(Evolutionary) Theories of warfare in preindustrial (foraging) societies

I present an inventory of theories of war causation (and on the origin of war) in preindustrial (traditional, foraging, 'primitive', hunter-gatherer, band- and tribe-level) societies, with emphasis on the roles of natural selection, sexual selection and kin selection. Also the school of sociocultural evolution is briefly discussed.     

Demography and competition in Hyrax

Summary This study investigates the demography and interspecific interactions of 6 Heterohyrax brucei and 4 Procavia johnstoni populations, which inhabited 6 kopjes (rock outcrops) in the Serengeti National Park, Tanzania over a period of 17 years. On three kopjes (H1, O2 and PH2) both species lived sympatrically, while on the other three (H2, H3 and P1) either one or the other species occurred allopatrically. The rainfall pattern between 1971/1972 and 1981/1982 had undergone major changes. There was significantly less precipitation in the wet season and the dry season had become extremer and longer. The kopje vegetation, recorded as percentage crown cover of browse from bushes and trees, had changed also, decreasing between 1971/72 and 1982 on 5 kopjes. In three kopjes (H1, O2 and PH2), which were shared by both hyrax species, an increase in the number of P. johnstoni, a decrease (H1 and O2) and extinction (PH2) of H. brucei took place between the time periods 1971–1976 (good browse years) and 1982–1984 (poor browse years). These changes are probably the result of indirect interspecific competition (exploitative competition) for browse material, which is the main food source for both species during the dry season. When the browsing resources are abundant coexistence on sympatric kopjes between both species is possible, but when browse is scarce the result is competitive exclusion of H. brucei by P. johnstoni. It is also argued, that P. johnstoni by being the larger species and by also being a grazer had several ecological advantages over H. brucei. In kopjes H2, H3 and P1 where H. brucei occurred allopatrically the population increased (H2, P1) or decreased (H3) during the same time period. Natural extinction was observed in kopje P1 in the allopatrically living P. johnstoni group through mange. Dispersal of 7 females and 7 males H. brucei as well as 1 female and 4 male P. johnstoni were recorded in kopjes H1, H2 and P1. One female H. brucei that immigrated successfully into kopje H2 reached an age of over 11 years. One H. brucei family group (H2) shifted its birth season from December–January to May–August. Successful natural colonization was recorded for H. brucei in kopje P1 and for P. johnstoni in H1. Two kopjes (PH2 and H3), which had no hyrax and are located far away from other hyrax populations, were experimentally colonized in 1971 and 1972 respectively. The P. Johnstoni on PH2 and the H. brucei group on H3 had been under breeding isolation for 16 years. Under such conditions P. johnstoni males can reach an age of over 8.5 years. These long-term observations have shown that the occupancy of the kopjes by both species is a dynamic process depending on the combination of several abiotic (rainfall and availability of holes and hiding places) and biotic factors (interspecific and intraspecific competition for food, interspecific cooperation, predation and parasites) as well as the degree of inbreeding dependent on the geographic isolation of the kopje.SWRC publication No. 448     

Human Demography and Disease

Human Demography and Disease. Susan Scott and Christopher J. Duncan. New York: Cambridge University Press, 1998. 354 pp.     

Evolutionary demography of agricultural expansion in preindustrial northern Finland

A shift from nomadic foraging to sedentary agriculture was a major turning point in human evolutionary history, increasing our population size and eventually leading to the development of modern societies. We however lack understanding of the changes in life histories that contributed to the increased population growth rate of agriculturalists, because comparable individual-based reproductive records of sympatric populations of agriculturalists and foragers are rarely found. Here, we compared key life-history traits and population growth rate using comprehensive data from the seventieth to nineteenth century Northern Finland: indigenous Sami were nomadic hunter-fishers and reindeer herders, whereas sympatric agricultural Finns relied predominantly on animal husbandry. We found that agriculture-based families had higher lifetime fecundity, faster birth spacing and lower maternal mortality. Furthermore, agricultural Finns had 6.2% higher annual population growth rate than traditional Sami, which was accounted by differences between the subsistence modes in age-specific fecundity but not in mortality. Our results provide, to our knowledge, the most detailed demonstration yet of the demographic changes and evolutionary benefits that resulted from agricultural revolution.     

Age-specific genetic and maternal effects in fecundity of preindustrial Finnish women

A population's potential for evolutionary change depends on the amount of genetic variability expressed in traits under selection. Studies attempting to measure this variability typically do so over the life span of individuals, but theory suggests that the amount of additive genetic variance can change during the course of individuals' lives. Here we use pedigree data from historical Finns and a quantitative genetic framework to investigate how female fecundity, throughout an individual's reproductive life, is influenced by "maternal" versus additive genetic effects. We show that although maternal effects explain variation in female fecundity early in life, these effects wane with female age. Moreover, this decline in maternal effects is associated with a concomitant increase in additive genetic variance with age. Our results thus highlight that single over-lifetime estimates of trait heritability may give a misleading view of a trait's potential to respond to changing selection pressures.     

