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Written by Tsirha Adefris

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Primatology

3.1 Mammals and Primates

        3.1.1 vertebrates and mammals

Major adaptations for terrestrial life have been proposed, including the acquisition of fingers and toes for support and movement on land and lungs in order to breathe oxygen out of water. However, many of those so-called adaptations are likely exaptations that were developed before a terrestrial lifestyle. In fact, even the loss of internal gills may have preceded terrestriality. On the other hand, there are features that most terrestrial vertebrates share that clearly are adaptations to life out of water. Some of the highlights include structures of the middle ear and tympanum for the reception of airborne sounds plus the lachrymal duct and eyelid to keep the eyeball moist. Unfortunately, many structures that would provide evidence of terrestriality in early tetrapods, such as the sophisticated water retention structures in the kidneys, are composed of soft tissues that commonly do not preserve in the fossil record. (Ekdale, 2011)

The most distinctive thing about the mammals of the Mesozoic Era is how small they were. Although some of their therapsid ancestors attained respectable sizes. For example, the late Permian Biarmosuchus was about the size of a large dog. Very few early mammals were larger than mice, for a simple reason: dinosaurs had already become the dominant terrestrial animals on earth.

The only ecological niches open to the first mammals entailed a) feeding on plants, insects and small lizards, b) hunting at night (when predatory dinosaurs were less active), and c) living high up in trees or underground, in burrows. Eomaia, from the early Cretaceous period, and Cimolestes, from the late Cretaceous period, were fairly typical in this regard.

This isn't to say that all early mammals pursued identical lifestyles. For example, the North American Fruitafossor possessed a pointed snout and mole-like claws, which it used to dig for insects. And, the late Jurassic Castorocauda was built for a semi-marine lifestyle, with its long, beaver-like tail and hydrodynamic arms and legs. Perhaps the most spectacular deviation from the basic Mesozoic mammalian body plan was Repenomamus, a three-foot-long, 25-pound carnivore that is the only mammal known to have fed on dinosaurs (a fossilized specimen of Repenomamus has been found with the remains of a Psittacosaurus in its stomach).

Recently, paleontologists discovered conclusive fossil evidence for the first important split in the mammal family tree, the one between placental and marsupial mammals. Technically, the first, marsupial-like mammals of the late Triassic period are known as metatherians. From these evolved the eutherians, which later branched off into placental mammals. The type specimen of Juramaia, the "Jurassic mother," dates to about 160 million years ago, and demonstrates that the metatherian/eutherian split occurred at least 35 million years before scientists had previously estimated. (Strauss, n.d.)

In the transition from early synapsids to mammals, the phalangeal formula was reduced from 2-3-4-5-3 (manus) and 2-3-4-5-4 (pes) to 2-3-3-3-3. The standard view is that certain phalanges in digits III and IV were reduced in length to form disc-like elements which were ultimately lost within each non-biarmosuchian therapsid group. This implies at least five separate origins of the mammalian phalangeal pattern. (Hopson, 1995)

        3.1.2 primate classification

There is a debate that stems from the common opinion that taxonomy should reflect evolution in some manner, combined with a disagreement about the practical details of how to do this. Although some authors have provided suggestions for making the Linnaean system of taxonomy work within the context of a cladistic approach to phylogeny reconstruction. (Silcox, 2007)

 A highly resolved primate cladogram based on DNA evidence is congruent with extant and fossil osteological evidence. A provisional primate classification based on this cladogram and the time scale provided by fossils and the model of local molecular clocks has all named taxa represent clades and assigns the same taxonomic rank to those clades of roughly equivalent age. Order Primates divides into Strepsirhini and Haplorhini. Strepsirhines divide into Lemuriformes and Loriformes, whereas haplorhines divide into Tarsiiformes and Anthropoidea. Within Anthropoidea when equivalent ranks are used for divisions within Platyrrhini and Catarrhini, Homininae divides into Hylobatini (common and siamang gibbon) and Hominini, and the latter divides into Pongina for Pongo(orangutans) and Hominina for Gorilla and Homo. Homoitself divides into the subgenera H. (Homo) for humans and H. (Pan) for chimpanzees and bonobos. There are differences between this provisional age-related phylogenetic classification and current primate taxonomies. (Goodman, et al., 1998)

