search image" of particular prey, individuals that differ from the most abundant form(s) can experience an advantage (Tinbergen 1960). Selection of this sort could be not only frequency dependent, conferring an advantage on distinct, rare morphs, it could also favor a continuously varying array of phenotypes, behavioral no less than physical. Individual differences could also be the result of sexual selection, if mate preferences (presumably on the part of females in particular) favored individuals who differed from the chooser, as a means of reducing inbreeding and its attendant fitness disadvantages (Potts et al. 1991). Several other factors could select for behavioral individuality and (along with those described above) they are not mutually exclusive. Thus, individual recognition between parent and offspring appears to be adaptive, predictable, and widespread (Beecher 1991). Especially when possible mix-ups could occur, selection should favor parents whose offspring are unique and distinctive, hence not easily mistaken. The inclusive fitness benefits of being able to identify kin, beyond offspring/parents, could also select for individual differences. The study of "kin recognition" has in fact become a successful cottage industry among students of animal behavior (e.g., Sherman and Holmes 1985). When such recognition is demonstrated, attention is then typically directed to the mechanism whereby it is achieved, with "phenotypic matching" (Holmes and Sherman 1983) and the "green beard effect" (Dawkins 1989) among the prominent contenders. Only rarely, however, is the chain of causation run the other way: to the possibility that phenotypic distinctiveness (i.e., individual differences) may have been

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selected as a means of providing for the adaptive dispensation of nepotistic benefits. Inclusive fitness benefits derived from group living more generally could also have contributed to the adaptive significance of individual differences. Animals should find it helpful to know, for example, whether a given individual is subordinate or dominant to one's self, what his or her prior behavior and aptitudes may be, etc. In most of these cases, an advantage would presumably accrue not only to the perceiver of individual differences, but to the sender as well; thus, selection would predictably generate not only sensitivity to the cues of individuality, but also the sending of such cues. It is also possible that the exigencies of reciprocity (Trivers 1971) have selected for individual differences among would-be reciprocators as a means of reducing the frequency of cheating, since reciprocating systems require that benefits be dispensed asymmetrically to those who previously provided assistance. In such cases, selection would probably be especially intense on the ability of initial donors to discriminate among their beneficiaries, so as to insist upon subsequent recompense. What may therefore appear as selection for heightened individuality may really be selection for enhanced ability to perceive whatever individuality already exists. On the other hand, insofar as individuals profit by being part of a successful reciprocating relationship, selection could also favor individual differences per se, analogous to Batesian mimicry in which models derive benefit from being distinguishable from their mimics. Finally, environmental heterogeneity--defined broadly, to include social as well as biological and physical environment--could result in proportionately more individual differences, both through the simple interposition of diverse experiences, and also by selecting for behavioral flexibility and variability as well (Nur 1987). We might consider the phylogenetically given, shared traits of a species or population as a kind of coarse adjustment in pursuit of fitness, and individual differences-however achieved--as the fine tuning.

THE ADVANTAGES OF SIMILARITY

Another way of gaining insight into the possible adaptive significance of individual differences would be to examine its converse, situations in which selection apparently favors similarity, if not identicality. For example, in cases of warning coloration, there is a benefit in presenting a "common phenotypic front" to potential predators, presumably because this facilitates learning and/or innate recognition of a pattern that de-

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notes dangerous or unpalatable prey. Also, the existence of speciesspecific traits such as courtship patterns suggests positive selection for similarity, or at least, selection against substantial divergence. Norrealizing or stabilizing selection of any sort would seem to exemplify the biological opposite of factors leading to greater individual distinctiveness. Generalizations about behavioral individuality are, at this stage in our knowledge, difficult to support. It is tempting, for example, to suggest that "higher" animals with more complex brains exhibit more individual variability than their "lower" counterparts, which rely comparably more on automatic, species-typical reactions to a narrow range of fixed stimuli. It would be surprising, for example, if jellyfish or barnacles turn out to demonstrate as much behavioral individuality as elephants or h u m a n beings. It may thus be significant that some of the most effective portrayals of behavioral individuality come from studies of large-brained animals, such as chimpanzees and gorillas (Goodall 1977; Harcourt and de Waal 1992). And yet, because vertebrates sequester their germ lines whereas hydroids, for example, do not, there is more variation-structural, although not necessarily behavioral--in the latter than in the former (Buss 1987). In their basic morphology, two oak trees are also likely to differ more from each other than two elephants, although no common metric exists at present with which to make such comparisons. Neither is there a satisfactory way of comparing the behavioral individuality of different species.

