Calorie restriction brings no benefits to lifespan under stochastic environments

Abstract

The impacts of resource availability on senescence –the loss of vitality with age– are formalised under the Calorie Restriction (CR) theory, which predicts that the onset of senescence is delayed and life expectancy prolonged due to the ultimate effects of restricted resource intake without malnutrition. However, CR studies are largely implemented in unrealistic environments that do not consider how interacting, stochastic drivers impact longevity. Indeed, little is known about the impact of stochastic resource availability on senescence, even though environmental stochasticity is the norm rather than an exception in natural populations. Here, we examine whether and how stochasticity in the quantity, quality, and frequency of resources impact lifespan, life history trait trade-offs, and population structure in two long-lived planaria: Schmidtea mediterranea and Dugesia tahitiensis. For each species, we estimate weekly population size, survival, and a size distribution metric that quantifies population structure and skew. Over the 19-week study, survival in S. mediterranea is lower than D. tahitiensis across all feeding regimes. However, for both species, CR does not diminish survival. There are also no clear shifts in population structure over time across the different feeding regimes. For S. mediterranea, in most treatments, population structure changed to fewer smaller than larger individuals (right-skewed). In the case of D. tahitiensis, only treatments where resources are provided frequently cause right-skewed population structures. Population size also varied between species, with that of D. tahitiensis never declining across treatments, and always becoming larger than S. mediterranea; in the case of S. mediterranea, most treatments show a decline in population counts over the study period. As before, no clear pattern emerges in the changes in population counts under CR conditions for both species. As such, we did not find evidence of CR providing benefits in terms of lifespan nor trade-off between population counts, survival, and body size. We call for the careful reevaluation of decades of CR work in short-lived species, by expanding and testing predictions in more realistic settings and across a wider range of life histories.