Dunbar (Reference Dunbar2025) offers novel analyses of the mechanisms underlying the social brain hypothesis, presenting new evidence that brain size variation in primates is driven by group size and the associated cognitive demands. The analyses oppose environmental explanations that propose complex foraging strategies – such as tool use, hunting, or food caching – require enhanced problem-solving abilities, lead to larger brains (DeCasien, Williams, & Higham, Reference DeCasien, Williams and Higham2017; González-Forero & Gardner, Reference González-Forero and Gardner2018; Schaik et al., Reference Schaik, Van, Schuppli, Drobniak, Heldstab and Griesser2023). While we do not dispute Dunbar’s findings, our aim is to place them within a broader context of vertebrate brain size and cognitive evolution. The social brain hypothesis has also been successfully applied to other clades, despite their smaller brain-to-body size compared to primates. For instance, the hypothesis can apply to fish, not only across species but also within the same species, thanks to the exceptional neural plasticity observed in fish (Bijl et al., Reference Van der Bijl, Buechel, Kotrschal and Kolm2018; Emery, Pessina, & Bshary, Reference Emery, Pessina and Bshary2025; Alejandro, Svante, & Niclas, Reference Alejandro, Svante and Niclas2009; Pollen et al., Reference Pollen, Dobberfuhl, Scace, Igulu, Renn, Shumway and Hofmann2007; Triki et al., Reference Triki, Zhou, Argyriou, Sousa de Novais, Servant and Kolm2024). Thus, the social brain hypothesis primarily explains some variation within taxa rather than the broader differences in brain size across taxa. Despite the sophistication of Dunbar’s analyses, they account for only a small fraction of the variation observed in brain size, leaving a significant portion of this variation unexplained. This remaining gap in understanding warrants further attention and investigation.

Fish cognition challenges existing hypotheses on brain and cognitive evolution (Triki, Van Schaik, & Bshary, Reference Triki, Van Schaik and Bshary2025). Despite having brains roughly ten times smaller than those of similarly sized endotherms (Jerison, Reference Jerison1973), many fish exhibit remarkable cognitive abilities, suggesting they possess cognitive toolkits comparable to those of average- and even large-brained endotherm species (Brown, Reference Brown2015, Reference Brown2012; Bshary, Wickler, & Fricke, Reference Bshary, Wickler and Fricke2002; Bshary and Brown, Reference Bshary and Brown2014; Kohda et al., Reference Kohda, Bshary, Kubo, Awata, Sowersby, Kawasaka, Kobayashi and Sogawa2023, Reference Kohda, Hotta, Takeyama, Awata, Tanaka, Asai and Jordan2019). Research on fish cognition has revealed that they excel in various tasks once thought to be exclusive to endotherms, particularly primates. Fish have demonstrated abilities such as social learning (Brown, Reference Brown, Tehrani, Kendal and Kendal2023), cognitive flexibility (Warren, Reference Warren1960), inhibitory control (Lucon-Xiccato, Gatto, & Bisazza, Reference Lucon-Xiccato, Gatto and Bisazza2017), numerical reasoning (Agrillo, Piffer, & Bisazza, Reference Agrillo, Piffer and Bisazza2010; Triki and Bshary, Reference Triki and Bshary2018), spatial memory (Aronson, Reference Aronson1951), transitive inference (Hotta et al., Reference Hotta, Ueno, Hataji, Kuroshima, Fujita and Kohda2020), object permanence (Triki et al., Reference Triki, Fong, Amcoff, Vàsquez-Nilsson and Kolm2023), generalization (Wismer, Grutter, & Bshary, Reference Wismer, Grutter and Bshary2016), cooperative hunting (Bshary et al., Reference Bshary, Hohner, Ait-el-Djoudi and Fricke2006) and even self-recognition in the mirror test (Kohda et al., Reference Kohda, Hotta, Takeyama, Awata, Tanaka, Asai and Jordan2019, Reference Kohda, Bshary, Kubo, Awata, Sowersby, Kawasaka, Kobayashi and Sogawa2023). Importantly for one of Dunbar’s arguments, fish show clear evidence of one-trial learning (Blank et al., Reference Blank, Guerim, Cordeiro and Vianna2009; Triki and Bshary, Reference Triki and Bshary2019). These findings challenge the notion that certain cognitive processes, including those for social behavior, require large brains. Instead, they suggest that many complex cognitive abilities can be supported by relatively small brains. This raises a key question: what similarities and differences between ectotherms and endotherms could account for this tenfold disparity in brain size?

