Infanticide

Comparative and Evolutionary Perspectives

Glenn Hausfater, Sarah Blaffer Hrdy

• Publisher: Routledge
• Year: 2017
• Print ISBN: 9780202362212
• eBook ISBN: 9781351512619
• Topic: Biological Sciences
• Subtopic: Evolution
• Index: Biological Sciences

Recent field studies of a variety of mammalian species reveal a surprisingly high frequency of infanticide - the killing of unweaned or otherwise maternally dependent offspring. Similarly, studies of birds, fish, amphibians, and invertebrates demonstrate egg and larval mortality in these species, a phenomenon directly analogous to infanticide in mammals. In this collection, Hausfater and Hrdy draw together work on animal and human infanticide and place these studies in a broad evolutionary and comparative perspective.Infanticide presents the theoretical background and taxonomic distribution of infanticide, infanticide in nonhuman primates, infanticide in rodents, and infanticide in humans. It examines closely sex allocation and sex ratio theory, surveys the phylogeny of mammalian interbirth intervals, and reviews data on sources of egg and larval mortality in a variety of invertebrate and lower vertebrate species. Dealing with infanticide in nonhuman primates, two chapters critically examine data on infanticide in langurs and its broader theoretical implications. By reviewing sources of infant mortality in populations of small mammals and new laboratory analyses of the causes and consequences of infanticide, this work explores such issues as the ontogeny of infanticide, proximate cues of infants and females which elicit infanticidal behavior in males, the genetical basis of infanticide, and the hormonal determinants.Hausfater and Sarah Blaffer Hrdy, through their selection of materials for this book, evaluate the frequency, causes, and function of infanticide. Historical, ethnographic, and recent data on infanticide are surveyed. "Infanticide" summarizes current research on the evolutionary origins and proximate causation of infanticide in animals and man. As such it will be indispensable reading for anthropologists and behavioral biologists as well as ecologists, psychologists, demographers, and epidemiologists.

II INFANTICIDE IN NONHUMAN PRIMATES: A TOPIC OF CONTINUING DEBATE

Infanticide in nonhuman primates: An introduction and perspective

Glenn Hausfater

Infanticide in animals and humans is a controversial topic. This is especially so when the discussion turns to nonhuman primates. The chapters in Part II have been selected to provide readers with a representative sample both of the nature of the data concerning infanticide among nonhuman primates and the nature of the debate concerning the interpretation of those data. However, as an unfortunate consequence of the latter consideration, readers who are not familiar with the “infanticide controversy,” especially as concerns langur monkeys, may very well be more confused than enlightened by the conflicting views expressed in these chapters. Thus, these brief introductory notes are intended to clarify certain basic issues and positions in the longstanding debate over the frequency and significance of infanticide among nonhuman primates. Although I obviously run the risk of making the situation worse, rather than better, I present here my own personal (and admittedly idiosyncratic) perspective on the infanticide controversy. In particular, I first discuss specific points of disagreement in the infanticide controversy and then consider some deeper issues that I believe to underlie the range of opinions held by primatologists concerning infanticide and its role in the social life of nonhuman primates in the wild.

With respect to specific points of disagreement, I view the infanticide controversy as basically reducing to the dual questions of (1) What level of detail of observation is required for an infant’s disappearance to be attributed to infanticide? and (2) What kinds of evidence are required to refute any given hypothesis concerning the events thus observed? Furthermore, I consider the extent of disagreement over the first of these issues to have been amplified by what I see as the use by some reviewers of much more stringent standards of evidence in regard to observations of infanticide than is the case in other realms of behavior, especially predation and feeding behavior. Nevertheless, it is also true that when one distills all reports of infanticide in langurs, as done by Vogel and Loch (Chapter 12, this volume) and by Boggess (Chapter 14, this volume), one finds that in only a handful of cases have fieldworkers actually observed a fatal attack on an infant.

Yet the crux of the issue is how one deals not with the cases in which observations are relatively complete but with those in which observations are incomplete due either to the habits of the monkeys, the habits of the observer, or both. How does one treat cases in which the observational record is truncated due to the fact that the infant and its attacker fell into foliage; or cases in which the wounding of an infant was clearly observed during daylight hours, but the infant’s disappearance occurred only on the subsequent night when, for understandable reasons, observations were not in progress; or cases similar to the preceding but in which the infant’s disappearance occurred several days after the observed attack? Even among those infant disappearances for which observations are relatively complete, how should one deal with the data in terms of hypothesis testing if, for example, the infant’s age was not known with certainty?