3.2 Primate Characteristics

        3.2.1 Skeleton

The issue of whether nails or claws were present on the digits of the last common ancestor of living primates is central to the understanding of the ecological context in which the order originated. Two lines of evidence are available, the shape (claw, nail, toilet-claw) and the histological structure (one or two horny strata). Here we review the existing data regarding the shape and histological structure of cheirideal appendages in primates and present new information from a wide range of living primates. We demonstrate the presence of a typical toilet-claw in Daubentonia madagascariensis and discuss its consequences, since the alleged lack of such structures in this species has long obscured the issue. The general view that primate nails, with the exception of those in New World primates, consist of only one layer is disproved by the presence of two distinct strata in the nails of the feet of three out of seven catarrhine species examined, as well as in Lemur catta. The combined new and old data indicate that the last common ancestor of the extant primates had lost the typical mammalian claws of its ancestors and developed nails on all pedal digits except digit II, which bore a toilet-claw. All nails as well as the toilet-claw originally consisted of two layers. (Soligo, Soligo, & Müller, 1999)

The hands and feet are in direct contact with a substrate, so their form would be expected to reflect substrate preference, whereas tooth shape is related directly to the functional demands of masticating foods having different mechanical properties. What remains unclear, however, is the role of developmental and genetic processes that underlie the evolutionary diversity of the primate body plan. Are variational tendencies a signature of constraints in developmental pathways? What is the genetic basis for similar morphological transformations among closely related species? These are a sampling of the types of questions we believe can be addressed by future research integrating evidence from paleontology, comparative morphology, and developmental genetics. (Chiu & Hamrick, 2002)

        3.2.2 Sense of touch

Tactile and proprioceptive signals are relayed by the peripheral nerves to the central nervous system, where they are processed to give rise to percepts of objects and of the state of the body. (Delhaye, Long, & Bensmaia, 2018)

The primate somatosensory cortex, which processes tactile stimuli, contains a topographic representation of the signals it receives, but the way in which such maps are maintained is poorly understood. Previous studies of cortical plasticity indicated that changes in cortical representation during learning arise largely as a result of Hebbian synaptic change mechanisms. It has been shown using adult owl monkeys trained to respond to specific stimulus sequence events, that serial application of stimuli to the fingers results in changes to the neuronal response specificity and maps of the hand surfaces in the true primary somatosensory cortical field. In this representational remodeling stimuli applied synchronously to the fingers resulted in these fingers being integrated in their representation, whereas fingers to which stimuli were applied asynchronously were segregated in their representation. Ventro-posterior thalamus response maps derived in these monkeys were not equivalently reorganized. This representational plasticity appears to be cortical in origin. (Wang, et al., 1995)

        3.2.3 Sense of smell

Recent years have seen a proliferation of physiological, behavioral, anatomical, and genetic investigations of primate olfaction. These investigations have begun to shed light on the importance of olfaction in the process of food acquisition. However, integration of these works has been limited. It is therefore still difficult to pinpoint large-scale evolutionary scenarios, namely the functions that the sense of smell fulfills in primates’ feeding ecology and the ecological niches that favor heavier reliance on olfaction. Here, we review available behavioral and physiological studies of primates in the field or captivity and try to elucidate how and when the sense of smell can help them acquire food. (Nevo & Heymann, 2015)

        3.2.4 Vision

It has been found that neurons encoding the binding relations between visual features, known as binding neurons, emerge during visual training when key properties of the visual cortex are incorporated into the models. These biological network properties include (i) bottom-up, lateral and top-down synaptic connections, (ii) spiking neuronal dynamics, (iii) spike timing-dependent plasticity, and (iv) a random distribution of axonal transmission delays (of the order of several milliseconds) in the propagation of spikes between neurons. After training the network on a set of visual stimuli, modelling studies have reported observing the gradual emergence of polychronization through successive layers of the network, in which subpopulations of neurons have learned to emit their spikes in regularly repeating spatio-temporal patterns in response to specific visual stimuli. Such a subpopulation of neurons is known as a polychronous neuronal group (PNG). Some neurons embedded within these PNGs receive convergent inputs from neurons representing lower- and higher-level visual features, and thus appear to encode the hierarchical binding relationship between features. Neural activity with this kind of spatio-temporal structure robustly emerges in the higher network layers even when neurons in the input layer represent visual stimuli with spike timings that are randomized according to a Poisson distribution. The resulting hierarchical representation of visual scenes in such models, including the representation of hierarchical binding relations between lower- and higher-level visual features, is consistent with the hierarchical phenomenology or subjective experience of primate vision and is distinct from approaches interested in segmenting a visual scene into a finite set of objects. (Isbister, et al., 2018)