Current brain evolution hypotheses do not account for the evolution of sensory and motor systems or their integration (Barrett, Henzi, & Barton, Reference Barrett, Henzi and Barton2021; Triki, Van Schaik, & Bshary, Reference Triki, Van Schaik and Bshary2025). A thorough examination of current knowledge and evidence suggests that, on a broader scale, the most significant differences likely lie in the sensory and motor systems and their integration (Triki, Van Schaik, & Bshary, Reference Triki, Van Schaik and Bshary2025), particularly in relation to the transition from water to land (MacIver & Finlay, Reference MacIver and Finlay2021). In fact, the sensory-motor skills of fish are less sophisticated than those of mammals and birds, where terrestrial and aerial life generally demand more advanced sensory-motor integration compared to aquatic life (MacIver & Finlay, Reference MacIver and Finlay2021). Importantly, some species stand as exceptions to this trend, such as elephant fish. These weakly electric fishes rely on complex electromagnetic sensing to navigate and forage in murky waters – a perceptual system that is highly demanding in terms of neural resources. This characteristic likely explains why their brain-body allometry falls within the range recorded for mammals (Nilsson, Reference Nilsson1996).

The emphasis on sensory-motor systems in vertebrate brain size evolution raises two questions. The first relates to how we define animal cognition. While the classic definition by Shettleworth (Reference Shettleworth2010) primarily focused on cognitive processes and perception, we have recently expanded the definition to provide a broader perspective and emphasize the role of sensory-motor systems: “Animal cognition is the array of skills animals utilize to gather information via their sensory systems, process that information through cognitive mechanisms, store it, make decisions, and execute goal-relevant behaviors through motor skills facilitated by sensory-motor integration” (Triki, Van Schaik, & Bshary, Reference Triki, Van Schaik and Bshary2025). The second question is whether the sensory-motor systems primarily evolved for social or environmental demands – and how we define “social” interactions. For instance, a gazelle’s ability to run swiftly over uneven terrain likely evolved as an adaptation to predation risk, yet predator-prey interactions can still be considered social (Oliveira & Bshary, Reference Oliveira and Bshary2021). Regarding more sophisticated sensory-motor systems, dexterity or anatomical features associated with dexterity – such as beak size in birds – accounts for a significant proportion of the variance in the data (Heldstab et al., Reference Heldstab, Kosonen, Koski, Burkart, van Schaik and Isler2016; Song et al., Reference Song, Drobniak, Liu, van Schaik and Griesser2024). In favor of selection on environmental intelligence, many intelligent species (e.g., primates, corvids, and dolphins) integrate visual, tactile, and motor systems to manipulate objects. Tool use, in particular, relies on the interaction between sensory feedback and motor planning (Merel, Botvinick, _& Wayne, Reference Merel, Botvinick and Wayne2019). On the other hand, many vertebrates engage in multimodal communication (e.g., visual, auditory, and chemical signaling), which requires the integration of multiple sensory inputs to generate appropriate motor responses. For example, in songbirds, the ability to mimic and learn songs depends on auditory feedback and precise motor control of vocal muscles (Adam et al., Reference Adam, Maxwell, Rößler, Hansen, Vellema, Brewer and Elemans2021). In relation to the target article, groomer-groomee interactions in primates likely rely on key sensory-motor system involvement and the integration of sensory feedback, highlighting the role of these systems in social behaviors.

In conclusion, brain evolution hypotheses, including the social brain hypothesis, should expand their scope to incorporate the role of sensory-motor systems in decision-making in order to capture the full range of vertebrate brain evolution.