Lethal Aggression Research Articles

A case of intercommunity lethal aggression by chimpanzees in an open and dry landscape, Issa Valley, western Tanzania

Intercommunity (lethal) aggression is a familiar component of the behavioural repertoire of many forest-dwelling chimpanzee (Pan troglodytes) communities. However, until now, the absence of intercommunity attacks – including killings – in communities that live in open, mosaic environments has supported hypotheses of reduced resource competition in drier habitats, and informed referential models of early hominin social dynamics in a similar habitat. In June 2020, we observed the first instance of intercommunity lethal aggression, a male-committed infanticide, by the Issa chimpanzee community, which live in a savannah-mosaic habitat in the Issa Valley, western Tanzania. The carcass was recovered by researchers after it was abandoned by the attackers. Here, we give a detailed account of the events leading up to and including the infanticide, and contextualise our observations with what has been described for other chimpanzee communities. Notably, in contrast to the majority of reported intercommunity infanticides, the infant male victim was castrated (and not cannibalised), making this the youngest reported castration. This observation of intercommunity aggression disproves its hypothesised absence in savannah-dwelling chimpanzees, which by extension, has implications for early hominin evolution. We suggest that the near absence of observations of intercommunity aggression in savannah chimpanzee communities is most likely due to the lack of long-term study communities, and in some cases geographic isolation. We hypothesise that food-rich areas within a habitat with otherwise widely distributed food sources may select for intense intercommunity aggression despite the low population density characteristic of savannah communities. Anecdotes such as this add to the comparative database available on intercommunity killings in chimpanzee society, improving our ability to draw inferences about their evolutionary significance.

Should Weaponised Moral Enhancement Replace Lethal Aggression in War?

AbstractSome have proposed the development of technologies that improve our moral behaviour – moral enhancement – in order to address global risks such as pandemics, global warming and nuclear war. I will argue that this technology could be weaponised to manipulate the moral dispositions of enemy combatants. Despite being morally controversial, weaponised moral enhancement would be neither clearly prohibited nor easily prohibitable by international war law. Unlike previous psychochemical weapons, it would be relatively physically harmless. I argue that when combatants are liable to lethal aggression to achieve an aim of war, they are also liable to weaponised moral enhancement to achieve that same aim. Weaponised moral enhancement will loosen just war requirements in both traditional and revisionist normative just war theories. It will particularly affect revisionist theories’ jus ad bellum requirements for humanitarian and preventive wars. For instance, weaponised moral enhancement could be more proportional and efficacious than lethal aggression to effect institutional changes in preventive and humanitarian wars. I will conclude that, despite evading international war laws and loosening normative just war requirements, the intuition that weaponised moral enhancement would gravely harm combatants can be defended by arguing that it would severely disturb personal identity, which could potentially ground future prohibitions.

Lethal Aggression in Captive Monkeys

Establishing the characteristics of monkey lethal aggression, assessing the age and sex of aggressors and victims, and the nature of the aggressors' actions seems necessary to identify the evolutionary roots of cases of human lethal aggression. In addition, the study of the features of lethal aggression can be considered as a condition for the development of measures aimed at preventing its manifestations in captive monkeys. The aim of the study is to study the species-specific features of lethal aggression in captive macaques, baboons and guenons. An analysis was made of cases of intra-group lethal aggression that occurred over a 10-year period in Rhesus macaques (Macaca mulatta), Crab-eathing macaques (Macaca fascicularis), Pig-tail macaques (Macaca nemestrina), Green monkeys (Chlorocebus aethiops), Hamadryas baboons (Papio hamadryas), Anubis baboons (Papio anubis) kept by social groups. It has been established that cases of lethal aggression are observed in all 6 studied species of monkeys. It was shown that in groups consisting of adult males, the highest frequency of cases of lethal aggression was noted in Rhesus macaques, and the lowest in Hamadryas baboons. Species-specific differences were found in the nature of lethal aggression in monkey family groups consisting of males, females, infants, and adolescents. It has been established that in Hamadryas baboons, Anubis baboons and Pig-tail macaques, that is, in species with pronounced sexual dimorphism in body size and social status, the main form of lethal aggression in family groups is infanticide committed by a single sexually mature male. In Rhesus monkeys, Crab-eathing macaques and Green monkeys, that is, those species that are characterized by relatively mild sexual dimorphism in body size and social status, the predominant form of lethal aggression in family groups is the collective actions of females aimed at young and adult members of the group.