Clearly, this is not a realm where evidenciai rules will come easily and in many ways is the obverse of the problem as to when life begins, discussed both by proponents of legislative restrictions on abortion and by Scrimshaw (Chapter 22, this volume). Here, of course, the issues become: when did a particular infant’s life end, and how does one assign the cause of death? Complications of a severe wound? Infanticide? Or, perhaps the former following the latter? Certainly, we should not dismiss altogether incomplete observations of a given event merely because they are so; afterall, no observational record can in reality be considered unequivocally “complete.” However, neither should we accept uncritically, for the sake of expanding our sample of “verified” cases of infanticide, an elaborate set of assumptions about what might have preceded or followed the brief sequence of events that was actually observed. In these regards, I consider the chapters by Crockett and Sekulic (Chapter 9, this volume) and by Leland et al. (Chapter 8, this volume) as exemplary both in their conservative treatment of very complex observational datasets and in their use of these data to test the predictions of specific alternative hypotheses.

With respect to such predictions, the two most strongly debated and mutually exclusive explanations of infanticide are the “sexual selection” and “social pathology” hypotheses. As outlined in the volume introduction, the sexual selection hypothesis predicts that infanticide among nonhuman primates should be directed primarily by adult males against unweaned infants sired by their competitors. In contrast, the social pathology hypothesis states that infant killing in primates is an aberrant behavior that is seen only in populations at high densities. Further, such populations are assumed to have reached abnormally high densities as a result, in most cases, of provisioning of food resources by humans or through other forms of human disturbance. Rather than summarize these various hypotheses (and evidence in support of them) any further, I will merely refer the reader to the volume introduction as well as to the position paper and detailed review of data by Boggess (Chapter 14, this volume), an advocate of the social pathology/enforced proximity hypothesis, and to the formal replies to that chapter by Hrdy (Chapter 16, this volume) and by Sugiyama (Chapter 15, this volume), strong proponents of alternative viewpoints (Sugiyama, 1965a; Hrdy, 1977a,b, 1979).

These latter three contributions to this volume, even given the brief nature of the reply chapters, should provide readers with a relatively good sense not only of the conviction with which positions are held in the infanticide debate but of the major points of disagreement concerning the correctness of various predictions drawn from various explanatory models. The magnitude of these disagreements is made even more striking when one realizes that draft copies of these three chapters were circulated among the authors prior to publication, and that ample opportunity was afforded to all concerned for modification and revision of their views in light of each other’s criticisms. Clearly, these authors read and interpret the existing literature on langur infanticide very differently, and much more interchange than provided either by the infanticide symposium itself or by the preparation of this volume will be required to produce any higher level of agreement.

The debate over the frequency and function of infanticide in langurs has had the unfortunate side effect of distracting attention from the burgeoning body of data that suggests that infanticide is an important element in the social organization of many nonhuman primate species other than langurs. The various field reports and data summaries included in this section provide readers with access to recent information on infanticide in these species. However, one finds here also, as in the case of langur monkeys, differences of interpretation concerning the data on infanticide. Thus, several chapters in Part II present somewhat different tallies both of the species for which infanticide has been reported and of the number of observed cases of infanticide in each of these species. However, I believe that these latter discrepancies are actually relatively minor, and reflect differences among the authors in access to the primary literature rather than any major disagreements over hypotheses or data. In fact, in my view, these latter chapters actually show quite substantial agreement concerning the kinds of data required to test particular hypotheses and the nature of the hypotheses themselves. More importantly, in several cases, the authors are themselves currently involved in long-term investigations that hold the promise of eventually providing such data.

Apart from considerations of hypothesis testing and data, the controversy over the frequency and significance of infanticide among nonhuman primates is, to my mind, also extremely interesting from the standpoint of the sociology of science. In many ways, the study of primate behavior has been the weak sibling of field zoology. Theoretical issues and concepts have been generated almost exclusively in the latter discipline (based on studies of nonprimate animals) and only subsequently assimilated by students of primate behavior. In the case of infanticide, however, the flow of ideas and issues has been almost precisely the opposite. Largely due to the writings of Hrdy (1974, 1977a) on infant killing in langurs, infanticide emerged from its status as a laboratory artifact to that of a phenomenon considered not only appropriate for study by zoologists (and psychologists, see Part III) but also amenable to evolutionary explanation. Obviously, however, this latter development cannot be considered apart from the widespread acceptance of sociobiology and other evolutionary paradigms in behavioral biology as a whole (Trivers, 1972; Wilson, 1975).

