A review of selected sociobiological principles: application to hominid evolution

J. SocialBiol. Struct. 1982 5, 69-90

A review of selected sociobiological principles: application to hominid evolution I. The development of group social structure Darius Baer and Donald L. McEachron

IBM, FieM Engineering Division, Boulder, CO 80302 and Department of Neurosciences, School of Medicine, University of California, San Diego, La Jolla, CA 92093, USA The first section of this paper reviews sociobiological theories about: (a) Sex and Mating Systems, (b) Kin selection and Inclusive Fitness, (c) Aggression and Mechanisms of Control, and (d) Dispersal, focusing mainly on terrestrial primates and social carnivores. The second section deals with applications of these principles to hominid evolution. It is suggested that the early hominids, prior to the development of weapons, lived in groups similar in structure to present-day terrestrial primates, with slight modifications caused by selection associated with co-operative hunting. The groups were structured around a stable core of interrelated females with a fairly stable, dominance-based group of associated males. Male emigration was slightly reduced due to the need for male-male co-operation. The early hominids were probably polygynous. With the development of weapons, it became necessary to reduce the expression of aggression with the group. This was accomplished by: (1) Decreasing or eliminating individual transfer between groups, thus increasing the number of adult males; (2) Strengthening the dominance hierarchies and changing the determinants of dominance from physical to social and mental skills; (3) Increasing the length of the females' estrous periods, thereby reducing the males' benefit per copulation and the need for male-male competition. As a result, m a l e female mating ties become stronger and more long-lasting; and (4) Developing in females as appeasement signals characteristics which mimicked offspring and mothers. These characteristics, being fitness-related, eventually became attractive to the male and were sexually selected. In summary, weapons had the effect of making the hominid groups more closed, with an extremely well organized dominance-based social structure. Only the more dominant males remained polygynous, with semi-permanent mating ties to several females. Some subordinant males were able to mate, and when they did so, were probably monogamous.

Introduction Sociobiology is the discipline which studies the implications o f natural selection and evolutionary biology on the social behavior o f organisms (Barash, 1977). The behavior o f an Send reprint requests to: D. L. McEachron, Department of Neurosciences (M-008), UCSD, La Jolla, CA 92093, USA. 0140-1750/82/010069 + 22 $02.00/0

O 1982 Academic Press Inc. (London) Limited

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animal can be examined for its effect on the animal's fitness and the resultant changes in the frequency of the genes associated with or causal to that behavior. From cost/benefit analysis of possible behaviors relative to their effects on fitness and gene frequencies, models of animal behavior can be derived. Properly understood and applied, sociobiological theory can give new insights into ethology, and can be used to predict the evolutionary pathways of animal species, including Homo sapiens. In this and a forthcoming paper to be published in this journal, the authors will review selected sociobiological principles and show how these principles might be used to explain certain facets of hominid evolution. The probability of a behavior evolving is, in sociobiology, related to the cost and benefit of that behavior in terms of an individual's inclusive fitness. It is the ratio of cost to benefit, or the difference between them, which determines whether or not a behavior is adaptive. Therefore, no behavior can be assumed a priori to be adaptive irrespective of social and physical environmental conditions. For a behavior to be attributed to a species in the past, there should be: (1) a theoretical or mathematical model which predicts it, (2) comparative ethological evidence for the model, and (3) evidence for predicted physical adaptations in fossil or modern members of the species in those cases where the model links behavioral and physical adaptations. In so far as there are more possible mathematical models than existent behaviors, the second and third conditions are particularly important. Some real behaviors will be excluded due to these conditions, since not all past behavior patterns have modem counterparts. However, these conditions will aid researchers to avoid developing large numbers of hypotheses which are untestable by virtue of their application to only one species (see Wilson, 1975). We hope to avoid this and other related problems when discussing hominid evolution by; (1) examining hominid adaptions, starting in the past with as few a priori assumptions as possible, (2) using sociobiological hypotheses tested with reference to comparative ethology, especially with reference to primates and social carnivores, (3) examining the physical adaptations of fossil hominids and physical and behavioral characteristics of modem humans against those predicted by our hypotheses. This first paper concerns the evolution of hominid social behavior with reference to group social structure which occurred after 2-3 millions years ago and prior to about 2000 generations or some 50,000 years ago. We chose this time period because of the changes in hominid brain size seen during it (Alexander, 1971, 1979). The two primary assumptions are: (1) most hominid evolution took place in a savannah or other terrestrial habitat and (2) at some point, after adapting to terrestrial existence, hominids began group hunting. Present-day primates provide an estimate of social and behavioral variability with which the hominids were endowed, while the lifestyle of the social carnivores gives an estimate of some of the selection pressures affecting the actual social system. The first section of the paper presents a review of selected sociobiological principles for those without formal training in ethology or sociobiology. The second section deals with some of the possible ways in which these principles might have affected hominid evolution.

I. A review of selected sociobiological principles fa) Sex and mating systems A female can be def'med as that sex which has evolved to produce large gametes, investing relatively great amounts of energy and nutrients into each gamete to increase its probability

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of survival (Parker, Baker & Smith, 1972). In female mammals, fertilization obligates the individual to increase the already large initial investment with long periods of gestation and lactation (Wittenberger, 1979). Thus, even before an offspring can contribute to the mother's fitness (an individual's fitness is def'med as the number of offspring produced by that individual's own young), that offspring represents an enormous energy investment. Because resources for reproductive effort are limited, a female is restricted to only a few such investments. Thus, to the female, each offspring represents a very high-risk venture. If an offspring were to fail to reproduce or be unfit for some other reason, the female not only loses the energy and time she invested, but also loses a high proportion of her reproductive capacity (Bateman, 1948: Trivers, 1972). In the cost-benefit analysis of natural selection, the behavior which evolved for females is to balance high cost with high benefit. In other words, in order to increase her own fitness, the female must ensure that her offspring will be maximally fit: One way this can be done is to mate only with males likely to produce highly fit offspring. Thus, female choice can be predicted to evolve in mammals (Orians, 1969; Trivets, 1972). On the other hand, males produce a virtually unlimited number of smaU gametes which are energetically inexpensive, and thus can take advantage of the large energy input of the female (Parker etal., 1972). Mating represents a low-risk venture for males - a failure to create a fit offspring from a single reproductive effort does not represent a critical loss in terms of gamete investment, nor does it prevent other matings. Theoretically, a single male could mate with every female in his troop. This represents a male's maximum fitness, disregarding other environmental and social factors (Bateman, 1948; Orians, 1969; Trivers, 1972). Thus, polygyny can be expected a priori as a basic mammalian pattern (Orians, 1969), and observations on mammals indicate that it is widespread (Barash, 1977; CluttonBrock & Harvey, 1977; Orians, 1969; Wittenberger, 1979; Wilson, 1975). While male gametes may be unlimited, female gametes deffmitely are not. Females, their gametes,-gestation, lactation and parental care represent a limited resource for the males (Dawkins, 1976). Williams, 1966). This situation often leads to male-male competition for females in polygynous societies (Barash, 1977; Dawkins, 1976; Orians, 1969; Williams, 1966; Wilson, 1975). Although obviously advantageous to the males, polygyny can also have advantages for the females. Polygyny allows easier access to more fit males for all females, reducing the need for competition [see Wittenberg (1979) for discussion of female competition]. In addition, since polygynous males tend to be more fit, females who mate with them will have more fit offspring than those who might mate with monogamous males (O'Donald, 1963; Williams, 1966). This latter concept, and the idea of female choice, can be criticized on the grounds that there will be a low correlation of fitness between parent and offspring (Maynard Smith, 1978). However, Maynard Smith (1978) argued that the additive component of fitness, the correlation of parent and offspring, is not zero and that the effects of harmful mutations and the establishment of favourable mutations could make female choice adaptive. In addition, preliminary data from observations on European wrens indicate that females mating with more polygynous males produce significantly larger clutches than those breeding with less polygynous males (Garson, 1980), indicating the adaptiveness of female choice and polygyny for both males and females. Environmental and social factors are important in determining whether or not the inherent tendency for polygyny is expressed in a given species. Factors which favor the development of a monogamous mating system include: (1) Environmental conditions such that the female cannot raise her offspring without aid (Lack, 1968; Clutton-Brock & Harvey, 1977; Emlen & Ofing, 1977; Orians, 1969),

