From the dense canopy of tropical rainforests to the arid savannas of East Africa, animals spend hours engaged in the meticulous, rhythmic parted parting of fur, combing of hair, and cleaning of skin. Far more than a mere hygienic necessity for the removal of ectoparasites, this ubiquitous behavioral pattern represents one of the most vital evolutionary mechanisms for navigating group life. Known in ethology and primatology as allogrooming, this tactile exchange operates as the fundamental currency of cooperation, coalitionary politics, and neurobiological stress reduction across diverse taxa.
Allogrooming
1. Concise Definition
Allogrooming refers to the behavior in which an individual animal grooms, cleans, preens, or maintains the body surface of a conspecific (another member of the same species). It stands in direct contrast to autogrooming, which denotes the self-directed cleaning and care of one’s own body.
In evolutionary biology and comparative psychology, allogrooming is recognized as a complex social behavior that serves dual functions: immediate sanitary maintenance (hygienic function) and the cultivation, negotiation, and stabilization of interpersonal bonds within social hierarchies (hedonic and social functions). While documented across insects, birds, and ungulates, allogrooming reaches its highest degree of structural sophistication and strategic utility among non-human primates.
Across mammalian lineages, the practice engages specialized neuroendocrine pathways, mitigating social stress and eliciting profound physiological down-regulation. Consequently, allogrooming is widely characterized by behavioral ecologists as a biological market good traded for social alliances, food access, mating opportunities, and infant handling privileges.
2. Etymology & Linguistic Origin
The term allogrooming is a modern scientific compound combining classical Greek and Middle English elements. The prefix derives from the Ancient Greek ἄλλος (allos), meaning “other,” “different,” or “external to oneself.” This prefix is widely used across biological taxonomy and genetics to indicate interactions or characteristics directed toward or sourced from another entity (e.g., allopatry, allopolyploidy, alloparenting).
The root word groom traceably emerges from the Middle English noun grom (attested circa 1200 CE), historically signifying a boy, youth, or male servant—specifically one charged with the maintenance, cleaning, and stabling of horses. By the late sixteenth century, the noun transitioned into an active verb meaning to tend, curry, and clean animals, before broadening into personal human hygiene and appearance maintenance.
The formal scientific synthesis of the two roots arose in mid-twentieth-century ethology as researchers such as C. R. Carpenter and Robert Yerkes sought objective, non-anthropomorphic vocabularies to catalog non-human primate behavioral repertoires. The term entered standardized academic discourse to cleanly bifurcate personal bodily upkeep (“autogrooming”) from interactive social maintenance (“allogrooming” or “social grooming”).
3. Pronunciation & Grammatical Form
Phonetic transcription: /ˌæl.oʊˈɡruː.mɪŋ/ (General American), /ˌæl.eəˈɡruː.mɪŋ/ (Received Pronunciation).
Part of Speech: Uncountable noun (mass noun); can also operate as a gerund or present participle of the transitive/intransitive verb to allogroom.
Morphological Variations:
- Allogroom (transitive/intransitive verb): e.g., “The dominant female will allogroom subordinate group members to secure political coalitions.”
- Allogroomed (past tense / past participle): e.g., “Subordinate males that were frequently allogroomed exhibited lower glucocorticoid levels.”
- Allogroomer (agent noun): The active provider of the grooming behavior.
- Allogroomee (recipient noun): The passive recipient experiencing tactile manipulation.
4. Detailed Conceptual Explanation
To fully conceptualize allogrooming, one must assess both its mechanistic execution and its multi-tiered functional architecture. At the primary motor level, allogrooming entails the meticulous scanning, parting, picking, scratching, and licking of an individual’s fur, feathers, or skin. In non-human primates, this is predominantly an optical and manual process involving fine motor control: the groomer uses their fingers to part hair, exposing parasites (such as lice and ticks), dried epithelial debris, dirt, and crystallized salt, which are subsequently plucked using the fingers or teeth and ingested or discarded.
The mechanical execution varies across the animal kingdom. Birds perform allopreening via precise bill adjustments, targeting inaccessible regions such as the crown, nape, and throat. Social ungulates—such as cattle, impalas, and horses—use reciprocal tongue licking and specialized incisor structures (such as the lemuriform toothcomb or impala dental apparatus) to dislodge ectoparasites. Social insects, including eusocial ants, bees, and termites, engage in allogrooming via specialized mouthparts and antennae to eliminate lethal fungal spores (such as Metarhizium) and distribute colony-identifying cuticular hydrocarbons.
