Foraging efficiency in the face of predation risk: A comparative study of desert rodents

Question: What is the adaptive significance of the heteromyid cheek pouch? Organisms: Two heteromyid rodents (Merriam's kangaroo rat, Dipodomys merriami, and desert pocket mouse, Chaetodipus penicillatus) from the Mojave Desert, and two gerbils (greater Egyptian gerbil, Gerbillus pyramidum, and Allenby's gerbil, Gerbillus andersoni allenbyi) from the Negev Desert, Israel. Site: An outdoor vivarium on the Sede Boqer campus of Ben-Gurion University of the Negev, Israel. Methods: We measured foraging time in seed trays for heteromyids and gerbils. We also measured the number of trips to food patches, and giving-up densities (GUDs, the amount of seed left behind when an individual left a seed tray). Predictions: We expected cheek pouches to confer improved heteromyid foraging efficiency by reducing the number of trips between food patches and caching sites. We further expected that, compared with the other species, kangaroo rats would be less inhibited by barn owls, by moonlight, and by risky microhabitats. Results: The two heteromyid species harvested more food per trip than the two gerbil species. Kangaroo rats had lower GUDs than any other species, particularly in risky microhabitats and at the full moon. Harvest rate curves for greater Egyptian gerbils and kangaroo rats indicated that these two larger bodied species were more vigilant than the two smaller bodied species. Conclusion: Adaptations such as body size and the external cheek pouch appear to allow kangaroo rats to manage risk and harvest food more effectively than smaller and nonheteromyid rodents.