Population differences and learning effects in walnut feeding technique by the Japanese squirrel

Abstract

Feeding efficiency is an important factor explaining diet selection in seed-eating animals. Behavioral adaptation to process and consume the seed is advantageous to improve their feeding rate. The Japanese walnut (Juglans ailanthifolia), which has a nutritious kernel surrounded by a hard shell, is an important food for the Japanese squirrel (Sciurus lis) in lowland mixed-species forests in Japan. In the present study, population differences in feeding technique were examined. In all squirrels captured from the Takao population (with wild Japanese walnuts in their habitat), the optimal feeding technique for quickly opening the walnut shell was observed. However, only 8% of the squirrels from the Fuji population (without walnuts in their habitat) processed the walnuts by the optimal feeding technique. By a repetitive provision of walnuts for 50 days, 14% of squirrels from the Fuji population acquired the optimal feeding technique. Learning effects did not differ significantly between solitary (without a demonstrator) and social conditions (with a demonstrator). The effects of age on learning the feeding technique were examined using individuals from a zoo population that had been naive for Japanese walnuts. All five individuals aged 6 months old, and half the individuals aged 1–2 years old, ate walnuts by the optimal feeding technique after a 2-month learning period, but none of the individuals older than 3 years did. Consequently, the walnut feeding techniques of the Japanese squirrel are not entirely genetically fixed behavior and can be improved by learning at a young age.