Meeting Abstract
When animals experience immune challenges, they often suppress a range of behaviors. This suppression of activities is termed sickness behavior and appears to be a ubiquitous response in controlled settings. However, when more than one physiological challenge is presented to an individual simultaneously, such as occurs in more natural settings, expressing sickness behavior may not always be advantageous. One hypothesis, the energy limitation hypothesis (ELH), predicts that animals simultaneously faced with an immune challenge and food restriction will suppress their immune response only once they reach a minimum threshold body mass. Experimental tests of the hypothesis are needed to support its utility. We challenged female zebra finches with immune activation (lipopolysaccharide (LPS) injection) alone or in combination with acute food restriction (FR) and compared behavioral and physiological measures of the immune response among these groups (VEH-only, LPS-only, LPS+FR and VEH+FR). Regardless of food availability, LPS injection elicited a similar immune response as measured via a bacterial killing assay (BKA). However, an increase in time spent resting (a typical sickness behavior) was only observed in LPS-only birds, whereas LPS+FR individuals were behaviorally indistinguishable from VEH+FR birds. Strikingly, LPS+FR females did not lose more mass that LPS-only birds, demonstrating that body mass was not the proximate signal causing the switch in behavior expression. Thus, our data do not provide clear support for universal utility of the ELH. Our data highlight the need for additional dimensions, including social and real-world environments, to be applied in the conceptual frameworks that predict and explain animal responses to environmental challenges.
