The developmental process of the nematode Caenorhabditis elegans is famously invariant; however, these animals have surprisingly variable lifespans, even in extremely homogenous environments. Inter-individual differences in muscle-function decline, accumulation of lipofuscin in the gut, internal growth of food bacteria, and ability to mobilize heat-shock responses all appear to be predictive of a nematode's remaining lifespan; whether these are causal, or mere correlates of individual decline and death, has yet to be determined. Moreover, few "upstream" causes of inter-individual variability have been identified. It may be the case that variability in lifespan is entirely due to stochastic damage accumulation; alternately, perhaps such variability has a developmental origin and/or genes involved in developmental canalization also act to buffer phenotypic heterogeneity later in life. We review these two hypotheses with an eye toward whether they can be experimentally differentiated.
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