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(2) The male attracting mates by defending a resource which cannot support more than one female (Clutton-Brock & Harvey, 1977; Emlen & Oring, 1977; Orians, 1969), (3) Aggression by the mated female or male preventing subsequent matings (Emlen & Oring, 1977;Wittenberg, 1976, 1979), and (4) When male reproductive fitness is reduced by mating with a second female, possible through adverse effects on the first female (Trivers, 1972; Wittenberge, 1979). Wittenberger (1979) argues that few if any mammal species are monogamous due to factor (1) (need for male parental care). Monogamy is fairly rare among primates, and when it is observed, it appears to be a consequence of factor (2), territorial defence of an area incapable of supporting more than the mated pair and their offspring (Clutton-Brock & Harvey, 1977). Monogamy has not been reported for primates in savannah habitats (Eisenberg, Muckenhim & Rudran, 1972; Clutton-Brock & Harvey, 1977). Among the social predators, monogamy~has been reported for wolves (Allen, 1979; Fiennes, 1976; Mech, 1970; Zimen, 1975, 1976, 1978) and wild dogs (Van Lawick-Goodall and Van Lawick-Goodall, 1971), but not in lions (Bertram, 1975, 1978; Schaller, 1972) or spotted hyena packs (Kruuk, 1972). In both wolves and wild dogs, there is usually only one breeding pair, which represents the dominant animals (Allen, 1979; Fiennes, 1976; Frame & Frame, 1976; Mech, 1970; Van Lawick-Goodall & Van Lawick-GoodaU, 1971; Zimen, 1975, 1978). Orian (1969)has argued that monogamy in the social canids represented the influence of the need of a male for provisioning of the pups. The phenomenon of communal pack care of the young in both wolves (Fiennes, 1976; Mech, 1970) and wild dogs (Kiihrne, 1965; Van Lawick-Goodall & Van Lawick-Goodall 1971) indicates that this is probably not the ease. One male, more or less, will not make a critical difference to the pups' survival. In wolves, and possibly wild dogs, monogamy is achieved by the active suppression of the mating attempts of subordinants by the dominant male and female, most especially the female (Fiennes, 1976; Frame & Frame, 1976; Mech, 1970; Van Lawick-GoodaU & Van Lawick-Goodall, 1971; Zimen, 1975, 1978). What is the selective advantage of such suppression to the breeding pair? Wolf packs tend to be limited in size by both social and resource factors (Allen, 1979; Jordan, Shelton & Allen, 1967; Rausch, 1967). Apparently, group size remains stable despite overall population increases (Rausch, 1967; Zimen, 1976). Because of this limitation, the possible number of offspring having maximum fitness is low. Thus, the alpha female and male probably suppress the mating activity of subordinates to increase the fitness of their own offspring by reducing competition. Because mating is pretty much limited to a single pair, a wolf pack is really a highly genetically related extended family (Fiermes, 1976). Thus the observed co-operation seen in hunting and caring for the young is most likely an expression of individual pack members seeking to increase their inclusive fitness through kin selection [see Section I(b)]. Other factors may include the need for male-male co-operation in hunting, preventing male desertion (Mech, 1970; Wittenberger, 1979), an extremely low probability of finding receptive females outside the group 0Yittenberger, 1979) and the fact that the social canids are monoestrus (breeding once per year) and thus the time which can be spent in both Finding a female and mating is very limited (Kleiman & Eisenberg, 1973; Wittenberger, 1979). It should be recognized that monogamy in the social canids is not completely strict. Not all subordinant mating efforts are prevented (Rabb, Woolpy & Ginsberg, 1967) and changes in the dominance hierarchy can change the identities of the mating pair (Zimen, 1975, 1976). For a more detailed examination of the issues discussed here, the reader is referred to Dawkins (1976), Clutton-Brock & Harvey (1977), Emlen & Oring (1977), Maynard Smith (1978), Orians (1969), Selander (1965), Wilson (1975) and Wittenberger (1979).

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(b) Kin selection and &clusive fimess Evolution can be defined as any change in gene frequencies in a population which occurs from generation to generation (Dobzhansky, 1970). Evolution and natural selection imply that genes exist in multiple copies. After all, it is not a change in the frequencies of individuals, but rather genes, which characterizes evolution. In addition, this definition of evolution does not specify any particular mechanism for changes in gene frequency - the simple fact of change is sufficient. Hamilton (1964) recognized that these factors meant that a definition of fitness limited to the number of offspring of an individual and his direct descendants was too narrow. If the only important factor were a change in gene frequencies, an individual could increase file representation of his genes in the next generation either by increasing his own fitness, or by increasing the fitness of his genetic relatives. In order to explore this idea further the concept of genetic relatedness is required, denoted by the symbol r. The genetic relatedness of two individuals is the proportion of genes which they share and are common by descent. In sexually reproducing species, such as mammals, the r value between a parent and its own offspring is 3, which is also the r value between full siblings. The r value of half-siblings is ¼, for uncles or aunts and nephews or nieces is ¼, for cousins is ~ and so on. For mammals, without considering inbreeding, the general formula is (3)N = r, where N is the number of generational links between any given two animals. As an example of kinship theory, and the influence of genetic relatedness, consider the actions of animal A when he sights an approaching predator. He can give an alarm call and warn his three fellow conspecifics, exposing himself to predation (an altruistic act), or he can hide himself, exposing his fellows to predation (a selfish act). Suppose that if A gives an alarm call, he is eaten; and if not, the three fellow conspecifics are eaten. Suppose further that all four are basically of equivalent fitness otherwise. Should A give an alarm call, or not? The r value of A with himself is 1. If the three conspecifics are all full sibs with A, each one has an r value of ½ with A, and can be thought of as being (½)A. If A gives the alarm call, the evolutionary equation can be symbolized (½)A + (½)A + (½)A - - A = + (½)A; whereas if he does not, the equation is A - ( ½ ) A - ( ½ ) A - (½)A = - (I)A. Clearly, the inclusive fitness of A is larger when he gives the alarm call, and thus he should do so. But what if the other conspecifics are first cousins of A? Then the equation for giving the alarm call becomes (~)A + (~)A + (~)A - - A = -- (~)A, and for not giving the alarm call it isA -- (-~)A -- (~)A (~)A = + (~)A. In this case, it is best from A's point of view not to give an alarm call. This kind of analysis is embodied in Hamilton's (1964) equation for altruistic behavior: K > l/r, where K is the ratio of recipient benefit to .altruist cost, which must be greater than l/r (the reciprocal of the genetic relatedness of recipient and altruist) in order for such a behavior to be selected. The above example is quite artificial, but there are observations, both in the field and in the laboratory, to confirm the reality of kin selection. Brown (1975) provided two examples of ecological testing of kinship theory in birds. In both scrub jays and superb blue wrens, the yearling has a choice of two strategies at the start of the breeding season. It can stay with its parents and help raise full siblings (altruist), or it can attempt to start a nest of its own (selfish). Which strategy is selected depends upon the size of that strategy's genetic contribution to the next generation. If the altruistic strategy is to be favored, the following rearrangement of Hamilton's (1964) equation must hold: Arl > Sr2, where A is the number of full siblings raised by the parents and the altruist in addition to the number which the parents could raise alone; S is the number of offspring raised by a single mated pair; r~ is the index of genetic relatedness between full sibs; and r2 is the index between parent and offspring. The equation can be further

