Biphasic mortality response of chipmunks in the wild to single doses of ionizing radiation: toxicity and longevity hormesis.

Based on survivalship data from Tryon and Snyder, wild chipmunks (Tamias striatus), captured, exposed to single doses of either 200 or 400 rad ionizing radiation, and subsequently returned to their natural habitat, exhibited a biphasic response in age-specific mortality rate (omega x). On the one hand, a residuum of unrepaired toxicity (injury) appeared to persist and manifest itself throughout life (enhancement of omega x values). A second response, termed longevity hormesis (of unknown mechanism), was also observed. This phenomenon initially reduced omega x values but was reversible. A relatively simple mathematical model characterizing differences in mortality experience between control and irradiated populations was formulated and tested. Although there were some shortcomings, the model characterized the data reasonably well.