Beyond hygiene, allogrooming serves as a potent vehicle for physical, neurochemical, and social homeostasis. When an organism is allogroomed, low-threshold mechanoreceptors in the hairy skin—specifically unmyelinated C-tactile afferents—fire in response to slow, gentle, stroking sensations. This sensory signaling projects directly to the insular cortex, bypassing the primary somatosensory cortex and activating the parasympathetic nervous system.
This autonomic shift yields immediate physiological benefits: resting heart rate plummets, systemic vascular resistance declines, respiratory rhythms deepen, and peripheral cortisol concentrations fall precipitously. Simultaneously, the recipient experiences a sustained release of central beta-endorphins and oxytocin, promoting sedation, positive affect, and profound inter-individual bonding. Thus, allogrooming constitutes an embodied, non-verbal dialogue that regulates collective physiology across social units.
5. Historical Development
Early naturalists initially treated grooming among animals as an incidental, self-evident reaction to ectoparasitic irritation. In The Descent of Man (1871), Charles Darwin observed that monkeys frequently inspect each other’s fur, interpreting the behavior primarily through the lens of functional utility and rudimentary mutual aid. However, the systematic ethological study of allogrooming did not accelerate until the advent of field-based primatology in the early-to-mid twentieth century.
A seminal paradigm shift occurred through the work of comparative psychologist Robert M. Yerkes in the 1930s. In his landmark 1933 paper, “Genetic Aspects of Grooming, A Social Behavior in Chimpanzees,” Yerkes demonstrated that chimpanzee grooming surpassed the mechanical needs of skin hygiene, identifying it as a powerful social mechanism that fosters positive inter-individual rapport, relieves tension, and facilitates social integration. Yerkes’ insights prompted researchers to shift their gaze from the skin to the social matrix.
During the 1960s and 1970s, field studies by pioneering ethologists—including Jane Goodall at Gombe, Hans Kummer studying hamadryas baboons in Ethiopia, and Alison Jolly observing prosimians in Madagascar—documented that allogrooming was disproportionately distributed among group members. Animals did not simply groom those who had the most parasites; they groomed kin, high-ranking matriarchs, and political allies. In the 1980s, Robert Seyfarth and Dorothy Cheney formalized cognitive and market-based interpretations of grooming, showing that female vervet monkeys selectively responded to the vocalized distress calls of unrelated individuals who had groomed them earlier that day.
In the 1990s and 2000s, British evolutionary psychologist Robin Dunbar cemented the role of allogrooming in theoretical anthropology by developing the Social brain hypothesis. Dunbar demonstrated that the percentage of time a primate species allocates to social grooming correlates directly with its mean group size and relative neocortex volume, establishing allogrooming as the core ancestral glue that preserved mammalian sociality prior to the emergence of symbolic human language.
6. Theoretical Foundations
Allogrooming is understood through three primary, non-mutually exclusive theoretical frameworks: evolutionary sociobiology, neurobiological attachment theory, and economic market models.
From an evolutionary perspective, allogrooming is parsed through the lens of kin selection and reciprocal altruism. Under Robert Trivers’ model of reciprocal altruism (1971), an individual incurs a non-trivial energetic and vigilance cost when grooming a conspecific: while looking down into another’s fur, the groomer cannot forage for food, remains blind to approaching predators, and risks opportunistic aggression. The behavior can only evolve if the initial cost is compensated over time by deferred fitness benefits returned by the recipient, such as reciprocal grooming, agonistic support in combat, or shared access to monopolized resources.
A second, highly predictive framework is Biological Market Theory, formulated by Ronald Noë and Peter Hammerstein (1994). This model conceptualizes allogrooming as a fungible biological service traded in an open social marketplace. Because grooming duration, quality, and recipient access can be quantitatively varied, animals use grooming as currency. Dominant individuals, possessing high social capital (e.g., resource defense capabilities, rank, infant possession), require significant grooming investments from subordinates to grant social tolerance. Subordinates, conversely, use grooming to purchase proximity, minimize the risk of unprovoked aggression, and secure future coalitionary backup.
