Endotoxin elicits a febrile response in laboratory-maintained but not free-living California ground squirrels

Abstract

California ground squirrels (Spermophilus beecheyi) in the natural environment maintain a higher mean body temperature (T(b)) compared with animals maintained in the laboratory. Our goal was to determine whether that increase was the result of an elevation in T(b) set point analogous to a fever induced by acute stress. Body temperatures were measured by telemetry in a paired experiment on 10 animals maintained in the laboratory, with each animal receiving an injection of sterile pyrogen-free saline and 50 μg/kg endotoxin of E. coli. Body temperatures also were measured by telemetry on 8 pairs of free-living animals, with 1 member receiving an injection of sterile saline and the other animal receiving an injection of 50 μg/kg endotoxin of E. coli. Animals maintained in the laboratory exhibited a fever following injection of endotoxin, with significant elevation in T(b) from 6 to 9 h after injection. Free-living animals that were injected with endotoxin did not exhibit an increase in T(b) compared with saline-injected controls over the same time. Absence of a febrile response in free-living California ground squirrels supported the hypothesis that this large squirrel had a pre-existing elevation in thermoregulatory set point.