MCLISTER, J.D.; SORENSEN-FORBY, J.S. ; DEARING, M.D.: Do woodrats use dietary toxins to reduce thermoregulatory costs?

Several woodrat species feed more extensively on toxic plants during the winter, thus exposing themselves to high toxic stress when thermal stress is also high and when energy availability is low. Because studies show that non-lethal exposure to toxins can reduce body temperatures and metabolic rates in laboratory rats and mice, we hypothesize that ingestion of toxic plants by woodrats may facilitate energy conservation during the winter by reducing the energetic cost of thermoregulation. To test this hypothesis, 8 woodrats (Neotoma albigula) were acclimated to warm and cold temperatures for several weeks during which they were fed either a non-toxic control diet or a toxic mixture of 50% control diet and 50% juniper (Juniperus monosperma). The energetic cost of thermoregulation for each wood rate was estimated from oxygen consumption rates across a range of ambient temperatures under each acclimation and diet treatment. Cold acclimation caused a significant increase in thermoregulatory cost over warm acclimation, presumably because of brown fat metabolism. Diet alone had no significant effect, but the diet by acclimation interaction effect was highly significant. When warm-acclimated woodrats ate juniper, oxygen consumption across a broad range of temperatures increased 20 to 25% relative to control, but, when cold-acclimated woodrats ate juniper, oxygen consumption decreased 20 to 25% relative to control. Juniper-related shifts in thermoregulation were associated with changes in the minimum conductance and the lower critical temperature. These data suggest that there is an energy benefit to eating juniper during the winter, when N. albigula is cold acclimated, but an energy disadvantage during the summer.