EVOLUTIONARILY STABLE STRATEGIES

Evolutionary biologists have shown considerable interest in one limited aspect of individual differences, namely those associated with alternative behavioral strategies. These are often analyzed in terms of "evolutionarily stable strategies" (ESSs), in which mathematical game theory is applied to explain the persistence of distinct behavioral morphs, with special attention to alternative mating strategies (Maynard Smith 1982). In such analyses, fitness returns are evaluated as frequency dependent evolutionary payoffs, determined not only by one's own phenotype but also by the phenotype of others with w h o m one somehow interacts. In such situations of evolutionary stability, there is no "best solution," but rather, a potentially stable equilibrium at which equal payoffs maintain more than one distinct morph simultaneously in the population. Much of the interest in ESSs originally derived from the question of whether this concept, derived originally from game theory, could be supported by empirical studies. But it was also driven by the theoretical

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question of how evolution could maintain diverse behavioral morphs rather than select for a single optimum. ESS theory is therefore conceptually related to an interest in behavioral individuality, in that both are concerned with the production, maintenance, and understanding of diversity. However, there is an important difference: Whereas the alternative strategies analyzed by ESS theory are distinct and discontinuous morphs (e.g., large territorial males vs. small "sneakers," or even males vs. females), a theory of individual differences must deal with continuous variability. Despite considerable research attention, the presence of undisputed morphs that lend themselves to productive ESS analysis remains so rare as to generate substantial interest whenever it is found. By contrast, continuous individual variability is ubiquitous and undeniable, regardless of the phenotypic dimension being measured. Indeed, part of the message of this article is that such variability is so widespread that it is generally taken for granted, and thus, rarely analyzed. Most of the time, it is not even described. I submit that ESS theory will not contribute substantially to an understanding of behavioral individuality, because continuous variability does not lend itself to game theoretic approaches. The finer-grained the analysis, the more impossible it becomes to specify the alternative "strategies" in question. Moreover, Evolutionarily Stable Strategies seem destined to be special cases, albeit fascinating ones, whereas behavioral individuality is the very stuff of life.

AN EXAMPLE: MARMOTS

Notable efforts at assessing individual behavioral profiles have been conducted in studies of yellow-bellied marmots (Marmota flaviventris), large, semi-fossorial rodents that inhabit the Sierra Nevada, southern Rocky, and Cascade mountains of North America. The technique employed is "mirror-image stimulation," in which animals were exposed to a mirror, and their subsequent behavior categorized as "approach," "avoid," or "sociable" (Svendsen and Armitage 1973). The behavior patterns observed for individual female yellow-bellied marmots were correlated with their fitness as well as their social roles, and were repeatable, as well as stable over time. Because it focuses on discontinuous behavior categories, such research is similar in approach to the search for ESSs. Long-term studies of hoary marmots (M. caligata) in the North Cascade Mountains of Washington State revealed continuous individual differences based on observed behavior in situ. I characterized the aggressiveness of five different adult females by the ratio of chases