Modeling preindustrial ANC and pH during the spring flood in northern Sweden

The natural preindustrial ANC and pH decline associated with 22 melt events from 11 streams during spring flood of 1997 and 1998 in Northern Sweden have been assessed using the Boreal Dilution Model (BDM). The results show that the spring flood pH decline of 0.5 to 2.5 pH units in the streams was largely caused by natural processes. The most important driving factors influencing pH were TOC increase in combination with ANC dilution. The study also demonstrates that pH in some streams can decline to pH values close to 4.5 as a result of natural processes alone. In general the anthropogenic component of the pH decline was between 0.1 and 0.3 pH units superimposed on the natural pH decline. Furthermore, the anthropogenic contribution to ANC and pH decline generally followed the gradient of anthropogenic S deposition in the region. The sites with the greatest inferred anthropogenic influence are also those for which the assumptions used in the BDM were most sensitive. Nevertheless, the results from this study suggest that the regional impact of anthropogenic acid deposition on the ANC and pH in northern Sweden is currently limited.     

Epilogue — Demography and anthropology

As a conclusion, this paper reviews briefly the content of the volume. The wealth of demographic data has not been adequately exploited in anthropology; this is why this publication is valuable in showing attempts to apply demographic data in a variety of anthropological problems. This symposium has explored many interesting points which we recall here. Yet it has also opened up a whole range of further questions on the material presented as well as in this broad field. Several directions of research could be developed, for instance, testing among human populations, over long periods, the ecological thoughts of ecosystems evolving as a cascade of instabilities, rather than a succession of equilibrium states. Let us also recall the pervasive nature of demographic facts in topics such as the energy cycle or the genetic structure and evolution of human populations.     

Structure and function in mammalian societies

Traditional interpretations of the evolution of animal societies have suggested that their structure is a consequence of attempts by individuals to maximize their inclusive fitness within constraints imposed by their social and physical environments. In contrast, some recent re-interpretations have argued that many aspects of social organization should be interpreted as group-level adaptations maintained by selection operating between groups or populations. Here, I review our current understanding of the evolution of mammalian societies, focusing, in particular, on the evolution of reproductive strategies in societies where one dominant female monopolizes reproduction in each group and her offspring are reared by other group members. Recent studies of the life histories of females in these species show that dispersing females often have little chance of establishing new breeding groups and so are likely to maximize their inclusive fitness by helping related dominants to rear their offspring. As in eusocial insects, increasing group size can lead to a progressive divergence in the selection pressures operating on breeders and helpers and to increasing specialization in their behaviour and life histories. As yet, there is little need to invoke group-level adaptations in order to account for the behaviour of individuals or the structure of mammalian groups.     

Fitness in stratified societies

When wealth or social status can be transmitted from parents to offspring and when fitness depends on wealth or social status, evolutionary consequences of individual transmission strategies can be described by a parameter, called long-term fitness by Rogers (1990), which is the expected relative contribution of an individual to the gene pool in the long-term future. We show how to measure and use this parameter in two models of general interest in sociobiology. First, we construct a system with social classes and hypergynous marriage. Our treatment includes a method for computing the fitnesses of the two sexes separately. As expected, upper-class males have the highest long-term fitness in this kind of social structure, followed by lower-class females, then lower-class males and upper-class females. Upper-class preference for sons would be favored by selection in this system, but not female unwillingness to marry down—in this sense such systems do not conform to a Darwinian model. We then study a system with one sex and three social classes, the poorest of which has very low single generation fitness. In this system, the class with the highest single generation fitness does not have the highest long-term fitness. We suggest that this system is a useful model for understanding the changes in reproductive behavior that occured during the demographic transition in Europe. We suggest that the absence of a destitute lower class in Africa may help explain the failure, so far, for signs of demographic transition to appear in Africa.     

Demography in an increasingly variable world

Recent advances in stochastic demography provide unique insights into the probable effects of increasing environmental variability on population dynamics, and these insights can be substantially different compared with those from deterministic models. Stochastic variation in structured population models influences estimates of population growth rate, persistence and resilience, which ultimately can alter community composition, species interactions, distributions and harvesting. Here, we discuss how understanding these demographic consequences of environmental variation will have applications for anticipating changes in populations resulting from anthropogenic activities that affect the variance in vital rates. We also highlight new tools for anticipating the consequences of the magnitude and temporal patterning of environmental variability.     

Teams in animal societies

We review the existence of teams in animal societies. Teamshave previously been dismissed in all but a tiny minority ofinsect societies. "Team" is a term not generally used in studiesof vertebrates. We propose a new rigorous definition of a teamthat may be applied to both vertebrate and invertebrate societies.We reconsider what it means to work as a team or group andsuggest that there are many more teams in insect societies than previously thought. A team task requires different subtasksto be performed concurrently for successful completion. Thereis a division of labor within a team. Contrary to previousreviews of teams in social insects, we do not constrain teamsto consist of members of different castes and argue that teammembers may be interchangeable. Consequently, we suggest thata team is simply the set of individuals that performs a teamtask. We contrast teams with groups and suggest that a grouptask requires the simultaneous performance and cooperationof two or more individuals for successful completion. In agroup, there is no division of labor—each individual performs the same task. We also contrast vertebrate and invertebrateteams and find that vertebrate teams tend to be associatedwith hunting and are based on individual recognition. Invertebrateteams occur in societies characterized by a great deal of redundancy,and we predict that teams in insect societies are more likelyto be found in large polymorphic ("complex") societies thanin small monomorphic ("simple") societies.