There Is More than One Way to Crack an Oyster: Identifying Variation in Burmese Long-Tailed Macaque (Macaca fascicularis aurea) Stone-Tool Use

We explored variation in patterns of percussive stone-tool use on coastal foods by Burmese long-tailed macaques (Macaca fascicularis aurea) from two islands in Laem Son National Park, Ranong, Thailand. We catalogued variation into three hammering classes and 17 action patterns, after examining 638 tool-use bouts across 90 individuals. Hammering class was based on the stone surface used for striking food, being face, point, and edge hammering. Action patterns were discriminated by tool material, hand use, posture, and striking motion. Hammering class was analyzed for associations with material and behavioural elements of tool use. Action patterns were not, owing to insufficient instances of most patterns. We collected 3077 scan samples from 109 macaques on Piak Nam Yai Island's coasts, to determine the proportion of individuals using each hammering class and action pattern. Point hammering was significantly more associated with sessile foods, smaller tools, faster striking rates, smoother recoil, unimanual use, and more varied striking direction, than were face and edge hammering, while both point and edge hammering were significantly more associated with precision gripping than face hammering. Edge hammering also showed distinct differences depending on whether such hammering was applied to sessile or unattached foods, resembling point hammering for sessile foods and face hammering for unattached foods. Point hammering and sessile edge hammering compared to prior descriptions of axe hammering, while face and unattached edge hammering compared to pound hammering. Analysis of scans showed that 80% of individuals used tools, each employing one to four different action patterns. The most common patterns were unimanual point hammering (58%), symmetrical-bimanual face hammering (47%) and unimanual face hammering (37%). Unimanual edge hammering was relatively frequent (13%), compared to the other thirteen rare action patterns (<5%). We compare our study to other stone-using primates, and discuss implications for further research.