Primates are the most visually adapted order of mammals. There is a rich history within anthropology of proposed explanations for the adaptive significance of binocular vision, especially pertaining to primate origins and evolution. Depth perception and orbit morphology have been hypothesized to be functionally related to specialized locomotor or feeding behaviors. Many of these arguments continue to this day. An understanding of specific primate visual adaptations, including binocular vision, can shed light on these long-term and heated debates. Primates exhibit highly derived neurological processing of spatial form and contrast, depth and motion perception, and multiple variants of color vision. One of the most con-spicuous primate visual specializations is the large area of overlap between the fields of vision of the two eyes. (Heesy, 2009)

3.3 Primate origins

       3.3.1 The timeline of primate origins

The mammalian order Primates made its first appearance in the fossil record during the Paleocene–Eocene thermal maximum (PETM), the global greenhouse warming event that marks the beginning of the Eocene. Two primate superfamilies, Tarsioidea and Adapoidea, dominate early and middle Eocene primate faunas. Warm climates enabled primates to thrive, and warming events within the Eocene facilitated cosmopolitan dispersal. Declining diversity at the end of the Eocene reflects environmental cooling. Fossils of earliest Tarsioidea and Adapoidea are similar dentally, often confused, and appear closely related as stem or crown Haplorhini. The superfamily Tarsioidea is represented by a single genus, Tarsius, living today, while Adapoidea appear to be ancestral to living Anthropoidea. Little is known of the Eocene history of strepsirrhine Lemuroidea and Lorisoidea. Temporal scaling of molecular clock ages suggests that Strepsirrhini appeared before Haplorhini in the Paleocene or possibly with Haplorhini at the beginning of the Eocene. Substantial skeletons of Eocene primates like those of adapoid Darwinius and Europolemur from Messel in Germany and Notharctus and Smilodectes from western North America constrain phylogenetic interpretation of primate relationships much more than dental remains ever can. A specialised grasping foot distinguishes early primates from other mammals. Traits associated in a functional complex include replacement of claws by nails on all digits; movement of the pedal fulcrum from the metatarsals to the tarsals; elongation of digit IV relative to digit III, with reduction of digit II and sometimes III; and then secondary development of a grooming claw or claws on digits II and sometimes III. The specialised grasping foot of early primates was later moderated in the emergence of anthropoid primates. (Gingerich, 2012)

In the last three decades investigations into the processes underlying molecular evolution have expanded our knowledge concerning the phylogenetic history of the primates. Advances in molecular evolution have coincided with exciting paleontological discoveries, especially the recent australopithecine and ramapithecine finds that question anew the events and timing leading to the origins of the hominine species. The molecular approach has yielded important insights into phylogenetic relationships among primates by sharpening our understanding in several areas. The possibility of correct placement for the enigmatic Tarsier with respect to prosimian and anthropoid lineages and understanding the evolutionary history of the gibbon and siamang with respect to the other ape lineages has been strengthened. Further insights into relationships within Old and New World monkey groups have also been gained. (Baba, Darga, & Goodman, 1982)

        3.3.2 The evidence for the origin of primates

The order Primates in the strict sense—Euprimates or primates of modern aspect—is defined by a familiar suite of synapomorphies. Some of these may represent adaptively neutral contingencies (for example, the formation of the auditory bulla by an outgrowth from the petrosal, rather than by a separate entotympanic bone). However, others appear to be telling us things about the basal adaptations of the order. Compared to primitive placental mammals, primates have a reduced sense of smell and an enhanced sense of vision. primate eyes point forward and are encircled by a ring of bone. The first toes of primates are stout, divergent grasping organs. All primates have reduced, flattened claws on the first toe, and most of them have them on the other digits as well. There are  adaptive meaning and origins of some of these morphological synapomorphies of the primate order. One behavioral synapomorphy of primates, which has received less attention in discussions of primate origins, is their distinctive walking gait. (Cartmill, Lemelin, & Schmitt, 2007)