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rearranged to: A/S >r2/rl, where rl =r2 = ½, and thus r2/rl = 1. Therefore, if the altruist strategy is to be favored,A/S > 1. For scrub jays, data of Woolfenden (1975) reported in Brown (1975)indicate that on the average, 1.3 young were raised to independence in nests with helpers, and 0.5 young by mated pairs alone. In this case, A = 1.3 -- 0.5 = 0.8, while S = 0.5; and thus, A/S = 0.8/0.5 > 1. Therefore, it is predicted that many birds will follow the altruistic strategy; and, in fact, almost all yearlings of the Florida scrub jay did so. The case in the superb blue wren is a bit more complex. Data from Rowley (1965) reported in Brown (1975) indicate, on the average, 2.83 independent young per year with helpers and 1.50 young without; thus,A = 2.83 -- 1.50 = 1.33. For females, 1.33/1.50 < 1, and therefore it is predicted that females will not become helpers. However, only 70% of the males f'md mates each year, resulting in two different analyses for mated and non-mated males. The former case is exactly analogous to that of the females, where 1.33/1.50 < 1. The latter case can be symbolized thus: 1.33/0 > 1. The overall prediction is that females should follow the selfish strategy along with the majority of males, but those males who cannot find a mate should stay and help their parents. Field studies indicate that superb blue wren behavior did indeed follow this prediction (Brown, 1975). Kin selection theory is both a powerful and a subtle concept. There is always difficulty in judging the exact amount and nature of the benefits and costs involved. Massey (1977) tested kinship theory in pigtail macaques (Macaca nemestrina). When disputes occur between individuals, bystanders could interfere by siding with either the attacker or the attacked. From kinship theory, one could predict that: (1) most interference will occur in the form of aiding the attacked party, since that individual would gain the most benefit; (2) such aiding behavior would be highly correlated with r, the amount of genetic relatedness between those aiding and being aided in agonistic encounters. These predictions agreed with the observations: most interference did take the form of aiding the attacked party, and such aiding was highly correlated with r. The correlation attained such a degree, in fact, that the number of aiding episodes differed significantly between cases where r = ¼ and where r = ~. There are further subtleties, however, which show up in the observations. Older animals, for example, aided more and got aided less. In terms of kinship theory and reporductive capacity, this phenomenon makes good sense older animals have less fitness to lose in aiding, and gain less from being aided than do younger animals. These and other kin-related behaviors have been observed in rhesus monkeys (Kaufmarm, 1967; Loy & Loy, 1974), Japanese macaques (Kurland, 1977; Yamada, 1963), and yellow baboons (Lee & Oliver, 1979). Kinship theory has been examined in great detail by several authors (Alexander, 1974; Hamilton, 1964, 1975; Vehrencamp, 1979; West-Eberhard, 1975). It is important to understand that natural selection predicts that altruistic or co-operative behavior will increase in proportion to the genetic relatedness of the animals involved. A word about group selection, the idea that animal groups might serve as units of selection. This idea has been severely criticized by some authors (Dawkins, 1976) on the basis that groups are not sufficiently long lasting and the individual members are not often genetically interrelated enough to serve as units of selection. However, Hamilton (1975) has analysed the possibility mathematically, indicating that group selection may indeed be possible in kin-groups (Brown, 1973) assuming high genetic relatedness and low levels of migration.

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(c) Aggression and mechanisms o f control In so far as active aggression is a means by which an animal can gain resources and increase his or her individual fitness, the question naturally arises as to why there is not more violence among animals than there is. The answer is, of course, that there are costs as well as benefits which arise from aggressive activity and the ratio of costs to benefits determines the adaptive value of aggression in any particular situation. There are a number of different factors which influence the amount and direction of aggressive activity in animals. 1. Factors controlling or decreasing aggression Maynard Smith & Price (1973) applied the tenets of game theory to animal conflicts in order to determine why such conflicts are not more violent. The idea was to match several different kinds of 'strategies' ranging from very violent (Hawk) to no violence (Mouse) in order to determine the evolutionary stable strategy (ESS). An ESS is a strategy such that if most of an animal population follows it, no 'mutant' strategy can be more fit (Maynard Smith & Price, 1973). Game theory and the concept of ESS will be discussed in much greater detail in the following paper. The strategy which proved to be an ESS in Maynard Smith & Price's (I 973) computer simulation was called Retaliator and was a strategy which became violent only in response to violence by its opponent. This idea makes good intuitive sence - if aggression is costly, it behoves animals to settle their differences without too much overt violence. On the other hand, if animals only bluffed and never actually became aggressive a mutant which called his opponent's bluff would win all his contests and that strategy would be selected and increase in the population. An interesting point was that when the probability of causing serious injury in a single move by either opponent became very high [0.9 in Maynard Smith & Price's (1973) simulation], Hawk, or all-out violent aggression, became the ESS. In many animal groups, especially in primates and social predators, membership in a particular group is restricted. It follows that the number of different possible conflicts is limited, also. This will often result in one of two outcomes: (1) animals will come to recognize opponents, or (2) animals will recognize that they can expect to win (or lose) a predictable proportion of agonistic encounters (Dawkins, 1976). Animals will compete for limited resources only so long as the cost of obtaining them does not exceed their realizable benifits. If the outcome of aggressive encounters can be predicted by the animals involved with a fair degree of accuracy then it is to the animals' benefit to act on the basis of these probable outcomes, and avoid risking injury by actually fighting. This recognition of probable outcomes results in the formation of the so-called dominance hierarchy, a set of sustained aggressive-submissive relations in a group which results in the formation of ranks in a semi-linear sequence (Gauthreaux, 1978; Wilson, 1975). Aggression tends to be reduced in the group after an initial sorting-out period (Bernstein, 1969; Southwick, 1969), and becomes significant only in the face of alterations in the status quo. Such alterations include removal of the dominant or alpha individual (Tokuda & Jensen, 1968; Oswald & Erwin, 1976; Vessey, 1971), addition of a new animal, with whom the probable outcomes of encounters are not yet known (Bernstein, 1969; Bernstein, Gordon & Rose, 1974; Dittus, 1977; Hall, 1964; Wade, 1976), or a large increase in the worth of a disputed resource. Appeasement displays serve the function of reducing aggression in conditions where escape is difficult or impossible (Manning, 1972). Such displays earl take advantage of pre-existing conditions by mimicking individuals of which the attacker is usually tolerant or by the attacked animal making itself as little threatening as possible (Manning, 1972). Wielder