Lethal Aggression in Captive Monkeys

Lethal Aggression in Captive Monkeys

Interspecific interactions between sympatric apes

SummaryGorillas reside in sympatry with chimpanzees over the majority of their range. Compiling all known reports of overlap between apes and augmenting these with observations made over twenty years in the Ndoki Forest, we examine the potential predation-related, foraging, and social contexts of interspecific associations between gorillas and chimpanzees. We reveal a greater diversity of interactions than previously recognized, which range from play to lethal aggression. Furthermore, there are indications that interactions between ape species may serve multiple functions. Interactions between gorillas and chimpanzees were most common during foraging activities, but they also overlapped in several other contexts. From a social perspective, we provide evidence of consistent relationships between particular chimpanzee-gorilla dyads. In addition to providing new insights into extant primate community dynamics, the diversity of interactions between apes points to an entirely new field of study in early human origins as early hominins also likely had opportunities to associate.

The evolutionary drivers of primate scleral coloration

The drivers of divergent scleral morphologies in primates are currently unclear, though white sclerae are often assumed to underlie human hyper-cooperative behaviours. Humans are unusual in possessing depigmented sclerae whereas many other extant primates, including the closely-related chimpanzee, possess dark scleral pigment. Here, we use phylogenetic generalized least squares (PGLS) analyses with previously generated species-level scores of proactive prosociality, social tolerance (both n = 15 primate species), and conspecific lethal aggression (n = 108 primate species) to provide the first quantitative, comparative test of three existing hypotheses. The ‘self-domestication’ and ‘cooperative eye’ explanations predict white sclerae to be associated with cooperative, rather than competitive, environments. The ‘gaze camouflage’ hypothesis predicts that dark scleral pigment functions as gaze direction camouflage in competitive social environments. Notably, the experimental evidence that non-human primates draw social information from conspecific eye movements is unclear, with the latter two hypotheses having recently been challenged. Here, we show that white sclerae in primates are associated with increased cooperative behaviours whereas dark sclerae are associated with reduced cooperative behaviours and increased conspecific lethal violence. These results are consistent with all three hypotheses of scleral evolution, suggesting that primate scleral morphologies evolve in relation to variation in social environment.

Drivers of intraspecific lethal aggression in mammals

AbstractAimTo assess whether interspecific variation in lethal intraspecific aggressiveness in mammals exhibits a geographical gradient and, if so, to identify which ecological, social and evolutionary processes could explain such patterns.LocationGlobal.TaxonMammalia.MethodsWe mapped intraspecific lethal aggressivity for 952 terrestrial mammal species and, through an assemblage‐based and phylogenetic approach, tested at a global scale four hypotheses that may explain differences in lethal aggressivity: the seasonal effects of photoperiod seasonality, food resource availability, social organisation and evolutionary history.ResultsPhotoperiod seasonality, which is linked to hormonal control, was the best predictor of lethal aggressivity. However, interactions between the environment and evolutionary factors explained the higher proportion of interspecific variation.Main conclusionsThe geography of behavioural traits, affecting multiple adaptive pathways, is an important emerging area of biogeographical research with high potential relevance for interpreting other macroecological patterns.

Lethal Coalitionary Aggression Associated with a Community Fission in Chimpanzees (Pan troglodytes) at Ngogo, Kibale National Park, Uganda.