Plesiadapiforms are central to studies of the origin and evolution of primates and other euarchontan mammals (tree shrews and flying lemurs). A comprehensive cladistic analysis using cranial, postcranial, and dental evidence including data from recently discovered Paleocene plesiadapiform skeletons (Ignacius clarkforkensis sp. nov.; Dryomomys szalayi, gen. et sp. nov.), and the most plesiomorphic extant tree shrew, Ptilocercus lowii. Our results, based on the fossil record, unambiguously place plesiadapiforms with Euprimates and indicate that the divergence of Primates (sensu lato) from other euarchontans likely occurred before or just after the Cretaceous/Tertiary boundary (65 Mya), notably later than logistical model and molecular estimates. Anatomical features associated with specialized pedal grasping (including a nail on the hallux) and a petrosal bulla likely Plesiadapoidea and Euprimates (Euprimateformes) by 62 Mya in either Asia or North America. Results are consistent with those from recent molecular analyses that group Dermoptera with Scandentia. There is no evidence here to support the hypothesis that any plesiadapiforms were mitten-gliders or closely related to Dermoptera. (Bloch, Silcox, Boyer, & Sargis, 2007)

3.4 Primate Behavior

       3.4.1 Primate sociality

The evolution and origin of primate social organization has attracted the attention of many researchers, and a solitary pattern, believed to be present in most nocturnal prosimians, has been generally considered as the most primitive system. Nocturnal prosimians are in fact mostly seen alone during their nightly activities and therefore termed ‘solitary foragers’, but that does not mean that they are not social. Moreover, designating their social organization as ‘solitary’, implies that their way of life is uniform in all species. It has, however, emerged over the last decades that all of them exhibit not only some kind of social network but also that those networks dyer among species. There is a need to classify these social networks in the same manner as with group-living (gregarious) animals if we wish to link up the different forms of primate social organization with ecological, morphological or phylogenetic variables. In this review, we establish a basic classification based on spatial relations and sociality in order to describe and cope properly with the social organization patterns of the different species of nocturnal prosimians and other mammals that do not forage in cohesive groups. In attempting to trace the ancestral pattern of primate social organization, the Malagasy mouse and dwarf lemurs and the Afro-Asian bush babies and lorises are of special interest because they are thought to approach the ancestral conditions most closely. These species have generally been believed to exhibit a dispersed harem system as their pattern of social organization (‘dispersed’ means that individuals forage solitarily but exhibit a social network). Therefore, the ancestral pattern of primate social organization was inferred to be a dispersed harem. In fact, new field data on cheirogaleids combined with a review of patterns of social organization in strepsirrhines (lemurs, bush babies and lorises) revealed that they exhibit either dispersed multi-male systems or dispersed monogamy rather than a dispersed harem system. Therefore, the concept of a dispersed harem system as the ancestral condition of primate social organization can no longer be supported. In combination with data on social organization patterns in ‘primitive’ placentals and marsupials, and in monotremes, it is in fact most probable that promiscuity is the ancestral pattern for mammalian social organization. Subsequently, a dispersed multi-male system derived from promiscuity should be regarded as the ancestral condition for primates. We further suggest that the gregarious patterns of social organization in Aotus and Avahi, and the dispersed form in Tarsius evolved from the gregarious patterns of diurnal primates rather than from the dispersed nocturnal type. It is consequently proposed that, in addition to Aotus and Tarsius, Avahi is also secondarily nocturnal. (Müller & Thalmann, 2007)

        3.4.2 aggressive and hedonic behaviors

A balanced social group of 17 rhesus monkeys, wild-trapped in northern India, was established in a colony cage of 1000 square feet in Calcutta. Quantitative data were obtained on behavioral repertoire, activity patterns and social interactions. Experiments were conducted to study the effect of certain environmental and social variables on the expression and intensity of intragroup agonistic behavior. A 25% food reduction resulted in no change in agonistic interactions, whereas a 50% food reduction and starvation regime resulted in a significant decrease in agonistic behavior. Investigative behavior increased, but grooming, sexual behavior, play, and aggressive behavior decreased. The monkeys became lethargic. The behavior of the monkeys resembled human behavior in famine and experimental starvation. A significant increase in agonistic behavior occurred when the distribution of the food was restricted, but the amount of food remained normal. Highly significant increases in the frequency and intensity of agonistic behavior occurred with the introduction of new monkeys who were social strangers. These results agree well with those of BERNSTEIN et al. on rhesus and KAWAI on Japanese macaques. The age and/or sex class corresponding to that of the introduced monkeys was the one which initiated most of the threat and attack behavior. A significant increase in agonistic behavior also occurred with a space reduction from 1000 square feet to 500 square feet. In general, social changes (i.e., changes in group membership) had a far greater impact on levels of intragroup aggression than did environmental changes such as starvation and crowding. (Southwick, 1967)