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(1967) has argued that the existence of female mimics in male animals lowers aggression by the appeasement mechanism, and in spotted hyenas, where females are the dominant sex, elaborate pseudo-penes and pseudo-scrotums have developed which appear to function in appeasement and greeting rituals (Kruuk, 1972). Genetic relatedness can serve to lower aggression in two ways. First, kinship lowers the cost of losing a conflict with a genetic relative in so far as the loser still increases his or her fitness by way of the increase in fitness of the relative. Second, the fact that genetically related individuals will often aid an attacked relative (Kaufmann, 1967; Kurland, 1977; Massey, 1977) may deter attacks by increasing the cost of attacking. 2. Factors increasing aggression One factor has already been mentioned - a tremendous increase in the cost of losing as demonstrated by Maynard Smith & Price's (1973) computer simulation. The other factor is a large increase in the worth of a disputed resource (Crook, 1970), exemplified by malemale competition for estrous females. Large increases in aggression at times of mating are reported for wolves (Fox, 1973; Mech, 1970; Zimen, 1975), rhesus macaques (Bernstein etal., 1974; Boelkins & Wilson, 1072; Drickamer, 1975; Kaufmann, 1967; Southwick, Siddiqi, Farooqui & Pal, 1974), chimpanzees (Kortlandt & Kooji, 1963; Pitcairn, 1974) and others (Dittus, 1977; Hausfater, 1975; Koyama, 1970; McGuire, 1974; Paterson, 1973). This conflict is a reflection of the change in the cost/benefit ratio at times of mating. Ordinarily, losing a dispute only reduces the loser's share of a contested resource (Loy, 1975; Richards, 1974; Suzuki, 1975), lowering the loser's reproductive potential, but not as much as a serious injury, which might well cancel it completely. A bit more food will increase an animal's reproductive potential indirectly and by only a small amount. Therefore, it is to an animal's benefit to resolve disputes without physical combat, even if it means occasionally losing the contested resource. On the other hand, a male gaining a successful mating directly raises his fitness by a large amount - after all, matings are a male's reproductive potential. Thus, while the cost of fighting remains flatly constant, the benefits of winning increase enormously during mating season, resulting in increased levels of aggression. Sexually receptive females are the cause of this increased male aggression and may promote it by limiting both sexual receptivity and sexual attractiveness to a short estrous period. The selective benefit to the females of promoting male-male competition is the resulting ability to exercise choice - the winners are, by definition, more fit. The idea that female choice is linked to gaining the attention of several males has been used to explain the correlation between estrus, sexual swellings and color changes, and living in multimale primate troops (Clutton-Brock & Harvey, 1976) and the extent of male group transfer in primates [Rasmussen, 1979; see Section I(d)]. For the male, estrus represents a time 'window' within which copulations stand an excellent chance of leading to fertilization, but outside of which, the probability of success becomes very low. Therefore, copulations during estrus are the single most valuable actions a male can take to increase his own individual fitness. It is not surprising, then, that mating seasons are the most aggressive periods in the existence of many mammal groups. Since females in a polygynous society represent a limited resource for the males and are all theoretically capable of mating with the most fit males, they are not subject to such increased aggressivity among themselves. (d) Dispersal Most animal groups are not strictly dosed systems within their respective populations, and usually one sex predominates in leaving the natal group (Greenwood, 1980). For mammals

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in general and primates in particular, the males are the emigrating sex (Clutton-Brock & Harvey, 1976; Greenwood, 1980). Chimpanzees are an exception in that females emigrate (Pusey, 1980), one possible reason for which will be suggested below. Several reasons have been suggested for male dispersal in primates including: (1) The need for females to instigate male-male competition in order to exercise choice (Rasmussen, 1979). Support for this idea comes from the observation that some female primates prefer to mate with 'transferred' rather than natal males (Packer, 1979a, b) (2) The need of males to gain access to estrous females (Greenwood, 1980; Packer, 1979a). Evidence for the hypothesis comes from the multiple transfer of male primates with higher than average reproductive capacity (Packer, 1979a) and comparisons of bird and mammal dispersal systems (Greenwood, 1980) (3) The effects of inbreeding depression in groups where the coefficient of genetic relatedness is high (Maynard Smith, 1978; Packer, 1979a) and when the adverse effects of inbreeding depression are greater than the costs of emigration (Packer, 1979a; Harcourt, Steward & Fossey, 1976). Evidence for this hypothesis comes from an analysis of inbreeding depression in yellow baboons (Packer, 1979a) and the fact that female chimpanzees often return to their natal group after estrus (Pusey, 1980). The reversal of the normal dispersal system in chimpanzees, females emigrating, may be related to the tendency of males to co-operate in attacking and repulsing strange males (Harcourt et al., 1976). The amount of male emigration in primates is by no means clear. Some observers emphasize the transient nature of the males in a troop (Kurland, 1977; Rasmussen, 1979; Sade, 1975) while others argue for a high level of group integrity (Angst, 1973; De Vore & Hall, 1965; MacRoberts, 1970; Packer, 1979b; Simonds, 1965). It seems apparent that in most primate societies, males leave their natal troop as they reach breeding age (Kurland, 1077; Packer, 1979a, b), although there is some evidence for the integration of a limited number of natal males (Kaufmann, 1967; Yamada, 1971). Packer (1976a, b) has reported that most males appear limited to one such transfer after which a fairly stable dominance hierarchy results involving transferred males in their new groups. The long-lasting tenure of the more dominant males has been emphasized by Packer (1979a), Rasmussen (1979) and Dittus (1977). Sade (1975) reported in a longitudinal study of rhesus macaques on Cayo Santiago Island that no adult male stayed with any troop more than four years. Bodkins & Wilson (1972) recorded a positive correlation between population density and rate of male transfers, however, and suggested that the unusually high population density of Cayo Santiago makes conclusions about natural populations problematic. Despite male emigration, the amount of genetic inter-relatedness in primate groups is high. Packer (1979a) estimated the coefficient of relatedness between individuals growing up in a yellow baboon troop to be between 0.12 and 0.17; for any two females, 0.08 ~
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semi-permanently with a troop after transfer from their natal group, and because of the large reproductive advantage of the dominant (Dittus, 1977; Hausfater, 1975; Packer, 1979b), they may show a disproportionate degree of genetic relatedness with juvenile males and females. This tends to increase further the relatedness among the juveniles. The subordinate males may also reside in a single troop for long periods, but are more likely to emigrate again than the dominants (Dittus, 1977; Hausfater, 1975; Kurland, 1977; Lee & Oliver, 1979; Packer, 1979a, b; Rasmussen, 1979; Sade, 1975; Vessey, 1971). Emigration can be very costly. Newcomers to yellow baboon troops are often chased and occasionally wounded (Packer, 1979a). Dittus (1977) observed that 72 per cent of adolescent male toque monkeys who tried to emigrate died making the attempt and Boelkins & Wilson (1972) reported an increase in male mortality in rhesus monkeys related to the frequency of male emigration and immigration. There is little doubt that primate groups are highly xenophobic (Bernstein, 1969; Bernstein et al., 1974; Deag, 1973; Drickamer, 1975; Hausfater, 1972) and this xenophobia increases the cost of group transfer. The social canids do not follow the mammalian rule of male emigration. Females emigrate in wild dogs and never transfer to a pack which already contains a breeding female (Frame & Frame, 1976). Both sexes emigrate rarely in wolves and the acceptance of a stranger into an established pack is even more rare (Allen, 1979; Mech. 1970; Zimen, 1978) although single packs can split and rejoin (Mech, 1970). The reason for this difference in dispersal probably involves the related factors of high genetic relatedness in the pack and the need for extensive male-male co-operation in hunting (Fiennes, 1976; Harcourt et al., 1976; Mech, 1970; Peters & Mech, 1975). By restricting membership in the packs, the genetic relatedness of the members is increased (Hamilton, 1975) and this could increase co-operative behavior (West-Eberhard, 1975). The limitation on the number of offspring [see Section I(b)] per pack may also increase the adaptiveness of a female emigrating to form her own group (Fiennes, 1976; Frame & Frame, 1976). II. Applications to hominid group structure [a} The effects o f primate phylogeny and social predation Present-day primate societies include solitary individuals, mated pairs, single-male harems, age-graded male troops, and multi-male troops (Brown, 1975; Eisenberg etal., 1972). As the protohominids moved from the forest onto the savannah, it is unlikely that solitary individuals or mated pairs would have survived. They would have faced powerful and efficient terrestrial predators as well as widespread, clumped resources difficult for the individual or pair to locate (Clutton-Brock & Harvey, 1977; Loy, 1975). Predator pressure alone can cause group formation, as Hamilton (1971) demonstrated in his 'selfish herd' model. Additionally there are other advantages in group living, such as co-ordinated defence (Clutton-Brock & Harvey, 1976). Groups can serve as information centers concerning the nature and location of resources (Ward & Zahavi, 1973) and would certainly become necessary when the hominids began to exploit big game (Fiennes, 1976; Mech, 1975; Peters & Mech, 1975). Therefore, it is evolutionarily reasonable to assume that the hominids lived in groups. The first level of organization observed on the savannah or terrestrial habitats is the uni-male troop, a group consisting of one male and several females and their offspring (Clutton-Brock & Harvey, 1977; Eisenberg et al., 1972). Based on the argument described in Section I(a), it is easy to see how such a group might evolve. From the male's point of view, he can insure polygyny by being the only male in a group of females. From the female's point of view, the very fact that this particular male can gain and maintain a group of