Finally, the neurobiological and evolutionary framework championed by Robin Dunbar links allogrooming directly to cognitive evolution. Dunbar argues that as primate social groups expanded to manage predation pressures, maintaining group cohesion required an increasingly substantial allocation of cognitive capacity and time. Because physical allogrooming can typically occur only between two individuals simultaneously, there exists an upper limit on group size—often termed the grooming time budget ceiling (roughly 20% of an animal’s active daylight hours). Dunbar posited that human vocal communication, laughter, music, and eventually verbal conversation evolved as “vocal grooming” mechanisms to overcome the physical throughput limitations of one-on-one allogrooming, allowing one speaker to simultaneously stroke the social endorphin receptors of multiple listeners.
7. Key Components, Types & Dimensions
To systematically classify allogrooming, behavioral researchers analyze its topography along several structural, social, and functional dimensions:
- Functional Subtypes:
- Hygienic Grooming: Targeted directed cleaning focused strictly on anatomically inaccessible zones (e.g., the upper back, crown, ears, and perianal regions) to extract parasites and detritus.
- Affiliative / Bond-Maintaining Grooming: Routine, calm tactile contact deployed between established bonded pairs (mothers and offspring, bonded pairs, long-term allies) to reinforce social closeness and reduce baselines of stress.
- Reconciliatory Grooming: Grooming episodes initiated within minutes following an agonistic conflict between the former combatants, intended to repair social rupture and attenuate renewed attack risk.
- Tension-Reduction (Appeasing) Grooming: Grooming directed up the dominance hierarchy by a subordinate toward a dominant animal displaying signs of agitation, intended to preempt prospective aggression.
- Transactional Grooming: Grooming immediately deployed as an exchange commodity to acquire access to specific monopolized resources, such as infant handling, access to scarce high-calorie foods, or copulation bouts.
- Directional and Symmetry Dimensions:
- Unidirectional Grooming: One animal exclusively provides grooming while the other passively receives it throughout the entire observational interval.
- Reciprocal (Bidirectional) Grooming: Animals exchange roles within the same bout, taking alternating turns as groomer and groomee.
- Simultaneous Mutual Grooming: Both interactants groom one another concurrently, a pattern common in cattle and birds but structurally rarer in higher primates due to manipulative hand positions.
- Quantitative Parameters:
- Bout Duration: The continuous temporal length of an allogrooming interaction before an interruption exceeding a predefined duration threshold (e.g., 30 seconds).
- Frequency: The raw number of distinct grooming episodes observed per individual or dyad over a standardized observation window.
- Topographical Site Distribution: The precise anatomical areas targeted during the bout, evaluated relative to the recipient’s bodily accessibility and emotional vulnerability (e.g., exposing the throat or belly indicates profound social trust).
8. Examples & Illustrative Cases
Allogrooming manifestations differ widely across species, reflecting distinct social ecologies:
Chimpanzees (Pan troglodytes): Male chimpanzees in patriarchal communities (such as Ngogo in Kibale National Park, Uganda) exhibit extensive, highly political allogrooming partnerships. Alpha and prospective challenger males engage in grooming bouts lasting over an hour. These sessions are punctuated by synchronized hand-clapping and leaf-clipping displays, forging defensive alliances that determine political dominance, lethal inter-group border patrols, and hunting meat redistribution.
Chacma and Olive Baboons (Papio cynocephalus / Papio anubis): In matriarchal baboon troops, allogrooming is strictly structured by maternal kin lineages. Matriarchs and daughters prioritize grooming among matrilineal relatives to preserve family rank. However, females also trade extensive grooming bouts to lactating mothers in exchange for permission to sniff, inspect, and temporarily hold newborn infants—a dynamic that vividly demonstrates the principles of biological market theory.
Vampire Bats (Desmodus rotundus): Among non-primate mammals, common vampire bats engage in intensive allogrooming coupled with reciprocal regurgitation of blood meals. In roosting colonies, bats groom conspecifics’ wings, faces, and bodies prior to and following regurgitation exchanges. Research demonstrates that allogrooming serves as a diagnostic precursor to food sharing, helping bats establish social bonds and identify partners who are likely to reciprocate when starvation looms.
Eusocial Insects (Honeybees, Apis mellifera): In the honeybee hive, worker bees execute specialized “grooming dances” to solicit allogrooming from nestmates. When infected with ectoparasites such as the Varroa destructor mite, a bee vibrates its body vigorously. Nearby workers respond by systematically examining the dancer’s thorax, chewing off and disposing of the mites. This behavior represents a form of social immunity that preserves the health of the superorganism.
