Eastern gray squirrels are consistent shoppers of seed traits: insights from discrete choice experiments

Abstract

Seeds of many hardwood trees are dispersed by scatter-hoarding rodents, and this process is often mediated by the traits of seeds. Although numerous studies have linked seed traits to seed preference by rodents, little is known about how rodents forage for seeds when multiple desirable and undesirable seed traits are available simultaneously. Here, we adopt a novel method of designing choice experiments to study how eastern gray squirrels (Sciurus carolinensis) select for 6 traits (caloric value, protein content, tannin concentration, kernel mass, dormancy period and toughness of shell) among seeds. From n = 426 seed-pair presentations, we found that squirrels preferentially consumed seeds with short dormancy or tougher shells, and preferentially cached seeds with larger kernel mass, tougher shells and higher tannin concentrations. By incorporating random effects, we found that squirrels exhibited consistent preferences for seed traits, which is likely due to the fitness consequences associated with maintaining cached resources. Furthermore, we found that squirrels were willing to trade between multiple traits when caching seeds, which likely results in more seed species being cached in the fall. Ultimately, our approach allowed us to compute the relative values of different seed traits to squirrels, despite covariance among studied traits across seed species. In addition, by investigating how squirrels trade among different seed traits, important insights can be gleaned into behavioral mechanisms underlying seed caching (and, thus, seed survival) dynamics as well as evolutionary strategies adopted by plants to attract seed dispersers. We describe how discrete choice experiments can be used to study resource selection in other ecological systems.

Mechanisms underlying detection of seed dormancy by a scatter‐hoarding rodent

“…The joint effect of the 2 cues was stronger than either cue alone and resulted in an increased likelihood of seed consumption. Although we did not examine here how individual-level attributes (including sex, reproductive and nutritional status) influence foraging decisions, previous work found no statistically significant variation in foraging decisions among individual eastern gray squirrels (specifically in the fall season when this study was conducted; Sundaram et al 2018). Furthermore, the large effect sizes of different cues on squirrel behavior recorded here, tested across different seed types and consistent with results of other studies (Steele et al 2001b), suggest that our conclusions are robust to study design and methodology.…”

Seed value influences cache pilfering rates by desert rodents

“…Naturally, animals prefer relatively large, nutritious seeds with short handling times (Hadj-Chikh et al 1996;Smallwood et al 2001). When handling costs are added, such as a hard seed coat or secondary compounds that need to be metabolized, animals are attracted to the food but often defer consumption by storing the food for later use (Jacobs 1992;Hadj-Chikh et al 1996;Steele et al 2001;Sundaram et al 2018). Due to the flexibility of hoarding behavior in many species, animals can evaluate existing conditions as well as anticipate future conditions and adjust their valuation of seeds accordingly .…”

Plant‐animal interactions: patterns and mechanisms in terrestrial ecosystems

“…The authors argue that the acorn serves as much as a fruit for dispersal and tolerance of partial seed predation as it does a source of energy for the developing seedling. Working in a similar system, Sundaram et al (2018) introduced a novel approach for evaluating seed choices by a scatter-hoarding rodent (Eastern gray squirrel; Sciurus carolinensis) when selecting seeds and nuts for both feeding and caching. Using a multivariate quantitative method, the investigators successfully quantified the simultaneous effects of numerous seed traits (e.g., caloric value, protein content, tannin and dormancy period) on behavioral decisions.…”