Foraging in groups affects giving-up densities: solo foragers quit sooner

Abstract

The giving-up density framework is an elegant and widely adopted mathematical approach to measuring animals' foraging decisions at non-replenishing artificial resource patches. Under this framework, an animal should "give up" when the benefits of foraging are outweighed by the costs (e.g., predation risk, energetic, and/or missed opportunity costs). However, animals of many species may forage in groups, and group size is expected to alter perceived predation risk and hence influence quitting decisions. Yet, most giving-up density studies assume either that individuals forage alone or that giving-up densities are not affected by group foraging. For animals that forage both alone and in groups, differences in giving-up densities due to group foraging rather than experimental variables may substantially alter interpretation. However, no research to date has directly investigated how group foraging affects the giving-up density. We used remote-sensing cameras to identify instances of group foraging in two species of Rattus across three giving-up density experiments to determine whether group foraging influences giving-up densities. Both Rattus species have been observed to vary between foraging alone and in groups. In all three experiments, solo foragers left higher giving-up densities on average than did group foragers. This result has important implications for studies using giving-up densities to investigate perceived risk, the energetic costs of searching, handling time, digestion, and missed opportunity costs, particularly if groups of animals are more likely to experience certain experimental treatments. It is critically important that future giving-up density studies consider the effects of group foraging.