9. Measurement & Assessment
In quantitative behavioral ecology, allogrooming is recorded through rigorous, standardized behavioral sampling regimes. Researchers avoid subjective interpretations by employing predetermined, operationalized ethograms—comprehensive inventories of discrete, observable behaviors.
The primary observational methodologies include:
- Focal Animal Sampling: An observer continuously watches a single target animal for a predetermined duration (e.g., 20 or 60 minutes), recording the exact second-by-second onset, termination, identity of partner, and spatial body site of every allogrooming interaction.
- Scan Sampling: At fixed, standardized intervals (e.g., every 5, 10, or 15 minutes), the observer records the immediate activity of all visible group members, yielding broad group-level time budgets (e.g., calculating the aggregate percentage of daily activity allocated to social grooming).
- All-Occurrences Sampling: The researcher documents every incidence of a specific, brief interaction—such as reconciliatory allogrooming following an aggressive scream or fight—across the entire troop.
Once gathered, grooming interactions are compiled into square sociomatrices representing dyadic interactions within the group. These matrices are subjected to advanced computational analysis, notably Social Network Analysis (SNA). Using metrics such as:
- Degree Centrality: The total number of distinct grooming partners an individual possesses.
- In-Degree Centrality: The total volume of grooming an individual receives from others (a reliable index of popularity, attractiveness, or social rank).
- Out-Degree Centrality: The total volume of grooming an individual directs outward to others (reflecting social effort and investment strategies).
- Eigenvector Centrality: A measure of an individual’s connectedness to other well-connected individuals in the network.
Alongside behavioral matrices, modern primatologists deploy non-invasive physiological monitoring to quantify the somatic effects of allogrooming. By collecting fecal, urinary, or salivary samples, researchers assay glucocorticoids (cortisol) and neuropeptides (oxytocin). Telemetric heart-rate monitors, thermal cameras detecting facial heat signatures (indexes of sympathetic autonomic activation), and portable respiratory belts provide real-time cardiovascular data during live grooming encounters.
10. Applications & Practical Significance
The study of allogrooming has practical implications across veterinary care, conservation strategies, human psychology, and urban design:
Captive Animal Welfare and Enrichment: In modern zoological parks, animal sanctuaries, and biomedical facilities, allogrooming rates serve as an essential indicator of psychological well-being and social stability. A complete cessation of allogrooming, or its sudden collapse into hyper-autogrooming (such as psychogenic alopecia or over-plucking), signals chronic environmental distress, pathologically high group tension, or social fragmentation. Enrichment designers construct habitats that offer adequate inter-individual distance and physical structures (such as communal platforms and protected feeding nodes) that actively facilitate peaceful allogrooming bouts.
Reintroduction Biology and Conservation: When rehabilitating trafficked or injured wildlife (such as orphaned gibbons or chimpanzees) for reintroduction into native ecosystems, conservationists use allogrooming network integration as a metric of social readiness. Reintroduction programs evaluate whether an orphan has formed durable grooming coalitions before releasing them as a unified cohort into territorial wild environments.
Human Somatosensory and Clinical Psychology: In human contexts, translational research on allogrooming provides insights into the necessity of non-sexual, somatic touch across human lifespans. Forms of professional touch therapies, including massage therapy, hair care rituals, somatic psychotherapy, and tactile social support, mirror the neurobiological mechanisms of non-human primate allogrooming. Interventions aimed at mitigating loneliness, reducing generalized anxiety, and supporting neurodiverse individuals with sensory processing challenges draw directly on the oxytocinergic and parasympathetic pathways identified in evolutionary allogrooming research.
11. Research & Empirical Evidence
Decades of empirical field studies have corroborated the multifaceted social and physiological value of allogrooming:
In a seminal study published by Joan Silk and colleagues in Science (2003) and expanded in subsequent papers, researchers tracked wild female baboons (Papio hamadryas ursinus) in the Moremi Game Reserve, Botswana, over an eighteen-year period. Silk et al. demonstrated that females who maintained consistent, equitable, long-term allogrooming partnerships had significantly higher infant survival rates and lived longer than isolated females, regardless of ecological variability and social rank. This provided some of the first direct evidence that social bonds forged through allogrooming confer measurable evolutionary fitness advantages.