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initiated to chases received, both before and after live-trapping and transplanting them into new colonies--chase ratio before/after transplantation: 0.48/0.35, 0.79/0.94, 1.!/0.9, 1.3/1.1, and 2.2/.08 (Barash 1989). Although there was a consistent trend for these females to show lower chase ratios after being transplanted, it is notable that in all but one case, individual profiles of aggressiveness also remained consistent: Aggressive individuals tended to remain comparatively aggressive and submissive individuals tended to remain comparatively submissive. Interindividual differences were significantly greater than "intraindividual" differences (Kolmogorov-Smirnov one-way analysis of variance, p < .05). A similar pattern was revealed when nontransplanted adult females were contrasted in years of comparable reproductive status: i.e., nonreproductive females compared with their next nonreproductive year, and contrasted similarly with other nonparous females. Reproductive females, by contrast, showed a relatively low "treatment effect" between individuals, presumably because a more relevant treatment effect (pregnancy) tended to equalize individual differences. (At the same time, it remains unclear whether other behavioral measures would reveal individual differences among reproductive females.) It seems certain that both yellow-bellied and hoary marmots have individual behavioral profiles, and that these profiles are relatively constant from year to year; another way of looking at it is that when individuals of the same social and biological role are considered, the differences between individuals are greater than the differences "within" individuals. There also appears to be an age-related component to marmot behavioral individuality. Among European Alpine marmots (M. marmota), individual differences among adults exceeded those of yearlings and between-colony individual differentiation exceeded differentiation within colonies, whereas comparatively asocial maintenance behaviors-foraging time, distance from the home burrow, frequency of selfgrooming, and frequency of alarm-calling--revealed no age-related trends in behavioral individuality (Barash 1976). I determined individual behavioral profiles for free-living M. caligata by recording the number of greetings, play-fights, and chases initiated per hour, based on 70 hrs observation for dominant males, 100 hrs for nonreproductive adult females, 100 hrs for yearling males, and 100 hrs for female young of the year. When individuals of the same age and sex class are compared, and the observed standard deviation for each individual is expressed as a single data point, the standard deviations are found to vary directly with the means of the behavior in question.

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An appropriate measure of individual variability therefore relates the absolute amount of variability to the magnitude of the trait itself, as by determining the ratio of standard deviation to arithmetic mean. This yields what might be called a "relative standard deviation," a measure of the observed standard deviation adjusted for the magnitude of the behavior in question. Taking this measure for each individual as a single data point, dispersion for the adult males was less than that for the yearlings, which in turn was less than that for the young, while the nonparous adult females were not statistically distinguishable from the adult males. Analysis of variance, using the data themselves rather than a transformation of those data, revealed that interindividual differences are greater than intra-individual differences for adult males and adult females but not for yearlings or young of the year (Barash 1989). These results show that older marmots are more consistent--i.e., show less interindividual variability--than are younger ones. It remains to be seen whether this pattern of age-related behavioral individuality is a more general phenomenon, characteristic of other species, since there is no a priori reason to suspect that marmots are any m o r e - - o r less-predisposed to behavioral individuality than any other animals. Accordingly, it may be noteworthy that among human beings, behavior generally becomes more predictable and consistent as a young child grows older (Kagan 1984).

STATISTICS AND INDIVIDUALITY When seeking to extrapolate from a sample to a larger population, life scientists typically present research results in terms of either mean or median, all the while knowing that there is no "average" individual. (The average human being, for example, has one ovary and one testicle.) The nature of statistical inference is such that results must be accompanied with reports of either standard deviation or variance, depending on whether the analysis is parametric or nonparametric. It might seem that such measures of dispersion effectively take notice of individual variability; indeed, it is precisely because of such variability that they are needed. But let's face it: we are overwhelmingly more interested in measures of central tendency. We take special note of the mean or the median as providing the information we are after, as effectively describing the phenomenon under study, whether it be the area of a panda's paw, the alarm calling frequency of a marmot, or the incarceration rates of young, unmarried men. We give at best only passing attention to statistical measures of dispersion, largely as unavoidable indices of irrel-

