In the target article, the author has provided a valuable framework for understanding the evolution of large brains (Dunbar, Reference Dunbar2024). However, this framework exhibits significant limitations when evaluated in light of comparative evidence. First, the Social Brain Hypothesis (SBH) predominantly relies on a correlation between neocortex size and group size in primates, positing that larger brains evolved to facilitate the management of larger social groups and complex social interactions. Yet comparative studies challenge this relationship. Research on non-primate species – including insects, flocking birds, fish, and cetaceans – demonstrates that taxa with relatively small brains can exhibit sophisticated social behaviors and sustain large group sizes. For example, paper wasps (Polistes fuscatus) display individual facial recognition and maintain complex dominance hierarchies despite their minimal brain size (Tibbetts, Reference Tibbetts2002). Similarly, certain ant species, such as Oecophylla smaragdina, engage in advanced cooperative behaviors, including coordinated nest-building and territorial defense (Patel & Bhatt, Reference Patel and Bhatt2020). Among birds, species like the cliff swallow (Petrochelidon pyrrhonota) form colonies comprising thousands of individuals, exhibiting intricate social dynamics such as communal nesting and collective information sharing (Brown & Brown, Reference Brown and Brown1996). Starlings (Sturnus vulgaris), known for their highly coordinated murmurations and complex communication systems, achieve these feats with brains significantly smaller than those of primates (Ballerini et al., Reference Ballerini, Cabibbo, Candelier, Cavagna, Cisbani, Giardina, Orlandi, Parisi, Procaccini, Viale and Zdravkovic2008; Hausberger et al., Reference Hausberger, Richard-Yris, Henry, Lepage and Schmidt1995). In fish, the cleaner wrasse (Labroides dimidiatus) demonstrates advanced social behaviors, including cooperation and conflict resolution (Pinto et al., Reference Pinto, Oates, Grutter and Bshary2011). Cetaceans, such as baleen whales, further challenge the SBH by displaying social learning and cultural transmission (Garland & Carroll, Reference Garland, Carroll, Clark and Garland2022), despite having small brains relative to their large bodies (Ridgway et al., Reference Ridgway, Carlin, Van Alstyne, Hanson and Tarpley2017). Conversely, some primates with large brains, such as orangutans, exhibit predominantly solitary lifestyles, suggesting that brain size alone does not predict social complexity (Van Schaik & Van Hooff, Reference Van Schaik, Van Hooff, McGrew, Marchant and Nishida1996). Moreover, a large-scale analysis of 140 primate species revealed that brain size was predicted by diet, rather than social factors (DeCasien et al., Reference DeCasien, Williams and Higham2017). Collectively, these findings indicate that the relationship between brain size and social group structure is not as straightforward as the SBH proposes.

Second, the author assumes that advanced cognitive abilities evolved to manage social relationships. However, many species exhibit rich cognitive repertoires that are not directly tied to social complexity or even exist in the absence of complex social structures. For instance, New Caledonian crows (Corvus moneduloides), which typically live in small family groups (Holzhaider et al., Reference Holzhaider, Sibley, Taylor, Singh, Gray and Hunt2011), exhibit advanced tool-making and problem-solving skills, such as crafting hooked tools (Bayern et al., Reference Bayern, Danel, Auersperg, Mioduszewska and Kacelnik2018; Gruber et al., Reference Gruber, Schiestl, Boeckle, Frohnwieser, Miller, Gray, Clayton and Taylor2019). Similarly, Clark’s nutcrackers (Nucifraga columbiana), which live in small groups or pairs, display exceptional spatial memory and the ability to compare and optimize caching strategies – abilities linked to ecological demands rather than social complexity (Clary & Kelly, Reference Clary and Kelly2011). Even cephalopods, particularly octopuses, exhibit advanced learning and memory despite lacking complex social interactions (Amodio et al., Reference Amodio, Boeckle, Schnell, Ostojíc, Fiorito and Clayton2019). These examples demonstrate that cognitive complexity can arise in response to ecological challenges, such as foraging demands or environmental unpredictability, rather than social pressures, underscoring the diverse evolutionary pathways to intelligence.

In this context, I propose that the SBH can be refined by shifting from a predominantly cortical-centered framework to one that integrates the functional contributions of subcortical structures, particularly the hippocampus – a phylogenetically conserved brain structure central to the episodic memory system. The episodic memory system not only underpins both ecological and social adaptive functions, such as recalling past interactions, alliances, and conflicts (Mettke-Hofmann, Reference Mettke-Hofmann2014; Rubin et al., Reference Rubin, Watson, Duff and Cohen2014), but also reflects evolutionary adaptations, as the size and organization of the primate hippocampus evolve in line social pressures (Todorov et al., Reference Todorov, Weisbecker, Gilissen, Zilles and De Sousa2019). The hippocampus also supports what Dunbar (Reference Dunbar2024) called “expensive cognitive skills,” such as one-trial learning (forming durable memories from single experiences; Squire, Reference Squire1992), causal reasoning, analogical reasoning, simulating strategic future outcomes, and mentalizing (inferring others’ intentions). By enabling one-trial learning and bridging past experiences with future goal-directed scenarios, the hippocampus underpins a core process vital for delaying gratification (e.g., “If I eat this now, I’ll regret it later”). During this process, automatic behaviors are overridden, and self-control is exerted (Edelson & Hare, Reference Edelson and Hare2023). Furthermore, it integrates self-relevant and other-relevant information to facilitate mentalizing via self-projection (e.g., “What would I do in their situation?”; Buckner & Carroll, Reference Buckner and Carroll2007). The hippocampus also offers valuable insights into the evolutionary trajectory of human cognition, such as language (Zhang & Shi, Reference Zhang and Shi2021). For example, hippocampal-dependent mental simulation and scene construction support mentalizing, critical for decoding communicative intent and the acquisition of complex syntactic structures (De Villiers & Pyers, Reference De Villiers and Pyers2002; Boeg Thomsen et al., Reference Boeg Thomsen, Theakston, Kandemirci and Brandt2021), while its one-trial learning capacity enables rapid word-referent mapping in vocabulary acquisition (Warren & Duff, Reference Warren and Duff2014). Additionally, hippocampal-prefrontal interactions regulate linguistic self-control, suppressing irrelevant associations during communication and fostering the referential function of human language (Badre & Wagner, Reference Badre and Wagner2007; Eichenbaum, Reference Eichenbaum2017). These functions demonstrate how the hippocampus anchors language in real-world referents (see also Shi, Reference Shi2024 for related discussion). In summary, by broadening the SBH to incorporate the hippocampus and its contributions to language evolution, we can achieve a more comprehensive understanding of the diverse evolutionary pathways that have shaped cognition across species. This refined perspective not only resolves inconsistencies in the current framework but also highlights the interplay between social and ecological pressures in driving brain evolution.