Original Article

Ecological Research

, Volume 23, Issue 3, pp 607-614

First online:

Geographic variation in walnut seed size correlates with hoarding behaviour of two rodent species

  • N. TamuraAffiliated withTama Forest Science Garden, Forestry and Forest Products Research Institute Email author 
  • , F. HayashiAffiliated withDepartment of Biology, Tokyo Metropolitan University

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Seeds of the Japanese walnut, Juglans ailanthifolia, are usually scatter-hoarded by two rodent species, the Japanese squirrel Sciurus lis and the field mouse Apodemus speciosus, but only by the latter in several areas where S. lis is absent. We examined seed-size-mediated interactions of these three species across a wide geographic range. Field tracking of walnuts with miniature radio-transmitters revealed that squirrels hoarded larger seeds more frequently and at greater distances than smaller seeds. In contrast, mice hoarded smaller seeds more frequently and transported them farther than larger seeds. These seed dispersers could affect the evolution of seed size because seeds hoarded at sites farther from source trees are known to survive better until germination and as seedlings. We expect that larger seeds may be advantageous in regeneration if the main seed dispersers are squirrels, whereas smaller seeds may be advantageous if mice are the dominant dispersers. These predictions were supported by the fact that seed size was smaller on islands inhabited only by mice and at the edge of the squirrel distribution, compared to areas where mice and squirrels are both common.


Apodemus Conflicting selection Geographic mosaic Sciurus Seed dispersal