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evant noise. Or--especially if our own data are at issue--we consider such measures with trepidation, since if they are too great, they threaten to keep our results from "reaching significance." Very rarely are such indicators of individual differences in our subjects seen as significant in themselves. An interesting exception involves concern among sociobiologists about the consequences of male-female differences in "parental investment," leading to the prediction that for most species, males will have a higher variance in reproductive success than will females (Trivers 1972). Overwhelmingly, however, prediction involves expectation concerning the "population as a whole," just as--paradoxically, for our purposes-in the above case, it is predicted that among most species the "mean" variance in male reproductive success will exceed that of females. Thus, even when individual variability is the focus, the very fact of "doing" science leads to predictions for a class of subjects, tending once again to obscure if not obliterate individuality itself! Enhanced concern about individual differences would be expected to affect research in various ways. At minimum, it should profitably lead to closer inspection of standard deviation and/or variance in research results, both experimental and those deriving from naturalistic observation. It might generate predictions based on measures of dispersion no less than on measures of central tendency. Finally, it could also be expected to produce finer tuning of such measures, by clarifying the meaning of intra-individual as opposed to inter-individual differences. Identical measures of dispersion could conceal two very different underlying patterns. On the one hand, there could be consistency in the actual traits of the same individuals, while on the other, different individuals could be distributed differently, but in a manner that produces the same mean and standard deviation as in the first situation. The former case speaks to a relatively low degree of intra-individual variability (or, another way of looking at it, a high level of individual consistency); the latter, a high degree of intra-individual variability (or, a low level of individual consistency). Either would produce the same measures of dispersion and central tendency, but with very different implications for the biology of the individual differences in question. By its consistency, the first case suggests that the observed individuality is a selected trait, whereas the second suggests that such individuality might be less biologically meaningful, resulting perhaps from the unavoidable vagaries and vicissitudes of life. It should not be difficult to present research results in ways susceptible to such analysis. For example, consider that under two conditions of phenotypic measurement, similar standard deviations are obtained. Different patterns of individuality could then be revealed by determining the Pearson cor-

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relation coefficient. The three extreme cases would be as follows: If the same individuals were responsible for the spread in each case, the correlation coefficient would be +1; if there was an exact inverse swap of positions, -1; and if the same standard deviation was obtained by a complete, random scrambling of the individuals involved, the correlation coefficient would be 0. I therefore now propose a clarifying distinction, between variability, defined as intra- or within-individual difference (in the sense of a mathematical variable, something that is acknowledged to be inconstant), and variation, defined as inter- or between-individual difference, and thus, a function of a population. The same degree of variation, in turn, could be achieved by a population of individuals, each of a given variability, or by different individuals, each of whom is relatively invariable. A theory of variability and variation, in the above sense, remains to be developed, but perhaps this clarification of terminology will help. Along these lines, a further distinction could profitably be made between "developmental plasticity" and "behavioral flexibility." The latter would refer to the capacity of individuals to be variable, with this capacity not limited or restricted by previous phenotypic events. The former refers to the potency of a genotype to produce more than one phenotype, but with the expectation that such differentiation may well constrain future phenotypes (Waddington 1962). In the case of developmental plasticity, the dice may be cast in a variety of ways, but once cast, it could be difficult or impossible to recall it; behavioral flexibility, on the other hand, permits the casting and recasting of each die within the lifetime of an individual. Either can contribute to the behavioral variation of a population, but only behavioral flexibility will yield substantial individual variability.

CONSEQUENCES OF INDIVIDUAL DIFFERENCES It seems clear that behavioral individuality matters. The social structure of coyotes (Canis latrans), for example, is apparently influenced by interactive patterns among litter mates (Bekoff 1977); it has also been suggested that wolves (Canis lupus) are predisposed to a social niche in their pack by the individual traits that characterize them as pups (Fox 1972). In yellow-bellied marmots, interactions among young of the year are strongly influenced by their individual behavioral profiles, which in turn appear to be more consequential than patterns of genetic relatedness per se (Armitage 1982). Similarly, the greatest part of the variance in female reproductive success is explained by variance among individual females,