Addressing the hygienic dimension, studies on wild Japanese macaques (Macaca fuscata) by Michael Huffman and colleagues documented that during peak seasonal infestations of lice (Pediculus), macaques drastically increased their allogrooming time. Furthermore, the anatomical distribution of grooming shifted directly to body regions that individuals could not self-groom, verifying that the hygienic utility remains an active, evolutionarily preserved driver alongside social functions.
In primate conflict management, Filippo Aureli and Frans de Waal established the pervasive occurrence of post-conflict reconciliation. In their extensive research on captive and wild macaques and chimpanzees, the researchers discovered that the probability of post-conflict allogrooming spiked significantly above baseline levels within the first ten minutes after a fight. This reconciliatory allogrooming directly attenuated elevated heart rates and stress hormone levels in both the former aggressor and the victim, preserving group cohesion.
On the neurochemical front, Crockford, Wittig, and colleagues (2013) examined urinary oxytocin patterns in wild chimpanzees following grooming bouts. Their findings revealed that oxytocin concentrations were significantly higher after chimpanzees groomed with bonded, long-term cooperative allies compared to when they groomed with non-allies. This indicates that the neuroendocrine benefits of allogrooming are not mere mechanistic byproducts of touch, but are moderated by the cognitive depth and emotional quality of the social bond.
12. Cultural & Cross-Cultural Considerations
While allogrooming is traditionally examined within animal ethology, comparative anthropological studies highlight its lingering presence and evolutionary substitutes across human societies.
In traditional human societies, physical allogrooming remains an overt and ubiquitous social practice. Ethnographic accounts of the San peoples of Southern Africa, indigenous Amazonian tribes, and diverse Oceanic communities describe communal groups spending hours picking through one another’s hair, braiding strands, removing ticks, applying body paint, and massaging limbs. Far from mere aesthetic preparation, these rituals function as explicit mechanisms for establishing social trust, resolving intra-tribal conflict, and managing stress.
In industrialized Western and urban contexts, direct physical grooming among adult conspecifics is largely privatized, restricted to romantic partners, parents, and dependent offspring. However, modern humans have heavily outsourced physical allogrooming to institutionalized commercial proxies. Barbershops, hair salons, nail parlors, spa services, and clinical massage clinics serve as institutionalized human equivalents of allogrooming hubs. Sociological studies of these spaces regularly emphasize that customers derive profound psychological validation, social support, and somatic stress relief from their interactions with practitioners, far exceeding the superficial aesthetic service rendered.
Furthermore, cross-cultural linguistic anthropology highlights Robin Dunbar’s contention regarding verbal grooming. Across every studied culture, casual small talk, humorous banter, and group storytelling serve the same neurochemical and structural functions as non-human allogrooming. By verbally “grooming” one another, human conversational circles stimulate endorphin and oxytocin release across groups larger than the physical dyads limited by non-human primate fur-picking.
13. Criticisms, Debates & Limitations
Despite its broad acceptance in primatology, the concept and analytical modeling of allogrooming remain sites of scholarly debate:
The Hygiene vs. Social Function Dichotomy: A longstanding methodological debate pits strict functionalist parasitologists against cognitive sociobiologists. Some researchers argue that cognitive primatologists have historically downplayed the fundamental anti-parasitic role of grooming in an effort to over-ascribe complex social calculations to non-human primates. Conversely, other scholars point out that primates in clean, captive laboratory settings—entirely devoid of ectoparasites for generations—continue to groom at frequencies comparable to or exceeding their wild counterparts, confirming that the social and hedonic motivations operate independently of parasite loads.
Cognitive Complexity in Grooming Markets: The Biological Market Theory of grooming has drawn criticism regarding the cognitive demands it imposes on animals. Critics ask whether an animal must track complex mental ledgers of reciprocal obligations over time, noting that this might exceed the working memory and executive planning capacities of prosimians or smaller-brained primates. Alternative explanations suggest that grooming reciprocation can be explained by simpler, emotionally mediated heuristics—such as “emotional bookkeeping” or partner-specific positive affect—without requiring deliberate cognitive scorekeeping.
