Cooperation in adult female western lowland gorillas: complex problem-solving through learned spatial affordances and...

Cooperative Problem-solving among non-human primates provides important insights into the evolution of social behavior and interaction. Although such behaviors have been documented in several great ape species, spontaneous, role-specialized cooperation between adult gorillas remains rarely reported. Here, we describe and analyze a recurring dyadic interaction between two captive adult female western lowland gorillas (Gorilla gorilla gorilla), Amelia (dominant) and Lisha (subordinate), who are half-sisters. The analysis is based on a primary event recorded via security cameras in 2024 and supplemented by opportunistic observations from 2021 to 2025. In the primary event, the two individuals combined an environmental feature with complementary body postures to overcome a spatial barrier: Lisha positioned herself quadrupedally atop a rock platform, enabling Amelia to climb onto her back and access a high-hanging Yellow Poinciana (Peltophorum dubium) branch (approximately 4.5 m above ground). Critically, both individuals shared the proximate goal of accessing and dislodging the branch, a goal that remained consistent across all documented events. This role differentiation was consistent across all documented observations. The 2024 event was followed by non-contested co-feeding and a branch-dragging display by Lisha, whereas the 2025 observation ended with exclusive consumption by Amelia, demonstrating variation in outcomes independent of the shared proximate goal. Although the interaction displays features consistent with elements of the social tool use (STU) framework, particularly voluntary participation and self-directed positioning, the shared proximate goal across all events and Lisha's proactive initiation establish this as cooperative behavior rather than STU, regardless of whether communicative signals could be verified. Across observations, Lisha's participation appeared voluntary, and no physical coercion by Amelia was observed. The precise mechanisms of communication remain unclear due to the absence of audio recordings. However, this does not undermine the classification as cooperation, which is fundamentally defined by shared proximate goals and functional interdependence, not by the mechanics of communicative exchange. This report contributes to the study of gorilla social behavior, highlighting the capacity for flexible, context-dependent cooperation in adult gorillas, providing important observations that may serve as a reference for future research on cooperative problem-solving among gorillas in captivity.