Partner choice in human evolution: The role of cooperation,foraging ability,and culture in Hadza campmate preferences

The ability to choose the partners we interact with is thought to have been an important driver in the evolution of human social behavior, and in particular, our propensity to cooperate. Studies showing that humans prefer to interact with cooperative others is often cited as support for partner choice driving the evolution of cooperation. However, these studies are largely drawn from Western samples, where conditions for partner choice to operate may be especially favorable. Here, we investigate qualities associated with being a preferred partner (i.e., campmate) in Hadza hunter-gatherers of Tanzania in 2016 and 2019. A total of 156 Hadza participants from 17 camps ranked their campmates on generosity, foraging ability, and their preference for them as future campmates. In 2016, Hadza preferred more generous people and better hunters as campmates, with evidence suggesting a stronger preference for better hunters; however, the relationship between generosity and being a preferred campmate was greater in 2019 than in 2016, such that the preference for generous people was stronger than the preference for better foragers, suggesting that campmate preferences are changing. These new findings contrast with reports on data from nearly a decade ago, suggesting that the Hadza do not prefer more cooperative campmates. Further, in 2019, there was anecdotal evidence that Hadza with greater exposure to outside cultural institutions (e.g., schooling, having a job, or living in a village) had a stronger preference for generous campmates than those with less exposure. Taken together, the results suggest that preferences for social partners may, in part, be culturally shaped.     

Life in the slow lane revisited: ontogenetic separation between chimpanzees and humans

This study investigates the evolution of human growth by analyzing differences in body mass growth trajectories among three populations: the Ache of eastern Paraguay, the US (NHANES, 1999-2000), and captive chimpanzees. The relative growth statistic "A" from the mammalian growth law is allowed to vary with age and proves useful for comparing growth across different ages, populations, and species. We demonstrate ontogenetic separation between chimpanzees and humans, and show that interspecific differences are robust to variable environmental conditions. The human pattern of slow growth during the lengthened period from weaning to the beginning of the adolescent growth spurt is found among the Ache (low energy availability and high disease load) and also in the US (high energy availability and low disease load). The human growth pattern contrasts with that of the chimpanzee, where absolute growth rates and relative "A" values are faster and less prolonged. We suggest that selection has acted to decrease human growth rates to allow more time for increased cognitive development with lower body-maintenance costs.     

Dynamics of the temporal structures of playing clusters and cliques among wild chimpanzees in Mahale Mountains National Park

The overall structure and temporally changing configuration of members of social play among the wild chimpanzees (Pan troglodytes schweinfurthii) of Mahale Mountains National Park, Tanzania, were described on both the microscopic ‘clique’ levels, conceptualized as directly connected configurations through social play behavior, and macroscopic ‘cluster’ levels, conceptualized as indirectly connected gatherings of members of adjacent multiple cliques at the same time and space. Most playing clusters started as dyads. Although the cumulative number of participants increased, each clique size remained at 2 for most of the observational units. Dyadic cliques were more stable and lasted longer than larger cliques. Of all clusters’ playing fields, 64.7 % had maximum diameters of 3 m. In summary, chimpanzees played stably in dyadic pairs in most of the time. As time passed, other chimpanzees often joined in the playing groups to form large polyadic clusters. Even when all chimpanzees in a cluster played socially at the same time, they normally did so in separate dyadic pairs, forming multiple dyadic cliques simultaneously in a small space. These social play dynamics may be explained assuming a hypothesis based on a balance model among socially playing chimpanzees, as the balanced cliques are limited only to those in which all the existing pairs form the mutual dyads, and they tend to avoid unbalanced and maintain balanced relationships during social play. As a result, larger cliques were difficult to maintain for long periods and tended to transition into dyadic mutual cliques. Thus, Heider’s balance theory can be one of the possible theories to explain not only human social phenomena, but also the proximate mechanism of the structure and the temporal change of social play among wild chimpanzees. Although both mutual and transitive relationships are known to be balanced in various human networks, only mutual relationships among socially playing chimpanzees were balanced.     