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rather than by the variance among colonies or among matrilines (Armitage 1986). It also appears that differential aggressiveness, as well as differential sensitivity to aggressiveness, strongly influence dispersal patterns, and thus, population structure (Armitage 1984). This appears to be true of other animal species as well (Lomnicki 1978). The impact of individual variability is probably especially great in highly social species (chimpanzees, elephants, h u m a n beings, hoary marmots) as opposed to relatively solitary species (aye-ayes, rhinoceroses, woodchucks), which are less likely to encounter such diversity. On the other hand, the fact that a species experiences a high level of social integration may paradoxically suggest precisely the opposite, namely, that individuals of such species are relatively unaffected by social vagaries. Species that are comparatively social may develop greater behavioral individuality, since they are likely to occupy social roles that are more clearly defined. But, since social species are more subject to the possibly homogenizing influences of other animals, it is possible that individual differences are more pronounced among solitary species. We simply do not know, for example, how eusocial and solitary bees compare in their individuality. Nor, at this stage, can we even make cogent predictions. Part of the thrust of this article has been to urge acknowledgment of the existence and importance of individual differences, and to disagree with Goethe's maxim "individuum est ineffabile" (individuality cannot be explained). Individual differences, I am confident, will eventually be explained. First, however, their existence must be acknowledged. For many evolutionary biologists, individual differences are something of an "anomaly," in the sense of Lightman and Gingerich: "Certain scientific anomalies are recognized as anomalies only after they are given compelling explanations within a new conceptual framework. Before this recognition, the peculiar facts are taken as givens or ignored in the old framework" (1992:693). I suggest that the "peculiar facts" of behavioral individuality have been taken as givens, and at the same time-ironically--also ignored in the current framework of most evolutionary studies. We probably do not need an entirely "new conceptual framework," but one elastic enough to incorporate individuality. On another level, part of the resistance encountered by h u m a n sociobiology, Darwinian psychology, evolutionary psychology--call it what you will--may be because essentially none of the "ultimate" interpretations thus far offered account for the enormous amount of (perceived or actual) individual variation that h u m a n beings identify among themselves (P. A. Gowaty, personal communication 1997). Perhaps there is something in the h u m a n psyche that cherishes and demands recognition of our own i n d i v i d u a l i t y . . , not only in our private, interpersonal

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interactions, but even in theoretical constructs purporting to help "explain" the nature of human nature! With some notable exceptions (e.g., MacDonald 1991; Tooby and Cosmides 1990), such "explanations" typically do not even acknowledge that individuality exists, never mind attempt to incorporate it into the framework of general theory. This may reflect the fact that, until recently, advances in applying evolutionary theory to human behavior have been almost entirely the work of biologists, who have typically given individuality short shrift. By contrast, psychologists--stimulated in part by the early work of Darwin's cousin, Francis Galton--have generally been more receptive to individual differences, with anthropologists generally occupying a more or less intermediate position (although with no small amount of individual differences!). Perhaps the growing involvement of these latter disciplines in attempts to flesh out a truly evolutionary theory of human nature will result in fuller incorporation of behavioral individuality. Western science since Aristotle has sought to identify and understand classes of phenomena, looking beyond the particular to organize knowledge into general categories. Accordingly, the present article's request for greater attention to individual differences may seem strangely retrograde. Maybe the best way to justify so perverse a preoccupation would be to substitute individual differences for the famed question about climbing mountains: "Why study individual differences?" Because they are there. David P. Barash Feceivedhis Ph. D. in zoologyfrom the Universityof Wisconsin (Madison) in 1970. He has been at the Universityof Washington since 1973, where he is professor of psychology. He has authored or co-authored thirteen books, with two others--one on the sociobiology of sex differences and another on the implications of inclusive fitness-forthcoming from lsland Press, as well as a textbook,Ideasof Human Nature (Prentice-Hall), to appear in late 1997. In addition to his continuing interest in sociobiology,Barash has been active in antinuclear activities and in the field of peace studies.

REFERENCES

Alcock, J. 1993 Animal Behavior. Sunderland, Massachusetts: Sinauer Associates. Armitage, K. B. 1982 Social Dynamics of Juvenile Marmots: Role of Kinship and Individual Variability. Behavioral Ecology and Sociobiology 11:33-36. 1984 Recruitment in Yellow-Bellied Marmot Populations: Kinship, Philopatry, and Individual Variability. In The Biology of Ground-Dwelling Squirrels, J. Murie and G. Michener, eds. Pp. 375-403. Lincoln: University of Nebraska Press.

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