Dunbar (Reference Dunbar2025) tells a story where primates with high brain-to-body ratios, like us, broke through the “glass ceilings” that limited our group size. He suggests that group size in primates is best explained by these cognitive limits, more than by other features like ecology. This ceiling-shattering narrative device may appropriately describe our evolutionary history. However, it also sets up a “tower” with humans at the top and all other animals somewhere below based on their more or less human-like socio-cognitive structure.

Applying this anthropocentric structure to other species implies that a more human-like social structure would benefit them. Dunbar (Reference Dunbar2025) does this when he renders social stressors as obstacles limiting stable group size and not adaptations. Dunbar notes, for example, that ungulates and some primates develop infertility in large groups, which they must overcome by having unstable groups. The same phenomenon can be described as an adaptation to prevent ecologically destructive large, stable groups.

This narrative positions cognitive and behavioral strategies as something that must develop to pass the obstacles, as opposed to imagining that obstacles arise to inhibit otherwise capable individuals. To conclude that we evolved large brains and grooming behavior to live in large, stable groups, we must rule out other explanations for the apparent lack of stability in large primate groups. In particular, we must set aside the possibility that large stable groups are sometimes less ecologically sustainable, and therefore less beneficial.

The first of Dunbar’s (Reference Dunbar2025) two ceilings for primate groups limits many, typically leaf-eating, primates to “cliquey” small groups of 10 or fewer, which only temporarily congregate in larger “herd” like groups. He hypothesizes that their foraging does not allow sufficient grooming time to maintain large stable groups; primates whose diets allow more free time would maintain larger groups of 15–20 individuals through reciprocal grooming. Such a hypothesis runs contrary to an ecology-focused hypothesis that grazing or leaf-eating animals might benefit from smaller or more disparate groups, only forming large groups temporarily to prevent lasting ecological harm.

These ecological constraints can be removed by providing an overabundance of food. If the constraints on group size were cognitive, not ecological, we should expect that provisioning animals would not increase group size. And, if the grooming time was limited by foraging, we should expect provisioned animals to increase grooming time. However, provisioning the macaque population in Japan has led to a doubling of group size while maintaining the same social structures, every 7 years from 1952 to 1976, with populations reaching over 1000 members (Yamagiwa & Hill Reference Yamagiwa and Hill1998). In laboratory studies, an overabundance of food (whole vs. chopped) did not mark a significant increase in grooming or self-grooming, with a reduced mean duration of grooming during the testing condition, but did mark increased time huddling in clusters and feeding (Sandri et al., Reference Sandri, Regaiolli, Vespiniani and Spiezio2017). In overabundance conditions in urban-dwelling macaques, there were no increases in grooming activities, but there were increases in behaviors that resembled play and leisure (Jayapali et al., Reference Jayapali, Perera, Cresswell and Dayawansa2023). Overabundance conditions likely change the underlying social dynamics that limit group size. These changes in ecological conditions likely lead to less strain on resource management and, therefore, relaxed social pressure. Their social maintenance strategies are not limited to grooming; they deployed various social maintenance strategies like increased play or huddling together while feeding.

Pluralistic social maintenance strategies other than grooming are widespread among animals, including those who live in “unstable and herd-like” social structures. For example, even vampire bats practice reciprocal food-sharing and live in large colonies (Wilkinson, Reference Wilkinson1984; Carter et al., Reference Carter, Farine, Crisp, Vrtilek, Ripperger and Page2020). There are also very low-effort ways to bond for primates and non-primates, including gentle touch (Monsó & Wrage, Reference Monsó and Wrage2020; Jablonski, Reference Jablonski2021), auditory signals (Teixeira da Cunha and Byrne, 2009; Xie et al., Reference Xie, Brask, Dabelsteen and Briefer2024), or merely being close while foraging or eating together. A pluralistic approach to social maintenance (Westra and Andrews, Reference Westra and Andrews2022) may reveal many behavior patterns that help group cohesion, leaving the grooming time constraint a permeable boundary, not a ceiling.

Dunbar’s (Reference Dunbar2025) ceiling narrative ends with the claim that humans, when living as hunter-gatherers, often form large stable groups of about 40 individuals, which in turn form larger societies. He implies that there is something cognitively manageable and “natural” to this “grade” of group size. However, human hunter-gatherers are famously holders of ecological knowledge; they intentionally set social boundaries that spread out a population to conserve animal and plant populations (Hensen et al., Reference Henson, Balkenhol, Gustas, Adams, Walkus, Housty, Stronen, Moody, Service, Reece, vonHoldt, McKechnie, Koop and Darimont2021; Inksetter, Reference Inksetter2018). Even human group size is interdependent with ecology.

3Early humans, and some of our closest great ape relatives, may have struggled to find the time and develop the skills needed to live in large, stable groups. We may have evolved large brains partly to overcome those obstacles. However, we risk problematic anthropocentrism if we insist that the social structures of other primates, or social animals more broadly, are limited by an incapacity for social maintenance of group cohesion. Instead, we might consider the mechanisms and stressors that limit their group size, like the infertility trap, as adaptations that evolved to prevent them from forming large stable groups with their existing social skills.