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females against other males may indicate the fitness of that male. Thus, social structure allows for female choice, and is adaptive for both female and male sexual strategies. Although widely distributed among forest species, the uni-male harem is relatively rare on the savannah compared to age-graded and multi-male troops (Clutton-Brock & Harvey, 1977; Eisenger etal., 1972; Struhsaker, 1969). There appear to be several reasons for this effect. First, the amount of time and energy necessary to prevent other males from joining the group increases as the ability of a solitary male to survive alone decreases (Clutton-Brock & Harvey, 1977) and on the savannah, the survivorship of a lone male primate will be poor (see above). The increased aggression needed to protect the harem will adversely affect the male by: (a) decreasing the time available for finding food, predator protection and reproduction and (b) exposing him to increasing probabilities of injury. These factors will also lower the fitness of the females in the group. A second reason for the rarity of one-male groups is that in areas of high predator pressure, there is a selective advantage to co-ordinated defence by the males for both males and females (Altmann & Altmann, 1970). When these factors outweigh the original advantages of the uni-male group, age-graded or multi-male groups may form as has been the case for most terrestrial primate species (Clutton-Brock & Harvey, 1977; Eisenberg etal., 1972; Struhsaker, 1969), Thus, it is a fairly good bet that the hominids formed age-graded or multi-male troops, at least early in their savannah existence. There is the possibility that the hominids formed open, non-competitive associations, even though the open group is not the basic social structure of any terrestrial primate (CluttonBrock & Harvey, 1976, 1977; Eisenberg et al., 1972). If such a hominid group existed, there would be a large selective advantage to any 'mutant' hominid who was aggressive and competed for resources (see Dawkins, 1976; Williams, 1966;Maynard Smith & Price, 1973). The number of individuals pursuing this 'mutant' strategy would increase in the population until most individuals were competitive. There would then be a selective advantage to associating with genetic relatives and modifying one's aggression accordingly. In order to maintain lowered levels of aggression due to kin selection and also maintain sufficient positive and negative reinforcement to modify aggression via a dominance system [Section I(c)], it would be necessary to restrict membership in hominid groups. Thus, not only are open primate groups seldom seen on the savannah (Clutton-Brock & Harvey, 1976, 1977) but there are good evolutionary reasons for why such groups are not often seen - reasons which should have applied equally well to the early hominid groups. The primates as a group are very adaptable and are capable of many different kinds of social organization in response to selection and depending on the phylogenetic history of the species involved (Chalmers, 1973; Eisenberg et al., 1972; Struhsaker, 1969). There is some indication that primates are more aggressive, with stronger dominance hierarchies, in a terrestrial habitat (Chalmers, 1973; Poirier, 1970) and terrestrial species are thought to form larger groups (Eisenberg et al., 1972; Kummer, 1977). Despite this variability, there appears to be a basic pattern to the age-graded or multi-male terrestrial primate troops, and there is no a priori reason why the hominids, as primates facing a terrestrial existence, should have been radically different. To review from previous sections, the terrestrial age-graded or multi-male primate troop appears to be built around a core of genetically related females with a stable dominance hierarchy (Hausfater, 1975; Kurland, 1977; MacRoberts, 1970; Packer, 1979a; Sade, 1975). The factors of relatedness and a stable dominance system increase the level of co-operation among the females for the exploitation of resources [Sections I(b), (c); Kurland, 1977; Trivets, 1972], although aggression is by no means eliminated (Koyama, 1970). Most, if not all, males leave their natal troop before breeding, transferring to other groups where males

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form their own dominance hierarchy, which is somewhat less stable than that of the females (Kurland, 1977; Packer, 1979a; Sade, 1975). Male competition is more for access to estrous females than for controlling other resources (Trivers, 1972) and dominance ranks are highly correlated with reproductive success in their polygynous systems (Dittus, 1977; Hausfater, 1975; Packer, 1979b). Although polygyny is a basic male sexual strategy hi mammals [Section I(a); Trivers, 1972; Orains, 1969], there is evidence that primate females, by maintaining and signalling a short estrous period and by soliciting stranger males, instigate malemale competition in order to exercise choice [Section I(a), (b), (d); Clutton-Brock & Harvey, 1976; Kudand, 1977; Packer, 1979a; Rasmussen, 1979; Wade, 1976). Males may demonstrate their fitness by being able to join an existing group and the dominants may do so simply by being able to resist challenges from subordinants and strangers. The dominant males, by virtue of their rank and the reproductive success that results from it, may spend long periods in a single troop [Section I(d); Dittus, 1977; Kaufmann, 1067; Packer, 1979b; Vessey, 1971] and large amounts of competition may aid the group's females in reevaluating their fitness. A long tenure for dominant males may result in high r values between those males and the troop's offspring and increases the r values among the offspring and juveniles. Other reasons for male migration in primates, inbreeding depression and access to estrous females, are discussed in Section I(d). Xenophobia is well known in primate groups (Bernstein et al., 1974; Dittus, 1977; Wade, 1976) and is mostly directed toward extra-group conspecifics of the same sex (Wade, 1976). Xenophobia against females may reflect the desire of a group's females to avoid rank-order conflicts and loss of inclusive fitness caused by sharing resources with an unrelated newcomer. The aggression of the resident males could also be a method of avoiding rank-order disputes or it could simply be a manifestation of male-male competition. To the existing primate social structure discussed above, the hominids added the selection pressures associated with co-operative hunting. The following discussion will focus mainly on the social canids and spotted hyenas since lions retain a harem social structure (Bertram, 1975, 1978; Schaller, 1972) which, for reasons discussed above, is probably not applicable to the hominids. Some of the major social differences between the primate structure given above and that of the social carnivores are: (1) a smaller group size with a smaller range of sizes; (2) greater levels of genetic relatedness and co-operation among the group's males (especially true for social canids); (3) decreased levels of emigration and female-based emigration in wild dogs; (4) increased levels of xenophobia and very restricted immigration; and (5) in wolves and wild dogs, a single monogamous breeding pair per pack (Allen, 1979; Cowan, 1947; Fiennes, 1976; Frame & Frame, 1976; Jordan, Shelton & Allen, 1967; Kruuk, 1972; Mech, 1970, 1975, 1979, Mowat, 1963; Rabbet al., 1967; Van LawickGoodaU & Van Lawick-Goodall, 1971; Zimen, 1975, 1976, 1978). The reader should realize that the selection pressures associated with the above behaviors acted on a basically primate social structure when influencing the hominids - in no way can it be expected that the hominids totally replaced their primate structure with one like that of the social carnivores. Although the smaller group size and range of social carnivores may be in part related to their position as secondary consumers, Mech (1970) suggested that social factors, including limits on the amount of social bonding possible, were the major factors limiting group size. This idea is supported by observations showing that increases in the amount of suitable prey and subsequent increases in the population density of wolves, results in more packs rather than packs with many more members (Allen, 1979; Jordan et al., 1967; Rausch, 1967). Although co-operative hunting may have lowered the average number of hominids in a troop, there are reasons for expecting this effect to have been minimal. First, the hominids

