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A general measure of the rate of senescence is the acceleration of mortality rate, represented here by the time required for the mortality rate to double (MRD). Rhesus monkeys have an MRD close to that of humans, about 8 years; their shorter life-span results mainly from higher mortality at all ages. In contrast, some groups with short life-spans (rodents and galliform birds) have shorter MRDs and faster senescence. On the basis of the Gompertz mortality rate model, one may estimate the MRD from the maximum life-span (t_max) and the overall population mortality rate. Such calculations show that certain birds have MRDs that are as long as that of humans. These results show that high overall mortality rates or small body sizes do not preclude slow rates of senescence.