Mutualism and manipulation in Hadza–honeyguide interactions

We investigated the ecology and evolution of interspecific cooperation between the Greater Honeyguide bird, Indicator indicator, and human hunter-gatherers, the Hadza of northern Tanzania. We found that honeyguides increased the Hadza's rate of finding bee nests by 560%, and that the birds led men to significantly higher yielding nests than those found without honeyguides. We estimate that 8–10% of the Hadza's total diet was acquired with the help of honeyguides. Contrary to most depictions of the human-honeyguide relationship, the Hadza did not actively repay honeyguides, but instead, hid, buried, and burned honeycomb, with the intent of keeping the bird hungry and thus more likely to guide again. Such manipulative behavior attests to the importance of social intelligence in hunter-gatherer foraging strategies. We present an evolutionary model for human-honeyguide interactions guided by the behavioral ecology of bees, non-human primates, and hunter-gatherers.     

The effect of peer separation in young chimpanzees (Pan troglodytes)

Ten chimpanzees (Pan troglogytes), aged 18–24 months, housed without mothers as two dyads and two triads, were subjected to social separation. Two issues were addressed: the effects of peer separation in chimpanzees; and differential responses by subjects living in dyads compared with those living in triads. Chimpanzees that were alone during separation reacted with high levels of “protest” alternating with “despair” throughout the separation period. The continued presence of one cagemate, during separation from a third, was a strong mitigating factor. Even when the primary attachment was formed with the absent cagemate, the remaining chimpanzees clung to each other and the levels of protest and despair, when present, were low. Upon reunion, neither “detachment” nor heightened levels of clinging were conspicuous, but there was increased social interaction. The data on separation of chimpanzees are intermediate between those of humans and monkeys separated from mothers or peers. The increased social interactions during reunion, including looking, are comparable to the visual vigilance reported for humans.     

Chimpanzees Cope with Temporary Reduction of Escape Opportunities

Many studies have focused on the responses of primates to changes in population density, but little is known about the responses to more subtle changes in the physical environment. Studies on primate responses to high density supported the use of various coping strategies depending on the circumstances. We investigated which strategy was adopted by a large group of captive chimpanzees in response to a special case of space restriction: the temporary reduction of escape opportunities. Due to work on the outdoor enclosure, the chimpanzees at Chester Zoological Gardens were confined to their indoor quarters for 1 mo. As it was winter, the chimpanzees spent most of their time indoors also during the 7-week control period, but had the option to go outside. Therefore, there was no major difference in the actual density between the two conditions, but by being confined indoors the chimpanzees had reduced escape opportunities. Under this condition they would be expected not to alter their behavior dramatically but to adjust it in ways to avoid conflict and to reduce tension. The chimpanzees did not change their overall aggression rates during the period with a reduction of escape opportunities versus the control period. The patterns of allogrooming and submissive greetings also did not differ between the two periods. However, they appeared to refrain from joining ongoing aggressive interactions during the period with reduced escape opportunities, whereas no change in counteraggression occurred between the two periods. Our findings suggest that the chimpanzees may adopt a selective inhibition strategy when escape opportunities are limited. Further support for such selectivity comes from the analysis of different types of aggressor-target dyads. During the period with reduced escape opportunities, aggression rates decreased in dyads characterized by high aggression rates at baseline, i.e., high aggressor-target dyads, whereas the same individuals did not change their aggressive tendencies in low aggressor-target dyads. Ours is among the few studies showing a decrease, albeit selective, in aggressive behavior during a situation of space restriction. The ability to rapidly adopt an appropriate strategy and to inhibit aggression during such restriction seems to confirm findings of chimpanzees under high density conditions. The inhibition of aggressive tendency may be especially developed in chimpanzees and may be related to the natural challenges posed by the routine changes in subgroup membership characteristic of their fission-fusion social organization.     

Preferences for symmetry in human faces in two cultures: data from the UK and the Hadza, an isolated group of hunter-gatherers

Many studies show agreement within and between cultures for general judgements of facial attractiveness. Few studies, however, have examined the attractiveness of specific traits and few have examined preferences in hunter-gatherers. The current study examined preferences for symmetry in both the UK and the Hadza, a hunter-gatherer society of Tanzania. We found that symmetry was more attractive than asymmetry across both the cultures and was more strongly preferred by the Hadza than in the UK. The different ecological conditions may play a role in generating this difference. Such variation in preference may be adaptive if it reflects adaptation to local conditions. Symmetry is thought to indicate genetic quality, which may be more important among the Hadza with much higher mortality rates from birth onwards. Hadza men who were more often named as good hunters placed a greater value on symmetry in female faces. These results suggest that high quality Hadza men are more discriminating in their choice of faces. Hadza women had increased preferences for symmetry in men's faces when they were pregnant or nursing, perhaps due to their increased discrimination and sensitivity to foods and disease harmful to a foetus or nursing infant. These results imply that symmetry is an evolutionarily relevant trait and that variation in symmetry preference appears strategic both between cultures and within individuals of a single culture.     

