The pace and shape of ageing

1. Humans age, but how much more or less do we age compared with other species? Do humans age more than chimps, birds more than fish or sheep more than buffalos? In this article, I argue that current methods to compare patterns of ageing across species are limited because they confound two dimensions of age-specific change - the pace and the shape of ageing. 2. Based on the two axes of pace and shape, I introduce a new conceptual framework to classify how species age. 3. With this method, I rank species according to how strongly they age (shape) and how fast they age (pace). Depending on whether they are ranked by pace or by shape, species are ordered differently. 4. Alternative pace measures turn out to be highly correlated. Alternative shape measures are also highly correlated. The correlation between pace and shape ranking is negative but weak. Among the examples here, no species is long lived yet exhibits negligible ageing - contrary to the commonly held view that long-lived species are good candidates for negligible ageing. 5. Analysis of species in pace-shape space provides a tool to identify key determinants of the evolution of ageing for species across the tree of life.