Rub-thin-fracture-extract: first records of hermit crab predation and non-tool shell processing by coastal long-tailed Macaques at Wat Takiap, Thailand

Coastal populations of long-tailed macaques (Macaca fascicularis) display remarkable behavioral flexibility in exploiting intertidal food resources. Tool-using groups in Thailand are known to crack marine shellfish with stones, yet the behavioral repertoire of non-tool-using populations remains poorly documented. Here, we present the first report of long-tailed macaques preying on aquatic hermit crabs without using tools, based on observations at Wat Takiap, Hua Hin District, Prachuap Khiri Khan Province, Thailand. During low tide, macaques were observed employing a sequence of substrate-assisted manipulations: repeatedly rubbing gastropod shells against rocks or concrete to thin and weaken the shell wall, followed by fracturing it with their teeth or by striking it against hard surfaces, and finally extracting and consuming the hermit crab's appendages. This behavior represents a novel, non-tool extractive technique for accessing encased prey-a strategy intermediate between manual shell handling and percussive tool use. Such manipulation indicates refined motor control and problem-solving in a foraging context where encased prey are abundant but challenging to access. Given the absence of long-term data, these observations provide a valuable record of a potential pre-emergent stage in the evolution of tool use. Continued longitudinal and comparative studies will be essential to determine whether such substrate-assisted behaviors may serve as precursors to habitual stone-tool use in coastal macaques.