Basis for Cumulative Cultural Evolution in Chimpanzees: Social Learning of a More Efficient Tool-Use Technique

Background

The evidence for culture in non-human animals has been growing incrementally over the past two decades. However, the ability for cumulative cultural evolution, with successive generations building on earlier achievements, in non-human animals remains debated. Faithful social learning of incremental improvements in technique is considered to be a defining feature of human culture, differentiating human from non-human cultures. This study presents the first experimental evidence for chimpanzees'' social transmission of a more efficient tool-use technique invented by a conspecific group member.

Methodology/Principal Findings

The chimpanzees were provided with a straw-tube, and spontaneously demonstrated two different techniques in obtaining juice through a small hole: “dipping” and “straw-sucking”. Both the “dipping” and “straw-sucking” techniques depended on the use of the same tool (straw-tube) for the same target (juice) accessible from exactly the same location (small hole 1 cm in diameter). Therefore the difference between “dipping” and “straw-sucking” was only in “technique”. Although the two techniques differed significantly in their efficiency, their cognitive and perceptuo-motor complexity were comparable. All five chimpanzees who initially performed the “dipping” technique switched to using the more efficient “straw-sucking” technique upon observing a conspecific or human demonstrate the more proficient alternate “straw-sucking” technique.

Conclusions/Significance

The social learning mechanism involved here was clearly not local or stimulus enhancement, but imitation or emulation of a tool-use technique. When there is no biologically relevant difference in cognitive or perceptuo-motor complexity between two techniques, and when chimpanzees are dissatisfied with their own technique, chimpanzees may socially learn an improved technique upon close observation of a proficient demonstrator. This study provides important insights into the cognitive basis for cumulative culture in chimpanzees, and also suggests possible conditions in which cumulative cultural evolution could arise even in non-human animals.     

Sexual Differences in Chimpanzee Sociality

Scientists usually attribute sexual differences in sociality to sex-specific dispersal patterns and the availability of kin within the social group. In most primates, the dispersing sex, which has fewer kin around, is the less social sex. Chimpanzees fit well into the pattern, with highly social philopatric males and generally solitary dispersing females. However, researchers in West Africa have long suggested that female chimpanzees can be highly social. We investigated whether chimpanzees in the Taï Forest (Côte d’Ivoire) exhibit the expected sexual differences in 3 social parameters: dyadic association, party composition, and grooming interactions. Though we found a significant sexual difference in each of the 3 parameters, with males being more social than females, the actual values do not reveal striking differences between the sexes and do not support the notion of female chimpanzees as asocial: females had dyadic association indices comparable to mixed-sex dyads, spent ca. 82% of their time together with other adult chimpanzees, and had a comparable number of grooming partners. Further, female associations can be among the strongest bonds within the community, indicating that both sexes can have strongly favored association partners. The findings are in contrast to reports on East African chimpanzees, the females of which are mainly solitary and rarely interact with other females. Our results suggest that researchers cannot generally regard chimpanzee females as asocial and need to redefine models deriving patterns of sociality from dispersal patterns to integrate the possibility of high female sociality in male philopatric systems.     

Interaction sequences between chimpanzees and human visitors at the Zoo

Data were collected on the behavior and physical characteristics of 259 human visitors and 24 chimpanzees at Chester Zoo. The successive responses of humans and chimpanzees to each other's behavior were recorded, the resulting long sequence being referred to as an interaction sequence. There was no particular set of characteristics that distinguished interactors from noninteractors in either humans or chimpanzees, although there was some evidence that chimpanzees were particularly likely to respond to men carrying objects. Chimpanzee responses were random with respect to the previous human behavior, but human responses were significantly associated with the preceding chimpanzee behavior. In particular, chimpanzee sounds were likely to be followed by human sounds, and begging was likely to be followed by the offer of food. Interaction sequences varied in length, but 9% of chimpanzee-initiated sequences went as far as a ninth interaction. Sequences resulted in the chimpanzees being given food in 25% of human-initiated, but only 8% of chimpanzee-initiated sequences. The results are consistent with the interpretation that humans and chimpanzees are motivated to interact with one another and that the chimpanzees do this primarily to obtain food. © 1995 Wiley-Liss, Inc.     

