Chimpanzees and bonobos maintain tight inner circles of preferred grooming partners inside their wider social networks, mirroring the layered friendship structures that scientists long assumed were unique to humans. A new study spanning 24 groups of both species found that these apes allocate grooming time the way people distribute social effort: concentrating on a few close bonds while keeping lighter ties with a broader circle. The findings challenge the idea that structured friendships required language or culture to evolve.
What is verified so far
Researchers led by a team at Utrecht University examined grooming data from 24 groups of chimpanzees and bonobos, both wild and captive, and found a consistent pattern. Each individual maintained a small set of heavily groomed partners embedded within a larger, less intensively serviced network. This layered allocation of social time closely resembles the concentric friendship circles described in human social psychology, where people invest most in a handful of close friends and progressively less in wider acquaintance rings.
The two Pan species, however, differ in how steeply they ration that grooming. Bonobos spread their grooming more evenly across partners, producing a relatively egalitarian distribution. Chimpanzees, by contrast, concentrated grooming on fewer individuals, creating sharper distinctions between inner-circle allies and peripheral contacts. Age amplified this selectivity in chimpanzees: older individuals appeared to narrow their grooming circles further, according to the iScience analysis. No equivalent age-driven narrowing was reported for bonobos in the same comparison, suggesting that social aging may follow different trajectories in the two species.
These results did not emerge in isolation. Earlier work applying resource-allocation models to chimpanzee grooming had already demonstrated that chimps organize their relationships in layered tiers comparable to human friendship structures. That study showed that when grooming time is treated as a limited resource, chimpanzee networks naturally fall into a few high-intensity ties, a broader ring of moderate partners, and a diffuse outer layer of low-intensity contacts. The new cross-species work extends this logic to bonobos and confirms that both great-ape species partition social effort rather than distributing it randomly.
Separate research on bonobos showed that individual personality traits predict which partners a bonobo prefers, confirming that bonobos form differentiated, non-random social preferences rather than distributing attention uniformly. That personality-driven selectivity, documented in a Royal Society study, adds biological depth to the newer comparison: traits such as sociability or boldness can bias which individuals end up in an ape’s inner circle.
Field observations of polyadic grooming sessions, where three or more individuals groom simultaneously, have also shaped how researchers interpret these networks. In wild communities, multi-partner grooming bouts affect which individuals gain access to new social contacts, broadening or constraining an ape’s network position depending on species-typical social rules. In chimpanzees, triadic grooming can cement alliances among males that later cooperate in conflicts, while in bonobos, more tolerant group grooming appears to ease tensions and facilitate female-centered coalitions.
Multi-group network analyses of zoo-housed bonobos have independently confirmed that age, sex, and rearing history all influence where an individual sits within the grooming web. Older females often occupy central positions, receiving grooming from younger animals, whereas individuals hand-reared by humans may show atypical patterns, either over-grooming a few favored partners or remaining on the periphery of the network. These findings fit the broader picture of structured, history-dependent relationships rather than fluid, interchangeable ties.
What remains uncertain
Several open questions limit how far these findings can be pushed. The primary studies supply grooming matrices and network-level metrics, but raw individual-level behavioral logs or timestamped video records from all 24 groups have not been made publicly available for independent verification. Researchers working outside the original teams would need access to those records to confirm that the layered pattern holds at the level of specific dyads rather than only as a statistical average across groups.
A second gap involves the human comparison itself. The claim that ape grooming layers mirror human friendship circles rests on structural similarity between the two patterns, not on a direct quantitative comparison published in the same paper. No side-by-side table matching ape grooming tiers to measured human friendship data appears in the focal iScience study. The parallel is inferred from the shape of the distribution rather than tested against a shared metric, leaving open the possibility that superficially similar layers arise from different underlying processes.
The role of ecology also remains loosely defined. Chimpanzee grooming selectivity may scale with group size, seasonal food scarcity, or habitat type, any of which could steepen or flatten the inner-circle gradient independently of species identity. The current dataset spans multiple groups but does not isolate these environmental variables in a controlled way. Whether bonobos would show chimp-like selectivity under resource stress, or whether chimps in small, stable captive groups would groom more like bonobos, has not been directly tested.
Age effects reported for chimpanzees lack accompanying statistical outputs in the institutional summary. The direction of the effect, older chimps grooming fewer partners, is stated clearly, but the magnitude and confidence intervals are available only in the primary paper’s supplementary materials, which limits quick independent assessment. Without those effect sizes, it is difficult to know whether aging reshapes the entire network or only trims the least important ties.
Another uncertainty concerns causality. The studies document correlations between grooming intensity, partner choice, and network position, but they cannot yet determine whether investing in a few strong ties causes better outcomes, such as improved support in conflicts or higher reproductive success. Experimental manipulations are ethically constrained in great apes, so researchers must rely on long-term observational data to connect grooming layers to fitness consequences, and those data are still being compiled.
How to read the evidence
The strongest evidence comes from the peer-reviewed comparative analysis in iScience, which used network metrics across both Pan species and multiple group contexts. That study sits at the top of the evidence hierarchy because it tested the layered-friendship hypothesis in a structured, cross-species design rather than reporting anecdotes from a single community. Its convergence with previous chimpanzee-only modeling work increases confidence that the observed layering is not a statistical fluke.
Supporting it are several independent primary studies. The earlier chimpanzee paper in Scientific Reports established the analytical framework by showing that continuum resource-allocation models fit grooming data and naturally produce inner and outer circles. The bonobo personality study in Proceedings of the Royal Society B demonstrated that individual differences drive partner choice, ruling out the possibility that bonobos simply groom whoever happens to be nearby. Field data on polyadic grooming added ecological realism by showing how multi-partner sessions reshape network access in wild settings and in captivity.
Taken together, these lines of evidence justify a cautious but clear conclusion: chimpanzees and bonobos structure their social worlds into layers of strong and weak ties, using grooming as a limited resource to manage those relationships. What remains to be clarified is how closely these layers map onto human friendship circles in function as well as form, and how ecological pressures, aging, and personality shape the precise contours of an ape’s inner circle.
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*This article was researched with the help of AI, with human editors creating the final content.
