Determinants of Seed Removal Distance by Scatter-Hoarding Rodents in Deciduous Forests

Abstract

Scatter-hoarding rodents should space food caches to maximize cache recovery rate (to minimize loss to pilferers) relative to the energetic cost of carrying food items greater distances. Optimization models of cache spacing make two predictions. First, spacing of caches should be greater for food items with greater energy content. Second, the mean distance between caches should increase with food abundance. However, the latter prediction fails to account for the effect of food abundance on the behavior of potential pilferers or on the ability of caching individuals to acquire food by means other than recovering their own caches. When considering these factors, shorter cache distances may be predicted in conditions of higher food abundance. We predicted that seed caching distances would be greater for food items of higher energy content and during lower ambient food abundance and that the effect of seed type on cache distance variation would be lower during higher food abundance. We recorded distances moved for 8636 seeds of five seed types at 15 locations in three forested sites in Pennsylvania, USA, and 29 forest fragments in Indiana, U.S.A., across five different years. Seed production was poor in three years and high in two years. Consistent with previous studies, seeds with greater energy content were moved farther than less profitable food items. Seeds were dispersed less far in seed-rich years than in seed-poor years, contrary to predictions of conventional models. Interactions were important, with seed type effects more evident in seed-poor years. These results suggest that, when food is superabundant, optimal cache distances are more strongly determined by minimizing energy cost of caching than by minimizing pilfering rates and that cache loss rates may be more strongly density-dependent in times of low seed abundance.

Scatter hoarding by the Central American agouti: a test of optimal cache spacing theory

“…This study revealed that cache spacing by scatter-hoarding rodents may vary with food abundance not only between subsequent years of contrasting food production at a single place (Jansen et al 2004;Moore et al 2007) but also between adjacent areas of contrasting food abundance at a single time (Hopewell et al 2008). Agoutis scatter-hoarded seeds of Astrocaryum further away in areas where this palm was less abundant.…”

“…Moore et al (2007) argued that, under OCST, the density dependence of cache pilferage should increase with the risk of pilferage. Thus, lowering cache density would be more effective in reducing pilferage under low food abundance, where adding caches may considerably increase the absolute density of caches and thereby increase the foraging efficiency of pilferers (Kraus 1983), than under high food abundance, where variation in the density of added caches will be nullified against the background of the numerous existing caches, and where adding caches hardly affects the absolute density of caches and the foraging efficiency of pilferers (Moore et al 2007). …”

“…Thus, under OCST, cache spacing and cache robbery should both respond not only to (1) variation in the nutritional value of food items, but also to (2) temporal and (3) spatial variation in food availability. Empirical support for response of cache spacing with temporal variation in food availability comes from studies that have found that the distance at which scatter hoarders cache seeds is greater in seed-poor years than in seed-rich years (Jansen et al 2004 and references therein; Moore et al 2007). Just one study has addressed how cache spacing is affected by spatial variation in food availability (Hopewell et al 2008).…”

How plants manipulate the scatter-hoarding behaviour of seed-dispersing animals

“…This change in caching dynamics during mast years is vitally important to the plant. Reduced secondary caching limits secondary dispersal (shorter primary-dispersal distance is also predicted in mast years because the value of individual nuts is lower; Moore et al 2007;but see Vander Wall 2002;Li & Zhang 2007), but this limitation also benefits plants because if animals handle nuts less often there are fewer opportunities for animals to eat nuts. Less handling means potentially greater survival of the seeds.…”

“…Roberts 1979;Smith & Reichman 1984;Kallander & Smith 1990;Vander Wall 1990;Smulders 1998;Vander Wall & Jenkins 2003;Dally et al 2006). Studies have ranged from descriptions of how animals prepare individual caches (Haftorn 1956;Kallander 1978;Bossema 1979;Kawamichi 1980;Pravosudov 1986), to how animals secure resources and thereby increase their competitive abilities (Stapanian & Smith 1978Clarkson et al 1986;Henry 1986;Daly et al 1992;Waite & Reeve 1995;Moore et al 2007), to how animals use stored resources to promote survival or reproduction (Balgooyen 1976;Kuhn & Vander Wall 2008;Landry-Cuerrier et al 2008), to investigations of spatial memory and cognition (Sherry et al 1981;Vander Wall 1982Kamil & Balda 1985;Jacobs & Liman 1991;Clayton & Krebs 1994;Bednekoff et al 1997;Pravosudov 2003). Most attention has been focused on how behaviour contributes to an adaptive syndrome for surviving periods of food scarcity.…”