Social grooming among wild bonobos (Pan paniscus) at Wamba in the Luo Scientific Reserve, DR Congo, with special reference to the formation of grooming gatherings

Chimpanzees (Pan troglodytes) groom in gatherings in which many individuals may be connected via multiple chains of grooming and they often exchange partners with each other. They sometimes groom another while receiving grooming; that is, one animal can play two roles (i.e., groomer and groomee) simultaneously. Although this feature of chimpanzees is notable from the viewpoint of the evolution of human sociality, information on our other closest living relative, the bonobo (Pan paniscus), is still lacking. In this study, I describe grooming interactions of bonobos at Wamba in the Luo Scientific Reserve, Democratic Republic of the Congo (DR Congo), with a particular focus on the formation of grooming gatherings. Like chimpanzees, the bonobos also performed mutual grooming (two individuals grooming each other simultaneously) and polyadic grooming (three or more individuals). However, unlike chimpanzees, these sessions lasted for only a short time. Bonobos rarely groomed another while receiving grooming. Because social grooming occurred not only in trees but also in open spaces, including treefall gaps, the conditions did not necessarily limit the opportunity to make multiple chains of grooming. However, bonobos also engaged in social grooming in different ways from chimpanzees; That is, many individuals were involved simultaneously at a site, in which they separated for dyadic grooming. Some cases clearly showed that bonobos preferred a third party not to join while grooming in a dyad, suggesting that bonobos have a preference for grooming in dyads and that immature individuals formed the preference that was shared among adults while growing up. Most members of the study group ranged together during the majority of the study period. Although bonobos show a fission–fusion grouping pattern, when group members frequently encounter one another on a daily basis, they may not be motivated to form multiple grooming chains at this site, as do chimpanzees.     

High adult mortality among Hiwi hunter-gatherers: implications for human evolution

Extant apes experience early sexual maturity and short life spans relative to modern humans. Both of these traits and others are linked by life-history theory to mortality rates experienced at different ages by our hominin ancestors. However, currently there is a great deal of debate concerning hominin mortality profiles at different periods of evolutionary history. Observed rates and causes of mortality in modern hunter-gatherers may provide information about Upper Paleolithic mortality that can be compared to indirect evidence from the fossil record, yet little is published about causes and rates of mortality in foraging societies around the world. To our knowledge, interview-based life tables for recent hunter-gatherers are published for only four societies (Ache, Agta, Hadza, and Ju/'hoansi). Here, we present mortality data for a fifth group, the Hiwi hunter-gatherers of Venezuela. The results show comparatively high death rates among the Hiwi and highlight differences in mortality rates among hunter-gatherer societies. The high levels of conspecific violence and adult mortality in the Hiwi may better represent Paleolithic human demographics than do the lower, disease-based death rates reported in the most frequently cited forager studies.     

A multilevel analytical framework for studying cultural evolution in prehistoric hunter–gatherer societies

Over the past decade, a major debate has taken place on the underpinnings of cultural changes in human societies. A growing array of evidence in behavioural and evolutionary biology has revealed that social connectivity among populations and within them affects, and is affected by, culture. Yet the interplay between prehistoric hunter–gatherer social structure and cultural transmission has typically been overlooked. Interestingly, the archaeological record contains large data sets, allowing us to track cultural changes over thousands of years: they thus offer a unique opportunity to shed light on long‐term cultural transmission processes. In this review, we demonstrate how well‐developed methods for social structure analysis can increase our understanding of the selective pressures underlying cumulative culture. We propose a multilevel analytical framework that considers finer aspects of the complex social structure in which regional groups of prehistoric hunter–gatherers were embedded. We put forward predictions of cultural transmission based on local‐ and global‐level network metrics of small‐scale societies and their potential effects on cumulative culture. By bridging the gaps between network science, palaeodemography and cultural evolution, we draw attention to the use of the archaeological record to depict patterns of social interactions and transmission variability. We argue that this new framework will contribute to improving our understanding of social interaction patterns, as well as the contexts in which cultural changes occur. Ultimately, this may provide insights into the evolution of human behaviour.     

