Fever and sickness behaviour vary among congeneric rodents

Fever and sickness behaviour are immune defences that most organisms engage to control bacterial and viral infections. Although generally beneficial, these defences can be energetically expensive, which may lead to variation within and among species. Here, we asked whether fever and sickness behaviour differ among five species of mice in the genus, Peromyscus. This comparison was motivated by our previous discovery of extensive, but systematic, immunological variation among many of these same rodent species. Some species were adept at controlling gram-negative bacteria whereas others were proficient at generating antibodies; no species was strongly capable of both. Such discrete variation suggested a continuum of immune defence strategies. We therefore predicted that variation in fever and sickness behaviour would mirror variation in bacterial killing capacity, as these defences are mediated by some of the same molecular pathways. To test this hypothesis, we characterized responses to lipopolysaccharide (LPS), a component of gram-negative bacteria that activates febrile responses without causing infection. Peromyscus species that showed little sickness behaviour post-LPS engaged fever; species that engaged sickness behaviour, however, either did not mount fevers or became hypothermic post-LPS. As predicted, species that were adept at killing bacteria in vitro mounted the largest fevers; species that were not as proficient at killing bacteria did not engage fever. These results further indicate a continuum of immunological strategies among Peromyscus species, which we expect applies to other taxa. We propose a few possible reasons for why species occupy specific points along an immune continuum; life-history orientation appears the most viable alternative at present.