UC San Diego

In many organisms sexual maturity occurs at an age approximately equal to one third of their maximal lifespan. This relationship has been shown to be at least partially genetically determined, as fruit flies specifically selected for delayed sexual maturity also tend to have an increase in lifespan (Luckinbill & Clare, 1985). However, it was previously unknown whether delaying sexual maturity via environmental manipulations would increase longevity compared to genetically similar animals undergoing puberty at an earlier age. An ideal species with which to test this hypothesis is the Siberian hamster (Phodopus sungorus), whose breeding season occurs during spring and summer: Pups born early in breeding season begin puberty at 25 days of age, whereas pups born late in the season instead transition to a winter phenotype and delay puberty until the following spring. Prior work with this species suggests physiological ramifications of the winter state (i.e., reduced caloric intake, daily torpor, enhanced immune function, sexual quiescence, increased melatonin biosynthesis) might potentially promote a "slow- aging" effect that could extend lifespan. The present study is the first to directly test these hypotheses by comparing lifespan among hamsters with drastically different life histories through exposure to one of six different photoperiod histories, accompanied by longitudinal collection of body weight, pelage, 24 h body temperature, and wheel running behavior, concluded with post-mortem organ weight analysis. Results reveal the juvenile life stage was increased from 3.2% to 20.2% of total lifespan duration, without consequence to longevity or rates of aging. In addition, induction into one or multiple winter states did not increase lifespan or slow aging. The degree of kidney hypertrophy was most predicted by the timing of puberty onset in male hamsters, suggesting the onset of reproductive hormones as a risk factor. A multivariate cox regression survival analysis accounted for 29% of lifespan variability among all hamsters, identifying body weight parameters to be the single most influential variable in predicting lifespan. While this study firmly demonstrates that the manipulation of photoperiod life history does not directly influence longevity, an explanation involving the potential degree of chronodisruption between photoperiod groups might account for observed differences in lifespan