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were probably omnivores and thus mixed primary and secondary consumption. Second, the hominids were prey as well as predators, and larger groups served as predator protection. And third, primates are capable of maintaining a social structure and sufficient social bonding in very large groups (De Vore & Hall, 1965; Kaufmann, 1967; Tsumari, 1967) and therefore the hominids were probably not subject to as strict a limitation in size based on social bonding. The differences (2)-(4) are in all probability related to maintaining a cohesive, cooperative social group necessary for big game hunting (Peters & Mech, 1975). The instability of the male dominance hierarchy in primates is due to the extent of males changing groups and the lack of genetic relatedness among the adult males [see Section I(d)]. As the hominids began group hunting, it would have become necessary to increase the level of male cooperation - presumably by enhancing the stability of the male dominance hierarchy and increasing the extent of relatedness among the adult males. This could have been accomplished by prolonging the tenure of the dominant males and allowing for a greater amount of integration of the natal males into the group's social hierarchy. At the same time, the amount o f group transfer was reduced. This would not be as radical a shift at it sounds in so far as primate groups can retain the dominant males for long periods (see above) and there is some indication that males can become a reproductively active part of their natal troop occasionally (Kaufmann, 1967). Thus, the changes required by co-operative hunting seem within the range of present-day primate variability. Monogamy in the social canids is apparently not related to the need for male parental care as such, but rather to an active suppression of other mating attempts by the breeding pair [see Section I(a), (d); Vehrencamp, 1979; Zimen, 1975]. It was argued in Section I(a) that this suppression was in part the result of restrictions on group size leading to a limitation on the number of pups per pack with maximum survivorship. Monogamy for this reason is not likely to have evolved in the hominids because (1), the large range of group size seen in terrestrial primates (Clutton-Brock & Harvey, 1977; De Vore & Hall, 1965) is likely to be reduced only slightly by the needs of co-operative hunting in the hominids and therefore there was no significant selection pressure for limiting mating and (2) in the larger hominid groups, where the females were unlikely to be monoestrous, it would have been difficult and energetically maladaptive for a single pair to attempt to suppress all other matings. It is predicted that hominid societies were based on dominance hierarchies and polygyny from the time of emergence onto the savannah until the development of tools and technology. Male group transfer became somewhat more restricted and the likelihood of adult males remaining in their natal troop was increased due to selection pressures associated with co-operative hunting. The overall r value of the hominid group was enhanced as a result. The dominance structure possibly consisted of overlapping male and female hierarchies such that at a given rank, males were dominant to females, but females dominated the males and females of lower ranks. This system has been reported for wolves (Zimen, 1975, 1976) and females dominating males of lower ranks has been reported for some primates (Sade, 1967; Yamada, 1963, 1971). In addition, we speculate that hominids formed multimale bands instead of age-graded troops because of the increased level of co-operation possible in multi-male groups (Eisenberg et al., 1972).

(b } The weapons effect Weapons presumably developed as a method of predator defenee in terrestrial habitats where throwing could be effective (Kortlandt & Kooij, 1963; Van Lawiek-Goodall, 1970). Weapons significantly altered the costs and benefits of aggressive behavior for two reasons.

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First, weapons could be developed faster than physiological protection against them could evolve. Under normal conditions, dangerous weapons and physiological counter-measures co-evolve, limiting the effect of aggressive behavior. This was pointed out by Lorenz (1963) and an example is the development of the male lion's mane as a counter-measure against canine teeth and claws. Artifical weapons, like those of the hominids, disrupt thts pattern. Second, and perhaps more important, the hominids' weapons could be thrown, removing the attacker from close physical proximity to the attacked. In effect, weapons lowered the costs of attacking while increasing the costs of being attacked. As a result, the chances of serious injury in a conflict were greatly enhanced. Many hominid groups, and perhaps whole species, may have become extinct because they were unable to deal with their new technology. Because of the high risk of serious injury, individual selection, ignoring other influences, would tend to favor 'Hawk' or an all-out aggressive strategy [Section I(c); Maynard Smith & Price, 1973]. In other words, as weapons increased the cost of losing an agonistic encounter, there was a selection pressure to develop 'pre-emtive strike' or attack before being attacked behavior (Maynard Smith & Price, 1973). In order for the hominid groups to survive (and the various selection reasoris for living in groups still applied), it was necessary to reduce aggression, i.e. to alter the cost/ benefit ratios of certain behaviors in order to reduce conflict. As discussed in Section I(c), aggression can be reduced by (1) increasing the genetic relatedness among potential adversaries, (2) a very stable dominance hierarchy and (3) appeasement signals or gestures. Conflict can be increased by (1) rank-order disputes, such as often occur when a stranger is introduced and (2) the females' short estrous period, which encourages male-male competition and group transfer. The selection for reduced conflict in the hominid groups would have acted on males and females since both sexes could be hurt by aggressive activity, either directly or indirectly. Directly, there was the chance of a serious physical injury. Indirectly, there was the loss of fitness which would occur due to reduced levels of hunting and predator protection if any substantial number of males were incapacitated. Therefore, behavioral changes can be expected to have occurred in both sexes. The first evolutionary step taken as weapons developed was probably severely to restrict individuals from changing groups. From the residents' point of view, the admission of an extra-group conspecific would lead to now dangerous rank-order confrontations as the stranger's status was determined [see Section I(c)]. In addition, the risks facing a hominid who attempted emigration or group transfer had increased dramatically. Running was no longer sufficient to escape the effect of weapons which were able to be thrown. An increased reluctance to admit strangers, coupled with greatly enhanced risks associated with emigration, would combine virtually to close the hominid groups. The dosing of the hominid groups would have resulted in two other beneficial effects. Because of the increased tendency of males to remain in their natal troop, the genetic relatedness among the adult males, and in the group as a whole, increased. This in itself would tend to reduce aggression [see Section I(b)]. Combined with a more uniform and predictable male membership in the group, the increase in r among the males would help establish a more stable and long-lasting male dominance hierarchy, reducing the chances of disruptive rank-order conflicts. The new high costs of overt aggression would also act to change the character of the dominance system. In so far as the dominant individuals could not afford to be injured in rank-order fighting, there would be art increased selection for social skill in attaining and maintaining status, and decreased emphasis on overt aggression. Presumably, these new social and mental skills would then become the basis for female choice. For subordinants,

