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

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 mechanism underlying detection of seed dormancy by scatter-hoarding rodents is unclear,although previous work suggests that the pericarp plays an important role in signaling dormancy status.Eastern gray squirrels(Sciurus carolinensis)consume early germinating seeds as they are more likely to perish immediately,whereas dormant seeds tend to be cached.To examine the mechanisms underlying dormancy detection,we characterized physical and chemical differences between germinating and dormant pericarps of northern red oak(Quercus rubra),American chestnut(Castanea dentata)and the BC3 hybrid of Chinese chestnut and American chestnut(Castanea mollissima×C.dentata)using scanning electron microscopy and gas chromatography-mass spectrometry.We found that,as seeds break dormancy,the wax layer on the pericarp degrades and is accompanied by the escape of lower molecular weight kernel compounds or lipid metabolism byproducts.Our field experiments showed that squirrels were 4-8 times more likely to consume seeds that were altered to remove pericarp wax coating or that were sprayed with seed chemicals.We argue that dormancy detection by scatter-hoarding rodents is a complex process involving physical cues such as loss of pericarp wax and chemical cues such as emission of olfactory cues.