Optimal distances for squirrels to transport and hoard walnuts

Walnuts, Juglans airanthifolia, are an important food item for Japanese squirrels, Sciurus lis, which hoard them for later use. We set up feeders in small walnut patches in secondary forest, mounted radiotransmitters on nuts, placed them on the feeders and tracked them when they were cached. Of 720 nuts placed on the feeders over a 3-year period, 32.6% were eaten immediately and 50.6% were scatter-hoarded 0-168 m from the feeders. With increasing distance to the cache site, the squirrels took longer to return to the feeders (range 0.35-35 min). However, the further away a cache was, the less likely it was to be stolen by conspecifics or other nut-eating animals: nuts we hoarded ourselves disappeared more quickly from near the source trees. Squirrels took nuts further away later in the season when caches were lost at a higher rate. Using a simple model maximizing the rate of energy gain per unit time taken to search for and hoard the nuts at different distances from the source, we discuss whether squirrels alter the transport distance of hoarding nuts according to cache survival rate and/or nut availability. Copyright 1999 The Association for the Study of Animal Behaviour.