Many animals engage in aggression, but chimpanzees stand out in terms of fatal attacks against adults of their own species. Most lethal aggression occurs between groups, where coalitions of male chimpanzees occasionally kill members of neighboring communities that are strangers. However, the first observed cases of lethal violence in chimpanzees, which occurred at Gombe, Tanzania in the 1970s, involved chimpanzees that once knew each other. They followed the only observed case of a permanent community fission in chimpanzees. A second permanent fission recently transpired at Ngogo, Kibale National Park, Uganda. Members of a large western subgroup gradually ceased associating peacefully with the rest of the community and started behaving antagonistically toward them. Affiliation effectively ended by 2017. Here, we describe two subsequent lethal coalitionary attacks by chimpanzees of the new western community on males of the now separate central community, one in 2018 and the second in 2019. The first victim was a young adult male that never had strong social ties with his attackers. The second was a high-ranking male that had often associated with the western subgroup before 2017; he groomed regularly with males there and formed coalitions with several. Other central males present at the start of the second attack fled, and others nearby did not come to the scene. Several western females joined in the second attack; we suggest that female-female competition contributed to the fission. This event highlighted the limits on protection afforded by long-term familiarity and the constraints on costly cooperation among male chimpanzees.

Prosocial Emotion, Adolescence, and Warfare : DNA Methylation Associates with Culturally Salient Combat Variables.

Examining the costs and motivations of warfare is key to conundrums concerning the relevance of this troubling phenomenon to the evolution of social attachment and cooperation, particularly during adolescence and young adulthood-the developmental time period during which many participants are first recruited for warfare. The study focuses on Samburu, a pastoralist society of approximately 200,000 people occupying northern Kenya's semi-arid and arid lands, asking what role the emotionally sensitized, peer-driven adolescent life stage may have played in the cultural and genetic coevolution of coalitional lethal aggression. Research in small-scale societies provides unparalleled opportunities for sharply defined variables, particularly in age generation societies in which all young men are initiated into "warriorhood." Proposing an epigenetic and component behavior approach, we examine whether raiding activities such as number of raids, killing, and sparing enemy lives associate with DNA methylation in two candidate genes: MAOA, linked to mood and arousal, and NR3C1, linked to stress and immune response. We report statistically significant associations between the epigenetic variables and the combat (exposure) variables of overall raiding activity and reportedly showing mercy to enemies. In contrast, epigenetic variables did not associate with post-traumatic stress disorder (PTSD) symptom scores (a potential outcome measure), and the only combat variable associated with PTSD (but not DNA methylation) was losing one's own livestock in a raid. These findings raise important questions concerning the mechanisms driving warfare's paradoxical mix of violent and altruistic behaviors.

Morphological Identification of Hair Recovered from Feces for Detection of Cannibalism in Eastern Chimpanzees

Chimpanzees (Pan troglodytes) are primarily frugivorous but consume a variable amount of meat from a variety of organisms, including other chimpanzees. Cannibalism is rare, usually follows lethal aggression, and does not occur following natural deaths. While chimpanzee cannibalism has been documented at multiple sites, many instances of this behavior go unrecorded. Identification of chimpanzee remains in feces, however, can provide indirect evidence of cannibalism. Hair, in particular, typically passes through the gastrointestinal tract undamaged and is commonly used for purposes of identification in wildlife forensics. Here we test the hypothesis that eastern chimpanzee (Pan troglodytes schweinfurthii) guard hair morphology can be reliably distinguished from the hairs of their most common prey species. Methods and results are presented in the context of a case study involving a suspected chimpanzee infanticide from Gombe, Tanzania. We find that chimpanzee guard hair morphology is unique among tested mammals and that the presence of abundant chimpanzee hair in feces is likely the result of cannibalism and not incidental ingestion from grooming or other means. Accordingly, morphological analysis of guard hairs from feces is a promising, cost-effective tool for the determination of cannibalistic acts in chimpanzees.