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the risks of fighting were even higher (they were, after all, more likely to lose) and selection would have acted to increase the individual's acceptance of the system and obedience to authority (individuals of higher rank). Such selection may be involved in the ejection of certain alpha and subordinate wolves from the pack, where it has been suggested that some animals are forced out because they didn't adjust to the pack's social structure (Fox, 1975; Zimen, 1975). Dittus (1977) has also noted that former alpha toque monkeys are sometimes forced to emigrate from their troops. In a similar manner, selection for conformity to the social system would act on hominid individuals of all ranks to reduce conflict. Females in estrus will sometimes be attacked by males in primate societies (Carpenter, 1974. Kortlandt & Kooij, 1963), an example of 'displaced' aggression (see Manning, 1972). In a species where males compete for females, a selective premium is placed on the females to reduce aggression directed toward them, often by looking very non-maleqike (Selander, 1965). Appeasement signals or gestures serve to reduce or redirect attack, often by the attacked anaimal mimicking some non-threatening stimulus or individual. These signals can become physically quite elaborate [see Section I(c); Kruuk, 1972]. It has been observed that the males of many species refuse to attack or are very tolerant to lactating mothers and young offspring especially members of their own group (Allen, 1979; Fiennes, 1976; Fox, 1975; Fausfater, 1974; Mech, 1970; Rabb etal., 1967; Southwick et al., 1974; Van LawickGoodall & Van Lawick-GoodaU, 1971; Zimen, 1975, 1976, 1978). An obvious ploy which would have gained the hominid females safety and a selective advantage, would have been to take advantage of this pre-existing response of the males by mimicking the appearance and behavior of mothers and children as an appeasement signal. Natural selection would then act quickly to select these characteristics since the females with them, being less prone to male attack and therefore likely to have a longer reproductive life, were more fit. During estrous or mating periods, the large benefits of mating would have still caused heightened aggressivity despite the costs of fighting with weapons. Even though male-male competition was reduced through restricted immigration, the problem of conflict involving the troop's males remained. In order to reduce aggression, one can increase the costs or lower the benefits. Since the costs were already high due to the development of weapons, the best strategy would have been to lower the benefits. The females' short estrous periods are a primary cause of male-male competition [see Section I(a), (c)]. With a short estrus, copulation is almost certain to lead to fertilization, and thus there is a high benefit-tocopulation ratio, and fighting for copulations is selected. If the estrous period were to lengthen, the chances of any given copulation leading to fertilization would decrease, and the benefit-to-copulation ratio would also decrease. As a result, the advantages of.fighting would diminish, and estrous period conflict would be reduced. Therefore, we propose that hominid females were selected for longer and longer estrous periods, until f'mally, estrus became tonic or continuous. A similar, though more limited, hypothesis has been proposed to explain certain aspects of lion reproductive behavior (Bertram, 1975). A much longer estrous period has some interesting implications. In a polygynous group with short-estrus females, all females are theoretically equipotent to the male. His investment in mating is so small that very occasional unsuccessful copulations hardly matter [see Section I(a); Orions, 1969; Wilson, 1975]. Female choice can be exercised by promoting male competition and mating with the winners. Since it is theoretically possible for the most fit males to perform all of a group's matings, female competition for mates is virtually nonexistent. In practice, the dominants do not perform all of the matings, and subordinates do obtain some successful copulations (Hausfater, 1975; Packer, 1979b; Rabb etal., 1967). Even so, there is little selective advantage in male choice or female sexual competition. As the estrous period lengthened, however, the number of copulations required to insure

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successful fertilization would have increased. Additionally, since the best time to fertilize the female was no longer signalled by a short estrous period, the male needed to control the female for a longer period of time, both to insure his own success and to prevent matings with other males. Thus, each male was forced to invest greater amounts of time and energy in each female. The result of this trend would have been that the females could n o longer be considered equipotent. The dominant males could control a number of females, given their priority over resources and the strengthening dominance system, but could not be expected to be able to control all females on a permanent basis. As a result, there is a possibility that male choice might have become selectively advantageous. The advantage arises from the fact that females who avoid male aggression are much more fit than those who do not. Males who mated with females with aggression-diverting characteristics would produce more fit female offspring, and thus dominants who were selected to choose such females would also be more fit. Since it was no longer even theoretically possible for all females to mate with the dominant males, mild female competition would have occured (mild because several females could still mate with one alpha male). Given male choice and female competition, aggression-diverting characters (such as protruding breasts, less body hair, high-pitched voices etc., which mimicked mothers and young) would have become sexually selected, a type of natural selection which often results in an exaggeration of the selected characteristics far beyond that needed for their original function (Darwin, 1871; O'Donald, 1963). Other characteristics, such as ability to raise offspring or social skill in maintaining rank, could also have become selected for in females. Subordinate males faced the same problem as the more dominant hominid males i.e. an increased investment necessary to insure successful reproduction. Given less access to necessary resources, and being less attractive to the females, only a few subordinates could mate. These matings might well have resulted in semi-permanent monogamy (see Trivers, 1972), depending upon the resources available and the position of the female in her hierarchy. The dosing of the hominid groups would have increased the genetic relatedness of the adult males. This may well have enhanced the selective value of slight differences in social or mental abilities in attaining dominance status. The same would have been true for the hominid females, now competing for mates. The result might have been an acceleration in the evolution of those social and mental skills which correlated with dominance rank. There are some difficulties with having a group as closed as the one described above. One problem is the effect of inbreeding depression. It is possible that this difficulty could have been partially avoided by both sexes recognizing very close relatives and refusing to mate with them (Maynard Smith, 1978). In fact, this may have contributed to the development of humans' ability to differentiate people and recognize relatives. Another serious problem is that, in the absence of emigration or high mortality rates (infanticide etc.), group size will increase. Possible solutions to both these problems will be examined in greater detail in our forthcoming paper. In summary, our analysis suggests that the hominids had evolved, up to 40,000--50,000 years ago, a strongly dominance-based polygynous society. There was marked selection for obedience to the social structure operating on individuals of all ranks, and consequently, ingroup conflict was minimal. Actual polygyny was restricted to the higher ranking males, and consisted of semi-permanent mating ties to several females. Subordinate males who did mate were monogamous. Dominance rank was based more on social than physical skill. Females were selected for, among other things, longer estrous periods and mother-and-childlike aggression-diverting characters. The surviving hominid groups were closed and members were highly related.

