Discrimination behavior mediates foraging quality versus quantity trade-offs: Nut choice in wild rodents

© 2016 The Author(s). Discrimination, the ability to distinguish sensory stimuli and respond accordingly, is a critical factor underscoring optimal foraging decisions. Nevertheless, little is known about how mammals discriminate between apparently similar foods of different quality. Here, we compared the foraging behavior of Chinese white-bellied rats, Niviventer confucianus, and Edwards's long-tailed giant rats, Leopoldamys edwardsi, under natural conditions in the field and in a captive enclosure without predation/competition. We examined the behavioral processes involved in discriminating between sound (i.e., undamaged) and insect larvae-infested nuts of seguin chestnuts (Castanea seguinii) and demonstrated that both rats could discriminate nut quality, where nut examination improved the rats' success rate at selecting sound nuts. Despite similar extents of discrimination-derived benefit in both settings for each species, differences between species-specific discrimination processes were identified. Chinese white-bellied rats engaged in a higher relative frequency and longer duration of nut examination in the enclosure than in the wild. This indicates that they alter their feeding strategy to trade-off selection for nut quality in captivity for a quantity-driven strategy in the field. In contrast, giant rats showed a consistent relative frequency of nut examination in both experimental settings. Their fixed strategy balanced food quality and quantity primarily to maximize caloric uptake without compromise when faced with predation or competition risk. We posit that this behavioral difference in optimal foraging between rat species is mediated by their differing, size-dependent energetic requirements as well as the higher competition pressure and predation risk faced by the approximately 8 times smaller whitebellied rats.