Intragroup Lethal Aggression in West African Chimpanzees (Pan troglodytes verus): Inferred Killing of a Former Alpha Male at Fongoli, Senegal

Lethal coalitionary aggression is of significant interest to primatologists and anthropologists given its pervasiveness in human, but not nonhuman, animal societies. Chimpanzees (Pan troglodytes) provide the largest sample of recorded lethal coalitionary aggression in nonhuman primates, and most long-term chimpanzee study sites have recorded coalitionary killing of conspecifics. We report an inferred lethal attack by resident males on a former alpha male chimpanzee (P. t. verus) at Fongoli in Senegal. We describe the male’s presence in the community, his overthrow, social peripheralization for >5 yr, and his attempt to rejoin the group as well as circumstances surrounding his death. We report attacks by multiple chimpanzees on his dead body, most frequently by a young adult male and an older female. The latter also cannibalized the body. Coalitionary killing is rare among West African chimpanzees compared to the East African chimpanzee (P. t. schweinfurthii). This pattern may relate to differences in population densities, research effort, and subspecies differences in biology and behavior.

Fear-Based Provocation

Psychological research has long found that anger may lead to aggression, sometimes even fatal one. The provocation doctrine corresponds to this finding by providing that murder charges may be mitigated to voluntary manslaughter if evidence establishes that the defendant acted under the influence of a “sudden heat of passion”, resulting from “adequate provocation”. The modern rationale underlying provocation doctrine rests on the idea that defendant’s intense emotion of anger had resulted in loss of self-control, therefore he or she ought to be partially excused. Case law demonstrates that defendants sometimes kill out of fear of serious physical violence threatened by the deceased. For example, victims of long-term physical abuse by the deceased may kill their abusers out of fear of future violence, even if at the moment of the killing the deceased was not posing an imminent threat to life. In circumstances where defendants are unable to satisfy the requirements of self-defense, provocation might be the only viable defense that would mitigate a murder conviction to voluntary manslaughter. Yet, existing provocation doctrine is unfit to capture the distinct features characterizing the reaction of fearful defendants. Commonly perceived as an anger-centric defense, the defense’s elements mostly accommodate typical the responses of defendants who acted quickly, immediately following a single and sudden triggering incident, before any lapse of time allowed them to regain control. The article offers three major contributions to challenge existing view of provocation: first, it considers psychological research that had found that fear, similarly to anger, may also significantly interfere with individuals’ decision-making processes by disturbing rational judgement, therefore sometimes leading to lethal aggression. Second, drawing on this research, the article’s key argument is that provocation doctrine should be reconstructed to also include a fear-based prong. Third, recognizing fear-based provocation calls for rejecting the “anger-loss of control” paradigm that currently dominates judges and jurors’ perception of the defense. In its place, the article advocates an alternative framework that would focus on the fearful defendant’s fear of violence threatened by the victim that caused a significant impairment in the defendant’s thought processes, resulting in obscured judgment and reasoning. The reconstructed defense would also include an objective component, under which, the defendant would have to prove that a person of ordinary disposition would also experience such emotion and respond rashly without exercising reason and judgment.

Resource scarcity drives lethal aggression among prehistoric hunter-gatherers in central California

The origin of human violence and warfare is controversial, and some scholars contend that intergroup conflict was rare until the emergence of sedentary foraging and complex sociopolitical organization, whereas others assert that violence was common and of considerable antiquity among small-scale societies. Here we consider two alternative explanations for the evolution of human violence: (i) individuals resort to violence when benefits outweigh potential costs, which is likely in resource poor environments, or (ii) participation in violence increases when there is coercion from leaders in complex societies leading to group level benefits. To test these hypotheses, we evaluate the relative importance of resource scarcity vs. sociopolitical complexity by evaluating spatial variation in three macro datasets from central California: (i) an extensive bioarchaeological record dating from 1,530 to 230 cal BP recording rates of blunt and sharp force skeletal trauma on thousands of burials, (ii) quantitative scores of sociopolitical complexity recorded ethnographically, and (iii) mean net primary productivity (NPP) from a remotely sensed global dataset. Results reveal that sharp force trauma, the most common form of violence in the record, is better predicted by resource scarcity than relative sociopolitical complexity. Blunt force cranial trauma shows no correlation with NPP or political complexity and may reflect a different form of close contact violence. This study provides no support for the position that violence originated with the development of more complex hunter-gatherer adaptations in the fairly recent past. Instead, findings show that individuals are prone to violence in times and places of resource scarcity.