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Testing the hypothesis Although the evolutionary scheme presented above makes good evolutionary sense, and has theoretical and some comparative ethological support, it is simply a hypothesis. In order to test it, we must make testable predictions. Dominance-based or competitive polygyny has certain distinctive characteristics. Some of these are (1) sexual dimorphism, with male attributes related to those actions necessary to win females, (2) delayed maturation of the male and (3) higher male mortality rates (Brown, 1975). Past mortality rates are difficult to determine, and especially sex differences therein, and therefore will not be discussed. Sexual dimorphism is the result of differential selection pressures on the sexes. In monogamous species, all males are likely to mate and therefore characters like large size, strong copulatory and aggressive drives and conspicuous sexual signals are held within strict limits (Selander, 1965). Individual selection is paramount, and it is expected to operate almost equally on both sexes. Ralls (1976) pointed out that, all things being equal, there is a selection pressure for increased size in the female in cases of prolonged gestation (the so-called 'big mother' hypothesis), and in humans, there is a small positive correlation between maternal stature and reproductive success (Thomson & Hytten, 1973). Thus, if other survival pressures are about equal, females might be somewhat larger than males. Polygyny, on the other hand, implies male-male competition for females, and then competition occurs for a limited resource, there is often selection for those characteristics, such as large size and aggressiveness, which aid in obtaining that resource (Wilson, 1975). A survey of monogamous primates, such as Callicibus moloch, and Callithrix, Saguinus and Symphalangus species agree with prediction in showing little or no sexual dimorphism (napier & Napier, 1967). Clutton-Brock & Harvey (1977) also surveyed the primates and concluded that all sexual dimorphism examined could be explained on the basis of differential breeding success. It is an important indication of past polygyny, therefore, that there is marked sexual dimorphism in both fossil hominids and present-day Homo sapiens. In an examination of the Australopithicine-like hominids, Wolpoff (1976) concluded that size differe._lces may have reached as much as 100 per cent of body size, males being larger. The Australopithecines apparently had tooth size dimorphism greater than any living primate, and canine tooth breadth is still sexually dimorphic in modem humans (Wolpoff, 1967). White (1980) also reported that early hominids were sexually dimorphic as did McHenry (1974) and modem human males tend to be larger, with a heavier muscle mass, than females. One reason why modem humans are less sexually dimorphic than the early hominids may involve the changing character of the dominance hierarchy. The effect of weapons was to limit the uses of overt aggression and switch the level of male-male competition from the physical realm to that of social and mental skills. As dominance was more and more determined by non-physical means, normalizing selection would act to reduce the differences between male and female (Maynard Smith, 1978). Another reason for decreased sexual dimorphism in modern humans will be discussed in the forthcoming paper. Considering the adaptability of behavior, and the probability of meaningful evolution in the past 2000 generations (during which the hominids may have had a very different social structure), it is significant that behavioral sexual dimorphisms still appear to exist. Moss (1966) discovered that a higher mean activity level exists for males as early as three weeks of age. Interestingly, although the correlation of irritability and maternal attention was significant and positive for both males and females at three weeks, it remained so only for females at three months - for males, the correlation had become negative (Moss, 1966). Thus, it is unlikely that the observed dimorphism was the result ~of a differential maternal response to male irritability. One-year-old children have also been observed to display some

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behavioral dimorphism. Boys tend to be more active and exploratory than girls (Goldberg & Lewis, 1969). Brindley and his co-workers (Brindley, Clarke, Hutt, Robinson & Wethli, 1973) observed that not only did boys from ages three-and-a-half to five years display significantly more aggression, but they elicited more aggression as well. Even more interesting was the observation that boys were significantly more aggressive toward other boys, whereas female aggression was evenly distributed towards other persons, teachers and objects (Brindley et al., 1973). This would be expected if the aggression displayed by the boys were an evolutionary remnant of male-male competition. Nor can these differences in aggression be entirely cultural. Boys are significantly more aggressive than girls in Bushman cultures, as well as in modem London society (Blurton-Jones & Konner, 1973). Delayed maturation of the male in dominance-based societies appraently served two purposes: (1) it gives the individual male time to develop the strength and ability necessary to compete and (2) it may give the male time to aid the group by co-operation prior to the onset of competition (Fox, 1975). Delayed maturation is seen in many dominance-based and non-monogamous animal groups, including primates (see Brown, 1975), birds (Selander, 1965) and wolves (Fiennes, 1975; Fox, 1975; Pulliainen, 1967). In light of this evidence, the delayed maturation of the human male [males reaching 95 per cent of adult height 1.8 years on the average later than females (Marshall, 1974)] is supportive evidence for an evolutionary history of dominance and polygyny. The size of the male penis has been suggested as necessary to reinforce the hominid pair bond (Morris, 1967). There is no comparative ethological evidence to support this ideal. The gibbon, human's closest monogamous primate relative, has a 4-cm penis when erect, for example (Michael, Wilson & Plant, 1973). Penile displays, however, are known to indicate dominance and aggression in some primate groups (Baldwin, 1968. Loy, 1975; Pruscha & Maums, 1976). It is possible that hominids, deterred from actual aggression by the cost of weapon use, needed a highly visible signal of dominance, such as the penis. Female secondary sexual characteristics cannot honestly be used to test the hypothesis which was, in part, created to explain them. This hypothesis, however, also predicts that estrus became continuous. It is logical to suppose that human female receptivity would then be under the control of tonically secreted hormones, instead of the cycling estrogen and progesterone levels seen in other mammals (Beach, 1976; Butler, 1974). While many mammalian females increase sexual responsiveness when administered high doses of androgens (Everitt & Herbert, 1969, 1970; Whalen & Hardy, 1970), the normal receptivity and sexual attractiveness of the species examined still remain under the control of estrogen and progesterone (Michael, Zumpe, Keveme & Bonsall, 1972; Whalen & Hardy, 1970). On the basis of several different lines of evidence, it has been proposed that human females' sexual receptivity is based on the tonic secretion of adrenal androgens (Money, 1961; Waxenberg, Finkbeiner, Prellish & Sutherland, 1960). Tonic estrus is a biological phenomenon as would be expected from the model discussed in this paper. The evidence for dominance in human societies has been reviewed by Tiger (1970) and Eibl-Eibesfeldt (1974) and need not be discussed here. Although by no means conclusive, the evidence does tend to support the existence of dominance hierarchies in the human evolutionary past. Also supportive is evidence of dominance-like group structures where such structures would not necessarily be expected. One such example was presented by Cartwright & Robertson (1961) who examined student achievement. They reported that an individual's achievement was correlated with the performance of the 'leader' of the group that person was in, even after IQ and need-to-achieve were partialled out (Cartwright & Robertson, 1961). Both the relationship of the followers' behavior to that of a leader, and the very existence of a leader, are reminiscent of dominance hierarchies.

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In conclusion, this model proposes that the early hominids, as both primates and social predators, lived in polygynous groups structured in overlapping dominance hierarchies. The advent of weapons placed a selective premium on the control of aggression within the group. This resulted in the hominid groups becoming virtually closed, stabilizing the male dominance hierarchy and increasing the genetic relatedness of the groups. The lengthened estrus and period of sexual receptivity of females served to minimize male-male and other types of conflict within the group. The development in females of less body hair, protruding breasts, high-pitched voices etc. as mimics of children and mothers particularly reduced aggression directed toward females. These characteristics, being fitness-related, eventually became attractive to the male. The dominance hierarchy became better organized and stronger as it was used to reduce intragroup aggression and promote co-operation in the necessary areas of hunting and predator defence. A selective premium was placed on accepting authority and responsibility, important for the evolution of early hominids and the precursor for future social and cultural systems. Consequently, the foundations of ethics and morality probably were established at the time of early hominid evolution. It is important to note that the authors believe that significant evolutionary changes have occurred in humans over the last 2000 generations or so. Thus, this model is not intended to be a complete history of hominid evolution. In addition, any attempt to justify present cultural practices on the basis of this model is unscientific conjecture. The purpose of sociobiology, and indeed, science in general, is not to provide moral justifications. The best that can be hoped for is a bit more understanding, and with it, perhaps a bit more control.

Acknowledgments This work was supported in part by the Environmental, Population, and Organismic Biology Department, University of Colorado, Boulder; the Institute for Behavioral Genetics, Boulder; the Neurosciences Department, School of Medicine, University of California, San Diego; and the Veterans Administration Medical Center, San Diego. We wish to thank Daniel F. Kripke, M.D. and Dorothy A. Doughman for their invaluable assistance.

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