The Circle of Friendship Among Monkeys: The Social Bonds of Chimpanzees and Bonobos - The Secret of the "Circle of Friends" Shared with Humans

"Circles of Friends" Are Not Exclusive to Humans

According to an article published on Phys.org on October 31, 2025, it has been shown that chimpanzees and bonobos form "concentric" friendships similar to humans. The research team analyzed the grooming behavior of both species from the perspective of how much social time (resources) is allocated to whom. The results revealed layers of a few individuals with dense connections and many others with looser ties.Phys.org

Research Design: 24 Groups × Mathematical Model = "Physics of Society"

This study was led by an international team centered at Utrecht University and Carlos III University of Madrid. Grooming data from 24 groups of chimpanzees and bonobos were collected and tested using a mathematical model describing how individuals allocate limited social resources. This approach, akin to the "physics of society," found a layered structure in both species consistent with human network research.Universiteit Utrecht

Bonobos "Broadly," Chimpanzees "Deeply"

The differences between the two species were clear. Bonobos tend to distribute grooming time relatively evenly among many individuals, while chimpanzees invest intensively in a few partners. Additionally, the larger the group size, the more selective they become, echoing the phenomenon known in human social network research where "the larger the group, the denser the inner circle."Phys.org

"Selective" Aging

It is known that human friendships narrow with age, and a similar trend was observed in chimpanzees. In contrast, the narrowing of inner circles with age was not clear in bonobos. The research team suggests that the more egalitarian and fluid social structure of bonobos may be a factor.Phys.org

Connections with Previous Research: Evolution of Memory and Cooperation

From the perspective of long-term memory, it has been experimentally shown that bonobos and chimpanzees can recognize "companions they haven't seen for years," supporting the existence of enduring social relationships. The current "layered structure" can be seen as the shape of networks built upon such long-term memory. Moreover, reports that bonobos can maintain cooperative relationships across group boundaries align well with their "broad and thin" connection strategy.pnas.orgPhys.org

Update on Bonobo "Equality"

Bonobos are often described as "peaceful," but recent reevaluations of their aggression are underway. However, long-term field studies have shown strong female solidarity and the ability to oppose males, which can influence the composition of networks and "who they strongly connect with." The result of "equality in time allocation" aligns with these social dynamics and matches the behavioral economics supporting "broad solidarity."AP News

Implications for Human Society and Social Media

Human social media is designed to coexist with inner circles (close ties) and outer circles (loose ties). This study shows that such layered structures are not unique cultural products of humans but can naturally arise from biological constraints and cognitive rules related to the allocation of limited attention and time. Therefore,

• Deep Investment to Protect Inner Circles (Chimpanzee Type) and

• Weak Connections that Bind Broadly (Bonobo Type)
  can be applied to community management, team formation, and online campaign design.Phys.org

Animal Welfare and Exhibit Design

In captive groups, the "use" of limited time is directly linked to well-being. Practical suggestions include ensuring individuals can interact sufficiently with preferred partners while adjusting group size and exhibit structures to prevent excessive selective pressure due to overcrowding. The research team itself emphasizes the potential contributions to understanding cooperation, social learning, and emotions.Universiteit Utrecht

Limitations and Future Directions

This study focused on grooming as a "social currency." Future research should integrate multiple social behaviors such as foraging, play, alliances, and post-conflict reconciliation to examine the functional significance of thelayered structure (e.g., impact on resource acquisition, stress indicators, learning speed). Additionally, exploring how individuality (combinations of personalities) and memory duration affect the thickness of layers is valuable in connection with related research.PMC

SNS Reactions (Excerpts/Summary)

• Academic Community's Announcement: Utrecht University Faculty of Science officially shared a press article, emphasizing the "deep continuity of social organization shared by humans and other primates."LinkedIn

• Expert Blog Commentary (Primatology.net): "They layer their social world, paying the most attention to intimate bonds, with weak ties spreading concentrically," succinctly capturing the key points. *Quote within 25 words.primatology.net

• Response to Dissemination: The relevant page on Phys.org shows78 shares, indicating a certain level of interest among general readers (as of November 1, 2025 JST).Phys.org

• Secondary Reporting by Overseas Media: Also introduced in the English version of Russia's Izvestia, gaining international attention.iz.ru

Quotes are limited to short excerpts from the original text, and summaries are reconstructed by the author.

Why They Seem "Human-like": Personal View

"Who to spend time with" is governed by trade-offs among constraints such as lifespan, attention, travel distance, conflict risk, and learning opportunities. Bonobos' "broad distribution" is compatible withcross-border cooperationandfemale solidarity, while chimpanzees' "focused investment" supportsalliance strengtheningandlong-term reciprocity. Both can be seen as "optimizations" adapted to their ecological and social environments. This study teaches us that such optimizations can be observed as aformof layered structure.science.org

Main Sources

• Phys.org "Research reveals chimpanzees and bonobos have 'circles of friends,' just like humans" (October 31, 2025)Phys.org

• Utrecht University Press Release (October 24, 2025)Universiteit Utrecht

• PNAS and Other Related Long-term Memory and Cooperation Studies (Reference)pnas.org

• Commentary by Primatology.net (October 31, 2025)primatology.net

• Introduction by Izvestia English Version (Published November 1, 2025)iz.ru
  
  

Note: Original Paper

iScience "The physics of sociality: Investigating patterns of social resource distribution among the Pan species" (DOI: 10.1016/j.isci.2025.113507). Accessible from the press release.Universiteit Utrecht