Scatterhoarding rodents favor higher predation risks for cache sites: The potential for predators to influence the seed dispersal process

Abstract

Scatterhoarding rodents often place caches in the open where pilferage rates are reduced, suggesting that they tradeoff higher risks of predation for more secure cache sites. We tested this hypothesis in two study systems by measuring predation risks inferred from measures of giving-up densities (GUDs) at known cache sites and other sites for comparison. Rodent GUDs were measured with small trays containing 3 L of fine sand mixed with sunflower seeds. In the first experiment, we relied on a 2-year seed dispersal study in a natural forest to identify caches of eastern gray squirrels (Sciurus carolinensis) and then measured GUDs at: (i) these caches; (ii) comparable points along logs and rocks where rodent activity was assumed highest; and (iii) a set of random points. We found that GUDs and, presumably, predation risks, were higher at both cache and random points than those with cover. At the second site, we measured GUDs of eastern gray squirrels in an open park system and found that GUDs were consistently lowest at the base of the tree compared to more open sites, where previous studies show caching by squirrels to be highest and pilferage rates by naïve competitors to be lowest. These results confirm that predation risks can influence scatterhoarding decisions but that they are also highly context dependent, and that the landscape of fear, now so well documented in the literature, could potentially shape the temporal and spatial patterns of seedling establishment and forest regeneration in systems where scatterhoarding is common.

Seed fate and decision‐making processes in scatter‐hoarding rodents

“…They also selectively cache in riskier, more illuminated locations after some of their caches have been pilfered (Preston & Jacobs, ). Eastern grey squirrels cache more valuable seeds outside tree canopies, where predation risks are likely to be higher, but pilferage rates are lower (Steele et al ., , ). They cache less‐valuable seeds under the canopy.…”

“…Several studies have noted an inverse relationship between the distance that rodents disperse seeds and per capita seed availability (Soné et al ., ; Jansen et al ., ; Moore et al ., ; Gálvez et al ., ; Liu et al ., ; Xiao et al ., , but see Vander Wall, ). Similarly, hoarders in more competitive settings also accept greater risks to place caches in habitats where they are less likely to be pilfered (Muñoz & Bonal, ; Steele et al ., ). Reciprocal pilferage likely plays a role in these patterns (Vander Wall & Jenkins, ; Jansen et al ., ), but individuals also invest more time and energy to disperse caches when resources are scarce (Hopewell, Leaver & Lea, ).…”

“…Hoarders may respond to variation in anti‐predator cover by preferentially moving seeds into deeper shelter (e.g. Giannoni et al ., ; Iida, ; Abe et al ., ; Zhang et al ., ) or by deliberately placing caches in risky locations (Preston & Jacobs, ; Muñoz & Bonal, ; Steele et al ., , ). The latter behaviour indicates that, at least in some circumstances, rodents will trade increased exposure to predation risk for lower pilferage risks.…”

Does scatter‐hoarding of seeds benefit cache owners or pilferers?

“…It is not fully understood why rodents adopt a scatter-hoarding strategy because establishing, protecting and retrieving numerous scattered caches may require considerable energy costs as well as increase predation risks to the hoarders (Jenkins & Peters 1992;Steele et al 2015). Several hypotheses have been proposed to explain the rationale of scatter-hoarding behavior.…”

The use of spatial memory for cache management by a scatter-hoarding rodent

“…Because these caches function as a primary food supply during periods of food shortage, scatter-hoarding animals are likely well adapted to recovering many of their caches as needed (Vander Wall 1990;Balda and Kamil 1992). Communicated by N. Clayton Despite their ability to recover cached food, scatter-hoarders are faced with the constant threat of cache pilferage by conspecifics and other competitors, which under some circumstances are projected to cause devastating losses of food stores (Vander Wall and Jenkins, 2003;Jansen et al 2012;Steele et al 2015). Indeed, food loss to competitors is considered a primary reason for the evolution of food hoarding and, in particular, scatterhoarding behavior.…”