Submitted to: Physiological Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/2/2008
Publication Date: 8/1/2008
Citation: Hahn, D.A., Martin, A.R., Porter, S.D. 2008. Body size, but not cooling rate affects supercooling points in the red imported fire ant Solenopsis invicta. Physiological Ecology. 37: 1074-1080. Interpretive Summary: Cold tolerance is an important factor limiting the northward expansion of the red imported fire ant, Solenopsis invicta. The rate of cooling is a critical factor that often affects insect cold tolerance. Models of cold tolerance usually also require consideration of body size. Scientists at the University of Florida and the USDA-ARS, Center for Medical, Agricultural and Veterinary Entomology in Gainesville, Florida tested the hypothesis that body size and cooling rate would interact to affect the point at which fire ants freeze and die. To do this they exposed different-sized workers to three different cooling regimes, a very fast cooling rate of 10oC/min, a slower rate of 1oC/min, and a slow ecologically-relevant rate of 0.1oC/min. As expected, body size had a considerable effect on the freezing point, with the largest workers freezing at temperatures approximately three degrees higher than the smallest workers. Cooling rate, however, had only a small effect on the freezing point and there was no interaction between the two factors. This information is important because it will allow scientists to better predict how climatic conditions will affect the survival of imported fire ants and where they will survive along the northern edge of their range.
Technical Abstract: The level of an animal’s stress resistance is set by multiple intrinsic physiological and extrinsic environmental parameters. Body size is a critical intrinsic parameter that affects numerous fitness-related organismal traits including fecundity, survival, mating success, and stress resistance. The rate of cooling is a critical extrinsic environmental factor that can affect thermal stress resistance. Workers of the red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), display considerable variation in adult body size. Therefore, developing ecologically-realistic models of thermotolerance in this species requires a consideration of body size. We tested the hypothesis that body size and cooling rate would interact to set the supercooling point in fire ant workers by exposing workers of a range of body sizes to three different cooling regimes, a very fast ramp of -10oC/min, a slower ramp of -1oC/min, and an ecologically-relevant very slow ramp of -0.1oC/min. Specifically, we asked whether large workers were more susceptible to differences in cooling rate than smaller workers. We found that body size had a considerable effect on supercooling point with the largest workers freezing at a temperature approximately three degrees higher than the smallest workers. Cooling rate had a very small effect on supercooling point and there was no interaction between the two factors. Therefore, the allometry of supercooling points across the range of worker body sizes does not change with cooling rate.