Non-human primates are marked by well-developed prosocial and cooperative tendencies as reflected in the way they support each other in fights, hunt together, share food and console victims of aggression. The proximate motivation behind such behavior is not to be confused with the ultimate reasons for its evolution. Even if a behavior is ultimately self-serving, the motivation behind it may be genuinely unselfish. A sharp distinction needs to be drawn, therefore, between (i) altruistic and cooperative behavior with knowable benefits to the actor, which may lead actors aware of these benefits to seek them by acting cooperatively or altruistically and (ii) altruistic behavior that offers the actor no knowable rewards. The latter is the case if return benefits occur too unpredictably, too distantly in time or are of an indirect nature, such as increased inclusive fitness. The second category of behavior can be explained only by assuming an altruistic impulse, which—as in humans—may be born from empathy with the recipient's need, pain or distress. Empathy, a proximate mechanism for prosocial behavior that makes one individual share another's emotional state, is biased the way one would predict from evolutionary theories of cooperation (i.e. by kinship, social closeness and reciprocation). There is increasing evidence in non-human primates (and other mammals) for this proximate mechanism as well as for the unselfish, spontaneous nature of the resulting prosocial tendencies. (Waal & Suchak, 2010)

3.5 Primate Culture

         3.5.1 Tool use

Any model for the evolution of the use of feeding tools must explain why tool use is found in only a small subset of primate species, why many of these species use tools much more readily in captivity, why routine reliance on feeding tools is found in only two species of ape, and why there is strong geographic variation within these two species. Because ecological factors alone cannot explain the distribution of tool use in the wild, we develop a model that focuses on social and cognitive factors affecting the invention and transmission of tool-using skills. The model posits that tool use in the wild depends on suitable ecological niches (especially extractive foraging) and the manipulative skills that go with them, a measure of intelligence that enables rapid acquisition of complex skills (through both invention and, more importantly, observational learning), and social tolerance in a gregarious setting (which facilitates both invention and transmission). The manipulative skills component explains the distribution across species of the use of feeding tools, intelligence explains why in the wild only apes are known to make and use feeding tools routinely, and social tolerance explains variation across populations of chimpanzees and orangutans. We conclude that strong mutual tolerance was a key factor in the explosive increase in technology among hominids, probably intricately tied to a lifestyle involving food sharing and tool-based processing or the acquisition of large, shareable food packages. (Schaik, Deaner, & Merrill, 1999)

         3.5.2 Language abilities

Primate vocal communication is very different from human language. Differences are most pronounced in call production. Differences in production have been overemphasized, however, and distracted attention from the information that primates acquire when they hear vocalizations. In perception and cognition, continuities with language are more apparent. We suggest that natural selection has favored nonhuman primates who, upon hearing vocalizations, form mental representations of other individuals, their relationships, and their motives. This social knowledge constitutes a discrete, combinatorial system that shares several features with language. It is probably a general primate characteristic whose appearance pre-dates the evolution of spoken language in our hominid ancestors. The prior evolution of social cognition created individuals who were preadapted to develop language. Several features thought to be unique to language—like discrete combinatorics and the encoding of propositional information—were not introduced by language. They arose, instead, because understanding social life and predicting others’ behavior requires a particular style of thinking. (Seyfarth & Cheney, 2008)

Neurophysiological research has supported the hypothesis of a close association between some aspects of human action organization and of language representation, in both phonology and semantics. Tool use provides an excellent experimental context to investigate analogies between action organization and linguistic syntax. Contributors report and contextualize experimental evidence from monkeys, great apes, humans and fossil hominins, and consider the nature and the extent of overlaps between the neural representations of tool use, manual gestures and linguistic processes. (Steele, Ferrari, Ferrari, Fogassi, & Fogassi, 2012)

Great apes give gestures deliberately and voluntarily, in order to influence particular target audiences, whose direction of attention they consider when choosing which type of gesture to use. These facts make the study of ape gesture directly relevant to understanding the evolutionary precursors of human language; here we present an assessment of ape gesture from that perspective, focusing on the work of the “St Andrews Group” of researchers. Intended meanings of ape gestures are relatively few and simple. As with human words, ape gestures often have several distinct meanings, which are effectively disambiguated by behavioral context. Compared to the signaling of most other animals, great ape gestural repertoires are large. Because of this, and the relatively small number of intended meanings they overlap in meaning. The great majority of gestures are innate, in the sense that the species’ biological inheritance includes the potential to develop each gestural form and use it for a specific range of purposes. Moreover, the phylogenetic origin of many gestures is relatively old, since gestures are extensively shared between different genera in the great ape family. Acquisition of an adult repertoire is a process of first exploring the innate species potential for many gestures and then gradual restriction to a final (active) repertoire that is much smaller. No evidence of syntactic structure has yet been detected. (Byrne, et al., 2017)