Antiquity and Social Functions of Multilevel Social Organization Among Human Hunter-Gatherers

Three levels of social organization are recognized among human hunter-gatherers: the community, the domestic unit, and the band. We describe the key features of these three levels and show how they are intimately connected. We hypothesize that, in the course of human social evolution, bands emerged as a level of social organization within existing communities. As predators, hunter-gatherers live at lower population densities than chimpanzees (Pan troglodytes), even where the two species are sympatric. We propose that band formation evolved in humans from the more transient fissioning behavior seen within chimpanzee communities as a solution to the conflicting pressures of sustaining higher levels of cooperation required in hunting and the division of labor in a more dispersed community. If disputes break out, or if resources in the band territory are temporarily depleted, the existence of a wider community continues to be adaptive. To reconstruct the evolution of band society, we draw upon four lines of evidence: group (or network) size predicted from neocortex ratios, the distance materials were moved from their source during various periods in hominin evolution, ethnographic data on hunter-gatherer daily foraging ranges and population densities collated by the authors, and fossil hominin morphology. From these data, we conclude that key features of modern human hunter-gatherer social organization probably appear in the course of the evolution of Homo heidelbergensis.     

Paleolithic public goods games: why human culture and cooperation did not evolve in one step

It is widely agreed that humans have specific abilities for cooperation and culture that evolved since their split with their last common ancestor with chimpanzees. Many uncertainties remain, however, about the exact moment in the human lineage when these abilities evolved. This article argues that cooperation and culture did not evolve in one step in the human lineage and that the capacity to stick to long-term and risky cooperative arrangements evolved before properly modern culture. I present evidence that Homo heidelbergensis became increasingly able to secure contributions form others in two demanding Paleolithic public good games (PPGGs): cooperative feeding and cooperative breeding. I argue that the temptation to defect is high in these PPGGs and that the evolution of human cooperation in Homo heidelberngensis is best explained by the emergence of modern-like abilities for inhibitory control and goal maintenance. These executive functions are localized in the prefrontal cortex and allow humans to stick to social norms in the face of competing motivations. This scenario is consistent with data on brain evolution that indicate that the largest growth of the prefrontal cortex in human evolution occurred in Homo heidelbergensis and was followed by relative stasis in this part of the brain. One implication of this argument is that subsequent behavioral innovations, including the evolution of symbolism, art, and properly cumulative culture in modern Homo sapiens, are unlikely to be related to a reorganization of the prefrontal cortex, despite frequent claims to the contrary in the literature on the evolution of human culture and cognition.     

Upper-body strength predicts hunting reputation and reproductive success in Hadza hunter–gatherers

Hunting is a characteristic feature of early human subsistence, and many theories of evolution have emphasized the role of hunting in hominization. Still today hunting ability continues to be selected for in extant foragers with better hunters experiencing greater reproductive success. Yet little is known about the traits that comprise a successful hunter traits that are presupposed to also be under selection. Two complementary empirical analyses were conducted to examine this question using data collected from Hadza hunter–gatherers in Tanzania. First, data on upper-body strength, running speed, target precision and visual and auditory acuity were collected to examine the traits that predict hunting reputation in men. Second, interview data were collected from Hadza informants regarding the traits they deem important for hunting. Results from the first study implicate upper-body strength as the strongest and most consistent predictor of men's hunting reputation. Hadza conventional wisdom also accord with these findings. Although informants stressed the importance of non-physical traits, such as “intelligence” and “heart”, strong arms were cited as the most important physical trait for hunting. Finally, men with stronger upper-bodies experienced greater reproductive success, a result that is largely mediated by hunting reputation. These findings suggest that selection for hunting ability may have acted on men's upper-bodies. Nevertheless, the importance of effort on strength and hunting success cannot be dismissed. This is also discussed.     

