Theory of Marginal Value

Information makes the world more predictable. Therefore, gaining information about the environment can reduce uncertainty in the decision making of foragers. However, acquiring information about the environment may be costly if it causes an animal to divert attention away from other fitness related activities. One such cost is that of increasing apprehension (multitasking). Apprehension occurs when a forger diverts its attention away from gaining information about resource patches to assessing predation risk. Here I present a study that quantified the cost of apprehension on the foraging ability of hispid cotton rats. I quantified the ability of cotton rats to forage efficiently and assess resource heterogeneity (patchiness) over a distance gradient that represents increasing predation risk. I provide evidence that apprehension does affect cotton rats’ ability to forage efficiently and in their assessment of patchiness. Although the increase in apprehension did reduce foraging efficiency in cotton rats, the benefit of being aware of predator presence and the potential risk of predation presumably outweighed that cost.
Eavesdropping (the use of information contained within a signal by an unintended receiver) for information contained within the communication and signals of other individuals (including heterospecifics) can often be beneficial. One source of information that may be acquired is through eavesdropping on signals (alarm calls) of conspecifics and heterospecifics. Alarm calls provide information about the presence of a potential predator. Thus, eavesdropping can potentially increase survival if an informed individual better manages the tradeoff between risks and safety. I present a study that quantified the ability of hispid cotton rats (eavesdropper) to change their perceived risk of predation when presented with information contained within heterospecific vocalizations, including the alarm and contact calls of blue jays, red-tailed hawk calls, American robin songs, and a silent control. I provide evidence that cotton rats (1) eavesdrop on the alarm vocalizations of blue jays and (2) increase their perceived risk of predation during jay alarms but not jay contact calls. There was no evidence that the direct cue of predator presence (hawk call) influences cotton rats’ perceived risk of predation. Eavesdropping behavior and the change in the perceived risk of predation potentially provides a benefit to the cotton rats as they balance the costs of predation risks and the benefits of foraging.

Acquiring information regarding resource patchiness can reduce uncertainty in foraging decisions. However, information on
patch quality may come at a cost if, when foraging within a risky (micro)environment, foragers allocate more attention to
predator detection and consequently allocate less attention to assessing resource patchiness, that is, the cost of multitasking.
Foragers may also benefit from alternative means of assessing predation risk that may not entirely conflict with foraging (e.g.,
vigilance postures are often incompatible with foraging and thus patch assessment). In particular, prey may eavesdrop on the
alarm calls of heterospecifics that provide public information about the presence of a potential predator. To investigate these 2
effects, multitasking and eavesdropping, we used giving-up densities (GUDs) to quantify the ability of cotton rats (Sigmodon
hispidus) to assess resource heterogeneity (patchiness) under different regimes of predation risk. GUDs were collected over
a distance from burrow gradient representing increasing predation risk and provide evidence that risk affected cotton rats’ patch
assessment abilities in 2 ways. First, under greater risk, cotton rats reduced their foraging time (i.e., higher GUDs) and hence
time available for patch assessment. Second, when the effects of foraging time are removed, cotton rats still showed poorer patch
assessment (i.e., ability to equalized GUDs between resource patches) in riskier habitats, that is, the cost of multitasking. In the
second experiment, we demonstrate that cotton rats eavesdrop on the alarm vocalizations of blue jays and increase their
perceived risk of predation during jay alarms, while simultaneously compromising their patch assessment performance. Key
words: apprehension, Bayesian forging, eavesdropping, giving-up densities, information, multitasking, patch assessment.