Comparative studies of foraging ecology: How the cheek pouch evolved in New World heteromyids

Background/Question/Methods

Direct comparisons of species adaptations can provide insight into their function.  Moonlight and predator avoidance are known to occur among nocturnal, granivorous rodents. Further, in systems where predators evolve improved prey sensing abilities, such as heat pits in New World vipers, one can expect that their prey might evolve counter-adaptations for predator avoidance. External cheek pouches should allow heteromyids to make fewer trips between food sources and the burrows where they store seeds, decreasing predator encounter rate. We examined the evolution of the heteromyid external cheek pouch by comparing rodent species from two continents. We measured giving up densities (GUDs), number of trips to a food patch, and foraging time in artificial food patches simultaneously for heteromyids (Merriam’s kangaroo rats, Dipodomys merriami, and desert pocket mice, Chaetodipus penicillatus) from the Mojave Desert and gerbils (greater Egyptian gerbils, Gerbillus pyramidumI, and Allenby’s gerbils, Gerbillus andersoni allenbyi) from the Negev Desert. Cheek pouches were predicted to confer improved heteromyid foraging efficiency by reducing the number of trips between food patches and burrows. Additionally, kangaroo rats were expected to forage at greater intensity in the presence of owls and snakes and to be less inhibited by moonlight than the other species. 

Results/Conclusions

The kangaroo rats foraged more intensively than the similarly-sized Egyptian gerbils, particularly in risky microhabitats, at the full moon, and in the presence of predators. Overall GUDs from desert pocket mice and Allenby’s gerbils did not differ from each other and fell in-between those of kangaroo rats and Egyptian gerbils.  Additionally, Egyptian gerbils and kangaroo rats foraged less intensively in the presence of predators than when there were none, but the GUDs indicate that these two species are less sensitive to the presence of predators than are the smaller desert pocket mice and Allenby’s gerbils.  Harvest rates and patch return frequency will be discussed. Body size and the external cheek pouch likely allow kangaroo rats to manage risk more effectively than smaller and non-heteromyid rodents.