Vocal responsiveness in male wild chimpanzees: implications for the evolution of language

Several captive chimpanzees and bonobos have learned to use symbols and to comprehend syntax. Thus, compared with other nonhumans, these animals appear to have unusual cognitive powers that can be recruited for communicative behavior. This raises the possibility that wild chimpanzee vocal communication is more complex than heretofore demonstrated. To examine this possibility, I investigated whether wild chimpanzee vocal exchanges exhibit uniquely human conversational attributes. The results indicate that wild chimpanzees vocalize at low rates, tend not to respond to calls that they hear, and, when they do respond, tend to give calls that are similar to the ones they have heard. Thus, chimpanzee vocal interactions resemble those of other primate species, and show no special similarity to human conversations. The results support the view that we need to explore cognitive and social continuities and discontinuities with nonhuman primates to understand the origin and evolution of language, but also emphasize the need for fine-grained analyses of wild chimpanzee vocal interactions.     

Why do chimpanzees have diverse behavioral repertoires yet lack more complex cultures? Invention and social information use in a cumulative task

Humans are distinctive in their dependence upon products of culture for survival, products that have evolved cumulatively over generations such that many cannot now be created by a single individual. Why the cultural capacity of humans appears unrivalled in the animal kingdom is a topic of ongoing debate. Here we explore whether innovation and/or social learning propensities may constrain the ability of one of our closest living relatives, chimpanzees (Pan troglodytes), to master an extractive foraging and tool-use task designed to afford opportunities for cumulative culture to develop. We further explore the potential demographic characteristics associated with novel task solutions. Chimpanzees (N = 53) were inventive, flexibly exploring the novel task, albeit complex inventions were rare and shaped by prior individual experience with similar tool-use tasks. However, they displayed no evidence of cumulative cultural learning. Communities displayed richer behavioral repertoires and had greater task success than chimpanzees tested in an asocial control condition, but their solution complexity did not surpass what individuals invented. The lack of social transmission of complex and beneficial solutions in contexts like those we studied provides one explanation for the limited cumulative culture observed in this species.     

Mother-infant relations and infant development in captive chimpanzees and orang-utans

Four chimpanzee (Pan troglodytes)mother—infant dyads and four orangutan (Pongo pygmaeus)motherinfant dyads were studied for the first 11 months of the infants’ lives. For both species, ventroventral contact and nipple contact decreased over time at a similar rate, but total contact decreased earlier in the orangutans and was 50% lower than for the chimpanzees at the end of the study. Social play between the mothers and the infants did not differ in frequency between the species, but orangutans played above the ground and chimpanzees on the ground. Solitary play differed in form between the species and, like social play, reflected their differences in arboreal and terrestrial proclivities. In addition, the orangutans engaged in solitary play considerably more frequently than the chimpanzees during the second half-year of life. The developmental differences in mother-infant contact and solitary play of these apes are consistent with the differences in their speciestypical social organization. The data may reflect, therefore, early development of species differences in the social and relatively solitary natures of chimpanzees and orangutans, respectively. An erratum to this article is available at .     

Dynamic interactions among badgers: implications for sociality and disease transmission

1. Direct interactions between individuals play an important part in the sociality of group-living animals, their mating system and disease transmission. Here, we devise a methodology to quantify relative rates of proximity interaction from radio-tracking data and highlight potential asymmetries within the contact network of a moderate-density badger population in the north-east of England. 2. We analysed radio-tracking data from four contiguous social groups, collected over a 3-year period. Dynamic interaction analysis of badger dyads was used to assess the movement of individuals in relation to the movement of others, both within and between social groups. Dyads were assessed with regard to season, sex, age and sett use pattern of the badgers involved. 3. Intragroup separation distances were significantly shorter than intergroup separation distances, and interactions between groups were rare. Within groups, individuals interacted with each other more often than expected, and interaction patterns varied significantly with season and sett use pattern. Non-mover dyads (using the main sett for day-resting on > 50% of occasions) interacted more frequently than mover dyads (using an outlier sett for day-resting on > 50% of occasions) or mover-non-mover dyads. Interactions between group members occurred most frequently in winter. 4. Of close intragroup interactions (< 50 m separation distance), 88.6% were associated with a main sett and only 4.4% with outlier setts. Non-mover dyads and non-mover-mover dyads interacted significantly more often at the main sett than mover-only dyads. These results highlight the importance of the main sett to badger sociality and support the suggestion that badger social groups are comprised of different subgroups, in our case based on differential sett use patterns. 5. Asymmetries in contact structure within a population will affect the way in which diseases are transmitted through a social network. Assessment of these networks is essential for understanding the persistence and spread of disease within populations which do not mix freely or which exhibit heterogeneities in their spatial or social behaviour.