This change of status for infanticide as a research topic is made even more interesting by the fact that within the subdiscipline of primate behavior, the first reports of infant killing in langurs (Sugiyama, 1965a, 1965b) were largely dismissed as having little general significance for our understanding of the social organization of primates in their natural environment. However, again directly attributable to Hrdy’s (1974) interpretation of infanticide among the langurs at Mt. Abu as an expression of sexual competition, this phenomenon came to be recognized as more widespread and important, at least among the Colobines, than was previously thought to be the case. Then, as an increasing number of reports of infanticide in other species were published (Struhsaker, 1977; Goodall, 1977; Rudran, 1979; Butynski, 1982) the possibility began to receive open consideration that infanticide may actually be an important phenomenon in the order Primates as a whole. The data-based chapters in Part II give a good indication of just how far this change in attitude toward infanticide as a research topic and as a normative feature of primate social organization has progressed.

However, in an intellectual and sociological process analogous to the second law of thermodynamics, every theoretical advance provides new impetus for the diametrically opposed school of thought and perhaps more strongly in the case of primate behavior than in most other fields. In part, the strength of this opposing reaction to the idea that infanticide might be a normative aspect of social behavior of some primate species was perhaps due to what may be called the “disciplinary schizophrenia” of primatologists themselves. By the latter term, I mean that students of primate behavior have been drawn from several very different intellectual traditions, primarily psychology, anthropology, and zoology. The anthropological tradition in particular has tended to approach the study of primate societies as a branch of ethnography. The focus of these studies has thus been social roles, relations, and norms and, more generally, the significance of social structure for the harmonious and adaptive functioning of societies.

In contrast, the zoological tradition has focused primarily on differences in reproductive success and survival among individuals within societies rather than on differences between societies per se. The distinction may not be so clear-cut as I have drawn it, for zoologists have certainly also attempted to understand within-species variability in social organization, but I believe that it is nonetheless a real and important one. Furthermore I believe that as a result of the peculiarities of their research questions and outlook, primatologists from the zoological tradition have generally been predisposed to accept the idea that a behavior could be good for the individual but detrimental to the group, population, or species as a whole. In contrast, primatologists in the anthropological tradition have been much more hesitant to accept explanations of behavior based on advantage to the individual as opposed to that of the group.

However, it is also the case that many fieldworkers, regardless of their training and tradition, have come to accept the importance of infanticide as a behavioral phenomenon among nonhuman primates based not on theoretical grounds but rather as a result of having actually observed the behavior in the wild themselves. Thus, in reading accounts of infanticide in Kibale Forest primates one cannot help but be struck by how much the acceptance or rejection of various explanations for this behavior seems to depend on the observers’ impressions of the specificity with which infants were targeted as victims by incoming males in contrast to the extent of agreement per se between these observations and the predictions of any specific hypothesis (Leland et al, Chapter 8, this volume). Likewise, recent observations of infanticide accompanying adult male replacement at Jodhpur have resulted in a rather substantial change of position by the members of that research team concerning both the nature of the langur mating system and the frequency of infanticide in that species (cf. Vogel, 1979 with Vogel and Loch, Chapter 12, this volume).

On the other hand, such changes of perspective and position can be seen as very encouraging developments for they indicate that the behavior of our study animals in the wild can be at least as important as our particular intellectual tradition in the evaluation of theory and hypotheses, if not more so. Still there remains a distressing tendency for field workers, regardless of tradition, to dismiss events reported at one site if similar events have not been observed at their own site. More generally, we all still far too frequently fail to appreciate how many situations, both ecologically and evolutionarily, can lead to certain behaviors being expressed by a species in one habitat or at one site, but not at others. Yet, despite such problems and biases, participants at the Wenner-Gren Foundation symposium that gave rise to this volume were still able to achieve substantial clarification of various hypotheses and to reach relatively close agreement on data and research strategies required to test those hypotheses.

However, as will also become clear in reading the following chapters, even given the extent of agreement already reached on the kinds of data and observational designs required to test specific hypotheses, we are still lacking sufficiently large and precise dataseis to actually carry out a rigorous evaluation of competing alternative hypothes...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Comparative and Evolutionary Perspectives on Infanticide: Introduction and Overview
• List of Contributors and Symposium Participants
• I. Background and Taxonomic Reviews
• II. Infanticide in Nonhuman Primates: A Topic of Continuing Debate
• III. Infanticide in Rodents: Questions of Proximate and Ultimate Causation
• IV. Infanticide in Humans: Ethnography, Demography, Sociobiology, and History
• Bibliography
• Index