3.6 Humans as Primates

         3.6.1 Shared derived features

The genetic similarity between humans and nonhuman primates makes nonhuman primates uniquely suited as models for genetic research on complex physiological and behavioral phenotypes. By comparison with human subjects, nonhuman primates, like other animal models, have several advantages for these types of studies: 1) constant environmental conditions can be maintained over long periods of time, greatly increasing the power to detect genetic effects; 2) different environmental conditions can be imposed sequentially on individuals to characterize genotype-environment interactions; 3) complex pedigrees that are much more powerful for genetic analysis than typically available human pedigrees can be generated; 4) genetic hypotheses can be tested prospectively by selective mating; and 5) essential invasive and terminal experiments can be conducted. Limitations of genetic research with nonhuman primates include cost and availability However, the ability to manipulate both genetic and environmental factors in captive primate populations indicates the promise of genetic research with these important animal models for illuminating complex disease processes. The utility of nonhuman primates for biomedical research on human health problems is illustrated by examples concerning the use of baboons in studies of osteoporosis, alcohol metabolism, and lipoproteins. (VandeBerg & Williams-Blangero, 1997)

         3.6.2 Features unique to humans

Darwin's claim ‘that the difference in mind between man and the higher animals … is certainly one of degree and not of kind’ is at the core of the comparative study of cognition. Recent research provides unprecedented support for Darwin's claim as well as new reasons to question it, stimulating new theories of human cognitive uniqueness. This article compares and evaluates approaches to such theories. Some prominent theories propose sweeping domain-general characterizations of the difference in cognitive capabilities and/or mechanisms between adult humans and other animals. Dual-process theories for some cognitive domains propose that adult human cognition shares simple basic processes with that of other animals while additionally including slower-developing and more explicit uniquely human processes. These theories are consistent with a modular account of cognition and the ‘core knowledge’ account of children's cognitive development. A complementary proposal is that human infants have unique social and/or cognitive adaptations for uniquely human learning. A view of human cognitive architecture as a mosaic of unique and species-general modular and domain-general processes together with a focus on uniquely human developmental mechanisms is consistent with modern evolutionary-developmental biology and suggests new questions for comparative research. (Shettleworth, 2012)

Multilevel (or modular) societies are a distinct type of primate social system whose key features are single-male–multifemale, core units nested within larger social bands. They are not equivalent to fission–fusion societies, with the latter referring to routine variability in associations, either on an individual or subunit level. The purpose of this review is to characterize and operationalize multilevel societies and to outline their putative evolutionary origins. Multilevel societies are prevalent in three primate clades: papionins, Asian colobines, and hominins. For each clade, we portray the most parsimonious phylogenetic pathway leading to a modular system and then review and discuss likely socioecological conditions promoting the establishment and maintenance of these societies. The multilevel system in colobines (most notably Rhinopithecus and Nasalis) has likely evolved as single-male harem systems coalesced, whereas the multilevel system of papionins (Papio hamadryas, Theropithecus gelada) and hominins most likely arose as multimale–multifemale groups split into smaller units. We hypothesize that, although ecological conditions acted as preconditions for the origin of multilevel systems in all three clades, a potentially important catalyst was intraspecific social threat, predominantly bachelor threat in colobines and female coercion/infanticide in papionins and humans. We emphasize that female transfers within bands or genetic relationships among leader males help to maintain modular societies by facilitating interunit tolerance. We still lack a good or even basic understanding of many facets of multilevel sociality. Key remaining questions are how the genetic structure of a multilevel society matches the observed social effort of its members, to what degree cooperation of males of different units is manifest and contributes to band cohesion, and how group coordination, communication, and decision making are achieved. Affiliative and cooperative interunit relations are a hallmark of human societies and studying the precursors of intergroup pacification in other multilevel primates may provide insights into the evolution of human uniqueness. (Grueter, Grueter, Chapais, Zinner, & Zinner, 2012)