The effects of different light regimes on the fitness of organisms have typically been studied using mean or median adult life span as the sole index of physiological well-being. It is, however, known that life span is inversely related to reproductive output in many species. Moreover, the effects of a given environmental treatment on life span can be due to effects on either age-independent mortality or the "rate of aging," or a combination of both. Drawing evolutionary inferences from the effects of light regime on mean or median adult life span alone is difficult and, at best, speculative. We examined the effects of constant light (LL), alternating light-dark cycles (LD 12:12 h), and constant darkness (DD) on the life span of reproducing and virgin flies in four populations of Drosophila melanogaster and also estimated lifetime fecundity in the three light regimes. The light regime effects on life span were further dissected by examining the age-independent mortality and the Gompertz rate of aging under the three light regimes. While mean adult life span of reproducing males and females and virgin females was significantly shorter in LL compared to LD 12:12 h and DD, life-time egg production was highest in LL. Life span of virgin males was not significantly affected by light regime. The rate of aging in reproducing females was higher in LL as compared to DD, whereas age-independent mortality was higher in DD. As reproductive output, especially early in life, is a far more significant contributor to fitness than is life span, our results suggest that the earlier reported deleterious effects of LL on fitness are partly an artifact of examining life span alone, without considering other components of adult fitness that trade off with life span. Our results suggest that detailed investigation of the effects of light regime on the physiological and behavioral processes that accompany reproduction is necessary to fully understand the effects of different light regimes on adult fitness in Drosophila.