Observational and Sampling Bias: Measuring allogrooming in wild populations is inherently susceptible to visibility bias. High-ranking, confident individuals often position themselves in prominent, unobstructed terrestrial locations, making their grooming bouts easy to record. In contrast, subordinate, stressed, or peripheral group members may retreat to dense undergrowth or high canopies, where delicate grooming interactions evade observation. This disparity can skew the resulting sociometric networks, artificially exaggerating the social dominance and interconnectedness of central figures.
14. Related Terms & Distinctions
To ensure diagnostic clarity across scientific literature, allogrooming must be differentiated from closely allied behavioral phenomena:
- Autogrooming: The self-directed cleaning, licking, scratching, or picking of one’s own body. While autogrooming shares basic motor patterns and hygienic utility with allogrooming, it is predominantly driven by immediate somatic maintenance or displacement behaviors (stress coping) and lacks dyadic social or transactional dimensions.
- Allopreening: The precise ethological term used for avian social grooming, involving the manipulation, realigning, and cleaning of feathers using the bill. Though functionally analogous to mammalian allogrooming, its physical execution and structural variations are shaped by avian physiology and flight requirements.
- Social Touch: A broad umbrella category encompassing all non-aggressive physical interactions between conspecifics, including hugging, huddling, holding hands, embracing, and leaning. Allogrooming is a specialized, structurally sustained subset of social touch characterized by systematic surface manipulation and focused attention.
- Allomothering (Alloparenting): The provision of parental care (such as carrying, feeding, and defending infants) by individuals other than the biological mother. While allomothering frequently incorporates allogrooming of the dependent infant, it represents a wider reproductive strategy and life-history phenomenon.
- Mutual Grooming: A subset of allogrooming where both participants actively groom each other simultaneously (such as two horses standing head-to-tail nibbling each other’s withers). In contrast, allogrooming more broadly encompasses unidirectional grooming and sequential, turn-taking exchanges.
15. Summary / Key Takeaways
Allogrooming stands as one of the most evolutionarily preserved, socially multifaceted, and neurobiologically complex behaviors in the animal kingdom. While originating as a functional, anti-parasitic hygiene measure, the behavior evolved into a critical social mechanism across group-living vertebrates, reaching its zenith in the complex social networks of non-human primates.
Driven mechanistically by the stimulation of low-threshold tactile fibers, allogrooming down-regulates sympathetic arousal, reduces circulating cortisol, and triggers the central release of oxytocin and beta-endorphins. In turn, these physiological responses facilitate reconciliation, consolidate defensive coalitions, appease dominant competitors, and maintain group cohesion. Interpreted through evolutionary models such as reciprocal altruism, biological market theory, and the social brain hypothesis, allogrooming serves as a bridge linking skin-level physiology to complex group politics and the evolution of human social communication.
References
- Aureli, F., & de Waal, F. B. M. (Eds.). (2000). Natural conflict resolution. University of California Press.
- Crockford, C., Wittig, R. M., Langergraber, K., Ziegler, T. E., Zuberbühler, K., & Deschner, T. (2013). Urinary oxytocin increases following allogrooming in chimpanzees. Proceedings of the Royal Society B: Biological Sciences, 280(1753), 20122765.
- Darwin, C. (1871). The descent of man, and selection in relation to sex. John Murray.
- Dunbar, R. I. M. (1991). Functional significance of social grooming in primates. Folia Primatologica, 57(3), 121–131.
- Dunbar, R. I. M. (1998). The social brain hypothesis. Evolutionary Anthropology: Issues, News, and Reviews, 6(5), 178–190.
- Noë, R., & Hammerstein, P. (1994). Biological markets: Supply and demand determine the effect of partner choice in cooperation, mutualism and mating. Behavioral Ecology and Sociobiology, 35(1), 1–11.
- Seyfarth, R. M., & Cheney, D. L. (1984). Grooming, alliances and reciprocal altruism in vervet monkeys. Nature, 308(5959), 541–543.
- Silk, J. B., Alberts, S. C., & Altmann, J. (2003). Social bonds of female baboons enhance infant survival. Science, 302(5648), 1231–1234.
- Trivers, R. L. (1971). The evolution of reciprocal altruism. The Quarterly Review of Biology, 46(1), 35–57.
- Yerkes, R. M. (1933). Genetic aspects of grooming, a social behavior in chimpanzees. Comparative Psychology Monographs, 10(2), 1–24.