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Walnuts and bur oak acorns were buried in alternate positions in a 10x -10 grid in a riparian forest with 10-m spacing between nuts. Bur oak acorns soaked in walnut extract and walnuts were buried in a second grid of similar spacing. Fox squirrels removed walnuts fastest and at the same rate in both grids, whereas acorns soaked in walnut extract were removed faster than unsoaked acorns but slower than walnuts. In the light of earlier studies, these observations are interpreted as showing that squirrels can smell buried walnuts from a greater distance than buried acorns. Fox squirrels (Sciurus niger) scatterhoard black walnuts (Juglans nigra) and bur oak acorns (Quercus macrocarpa) in the Great Plains riparian forests near Manhattan, KS (Stapanian and Smith, 1978, 1984). They are also the one species that digs up nuts buried by human investigators and also by competing individual squirrels (Monzyk and Smith, 1991). Thus, selective pressures on fox squirrels for cache spacing is exerted by the foraging behavior of squirrels with overlapping home ranges. In experiments using grids in which acorns and walnuts were buried in alternate positions such as the red and black squares in a checker board, Stapanian and Smith (1984) determined that squirrels removed walnuts faster than acorns. Because squirrels removed walnuts faster than acorns from single species grids and ultimately reduced the walnut grids to a lower nut density than acorn grids, Stapanian and Smith assumed that squirrels preferred walnuts to bur oak acorns. Stapanian and Smith (1978) had hypothesized that squirrels would search around the point where they located a nut in order to locate other nuts. Such searches would go on until the length of time between finds would be too long for it to be profitable to continue. This hypothetical view of their food preference and searching behavior was not consistent with squirrels locating more walnuts than acorns in mixed grids; upon locating a walnut they should be more motivated and hunt longer in the vicinity and be more likely to locate one of the four acorns that were the nuts closest to the walnut they first discovered. To resolve this inconsistency, Stapanian and Smith This content downloaded from 157.55.39.59 on Sat, 15 Oct 2016 04:15:17 UTC All use subject to http://about.jstor.org/terms 2 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE (1984) hypothesized that fox squirrels could smell buried walnuts from a greater distance than bur oak acorns and that rather than make searches around specific nuts, squirrels made generalities about which general areas, such as under a productive tree, had more nuts than others. Stapanian and Smith (1978) tested the possibility that squirrels discovered buried nuts by the smell of earth disturbed in the burial process or by the odor of tape or aluminum foil when they buried aluminum foil wrapped with tape near grids of buried nuts. There was no evidence of digging near the buried foil and tape. Here, we report on a test of the hypothesis that buried walnuts can be smelled from a greater distance than buried bur oak acorns. Two mixed square grids of 50 walnuts and 50 acorns each were established in the riparian forests on Konza Prairie Research Natural Area near Manhattan, KS. Each nut was half wrapped in aluminum foil, buried under 1 cm of soil or litter, and its position marked with a piece of strapping tape attached to the vegetation 1-2 m above the points of burial. Walnuts and bur oak acorns were alternated with 10 m between nuts, thus duplicating the conditions in mixed grids of Stapanian and Smith (1984). In the control grid nothing was changed, but in the experimental grid the acorns were soaked before burial in a commercial walnut extract made from the endosperm and embryos of walnuts. The grids were checked with a metal detector on days 2, 3, 5, 8, 9, 12, 14, 18, 22, and 29 after the nuts were buried on October 21. Survival of the nuts is plotted in Figure 1. Standard analyses of variance were conducted by using the SAS general linear computer model in which the day a nut was discovered missing was the dependent variable. The SAS models contained the following independent variables: (1) grid, (2) species of nut, and (3) scented-unscented nut contrast. Walnuts were taken more rapidly than control (P < 0.0001) or soaked (P = 0.029) acorns. There was no significant difference in the speed with which walnuts were removed from the two grids (P = 0.795), but acorns soaked in walnut extract were removed more rapidly than control acorns (P < 0.0001). Replication of the control and experimental grids would have eliminated the possibility of habitat effects on the results. The apparent similarity of habitats within the same forest for the two grids and the similar rate of removal of walnuts from the two grids (Fig. 1) gives us confidence that habitat had no significant influence on the results. The increased rate of removal of acorns soaked in walnut extract relative to unsoaked acorns is consistent with the hypothesis that walnut odor from buried nuts can be detected at a greater distance than acorn odor by squirrels. The difference resulting from soaking acorns also is consistent with the hypothesis that squirrels prefer walnuts to bur oak acorns and mistake the soaked acorns for walnuts. However, the second hypothesis seems more unlikely because squirrels cached equal numbers of walnuts and bur oak acorns when given This content downloaded from 157.55.39.59 on Sat, 15 Oct 2016 04:15:17 UTC All use subject to http://about.jstor.org/terms VOLUME 97, NUMBERS 1-2 3 |