Female infanticide and female-directed lethal targeted aggression in a group of ring-tailed lemurs (<i>Lemur catta</i>)

Abstract. We report on extremely rare events of lethal aggression in a semi-captive group of ring-tailed lemurs (Lemur catta) during the birth season 2014. This aggression was targeted against the two highest-ranking females. It led to their eviction from the group and following a final aggressive interaction four days later to their deaths caused by almost all the females and probably even two males of their former group. These events of targeted aggression erupted initially following an infanticide by the highest-ranking female directed at the offspring of a subordinate. Even for ring-tailed lemurs this is a very special case of changeover of power between two different matrilines in a group. In accordance with other studies we suggest that these events of targeted aggression were based on the growing group size and were acts of female reproductive competition during birth season. With the intensity of the events we add new aspects to the existing knowledge of aggressive interactions in ring-tailed lemurs.

Making sense of economic deprivation as a predictor of suicide and homicide: A nationwide register-study

IntroductionClassical work on lethal aggression often viewed suicide and homicide as sharing a common source.ObjectiveThe present investigation explores the association between measures of social deprivation on the relative incidence of suicide over homicide in Italian provinces.MethodsData refer to official government sources on lethal violence rates and measures of social deprivation. The central dependent variable is termed SHR or the suicide rate expressed as a proportion of the sum of the suicide and homicide rates Data were available for the 103 Italian provinces.ResultsThe SHR had three significant predictors. The greater the percentage of the population with low education, the lesser the tendency towards suicide. The tendency towards suicide was also predicted by rental housing, the greater the percentage of the population living in rental housing the less the tendency towards suicide. The inverse of the unemployment rate also predicted the SHR. Given that the measure follows an inverse function, the greater the unemployment rate the lesser the tendency towards suicide relative to homicide (SHR). We can interpret the results relative to a homicidal tendency in the SHR: the greater the low education percentage of the population, the greater the homicidal tendency, and the greater the rental housing percentage, the greater the homicidal tendency in the SHR.ConclusionThe results are consistent with a stream of previous research that connects deprivation with a relatively high probability for disadvantaged populations to direct aggression outwardly in the form of homicide rather than inwardly in the form of suicide.Disclosure of interestThe authors have not supplied their declaration of competing interest.

The dark side of the red ape: male-mediated lethal female competition in Bornean orangutans

Female Bornean orangutans (Pongo pygmaeus wurmbii) are mainly solitary and philopatric, leading to adult female relatives sharing adjacent and overlapping home ranges. Females tend to be intolerant of unrelated females, with whom they also may have overlapping home ranges. However, fights that lead to injuries are extremely rare and lethal aggression had never been observed. Here, we report the first case of lethal female-female aggression during over 26,000 h of focal data collected on adult females at Tuanan, Central Kalimantan: A young female, who had recently lost her infant, attacked an old resident female. The interaction’s unique feature was that the attacking female was supported by an unflanged male, who had been in consort with her during the week preceding the attack and was responsible for the lethal injuries to the victim. The victim received protection from a flanged male who was probably attracted to the noise generated by the fight. We conclude that even in a species in which coercion is frequently observed in male-female interactions, female leverage over males can coax males into providing services, such as coalitionary support.

Myths about hunter-gatherers redux: nomadic forager war and peace

Purpose– The purpose of this paper is to critique several studies that claim to show that nomadic foragers engage in high levels of inter-group aggression. This is done through exploring four myths: nomadic foragers are warlike; there was a high rate of war mortality in the Pleistocene; the nomadic forager data support the “chimpanzee model” of lethal raiding psychology; and contact and state influence inevitably decrease aggression in nomadic forager societies.Design/methodology/approach– Using exact criteria, a sample of 21 nomadic forager societies is derived from the Standard Cross-Cultural Sample. This sampling method minimizes the chance of sampling bias, a shortcoming that has plagued previous studies. Only the highest quality ethnographic data, those classified as Primary Authority Sources, are used, which results in data on 148 cases of lethal aggression. The specifics of the lethal aggression cases are then discussedvis-à-visthe four myths to demonstrate the disjuncture between the data and the myths.Findings– All four myths are found to be out of step with actual data on nomadic forager war and peace. Overall, the default interaction pattern of nomadic foragers is to get along with neighbors rather than make war against them. The findings contradict both assertions that there was a high level of war mortality among nomadic foragers of the Pleistocene and the chimpanzee model's proposal that human males have a tendency or predisposition to form coalitions and make lethal attacks on members of neighboring groups.Research limitations/implications– Consideration of nomadic forager war and peace should be contextualized in terms of social organization, contact history (including ethnocide, displacement, and other factors), and the current situation faced by extant forager populations. As in other contexts, the introduction of alcohol at contact or subsequently has increased nomadic forager aggression.Practical implications– Propositions as to the aggressiveness of nomadic foragers should be viewed with skepticism because they are contradicted by data and a contextual view of nomadic forager social organization and ethnohistory.Social implications– The debate over nomadic forager war and peace is connected to larger debates in modern society about the nature of human nature and has real-world implications regarding foreign policy and political approaches toward war and peace.Originality/value– A critique of sampling, methodology, and theory is provided in this area.

Lethal aggression in Pan is better explained by adaptive strategies than human impacts.

Observations of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) provide valuable comparative data for understanding the significance of conspecific killing. Two kinds of hypothesis have been proposed. Lethal violence is sometimes concluded to be the result of adaptive strategies, such that killers ultimately gain fitness benefits by increasing their access to resources such as food or mates. Alternatively, it could be a non-adaptive result of human impacts, such as habitat change or food provisioning. To discriminate between these hypotheses we compiled information from 18 chimpanzee communities and 4 bonobo communities studied over five decades. Our data include 152 killings (n = 58 observed, 41 inferred, and 53 suspected killings) by chimpanzees in 15 communities and one suspected killing by bonobos. We found that males were the most frequent attackers (92% of participants) and victims (73%); most killings (66%) involved intercommunity attacks; and attackers greatly outnumbered their victims (median 8:1 ratio). Variation in killing rates was unrelated to measures of human impacts. Our results are compatible with previously proposed adaptive explanations for killing by chimpanzees, whereas the human impact hypothesis is not supported.

Lethal Aggression in Mobile Forager Bands and Implications for the Origins of War

It has been argued that warfare evolved as a component of early human behavior within foraging band societies. We investigated lethal aggression in a sample of 21 mobile forager band societies (MFBS) derived systematically from the standard cross-cultural sample. We hypothesized, on the basis of mobile forager ethnography, that most lethal events would stem from personal disputes rather than coalitionary aggression against other groups (war). More than half of the lethal aggression events were perpetrated by lone individuals, and almost two-thirds resulted from accidents, interfamilial disputes, within-group executions, or interpersonal motives such as competition over a particular woman. Overall, the findings suggest that most incidents of lethal aggression among MFBS may be classified as homicides, a few others as feuds, and a minority as war.

Dernières nouvelles des bonobos: Les mythes et réalités de Pan paniscus

Bonobos are our closest living relatives along with chimpanzees. They attract much attention from anthropologists who want to better understand our primate origins and more recently from the public because of their remarkable behavior and matriarchal social system. New published insights from recent years allow us to better know Pan paniscus. This review describes the most recent findings: bonobos, chimpanzees, and humans ought to be part of the same genus (Homo or Pan) according to our genetics. bonobos have impressive cognitive ability to communicate with lexigram and sign-language, solve problems and use tools. Females have high social status in the group due to female association and coalition. The society is not really characterized by female dominant but rather by co-dominance of associated females. They are not purely egalitarian but non-violent and tolerant. Neither lethal aggression nor infanticide were observed and are not expected. Sex has a pivotal role in this pacifist society, which lacks sexual restrictions with the one exception of incest. Bonobos are probably a key species in forest ecology through their seed dispersal mutualism with plants whose fruits they eat. We continue to discover fascinating biological facts about our cousins who are in danger of extinction. A few of these are described here.