The basic tenets of the evolutionary theories of senescence are well supported. However, there has been little progress in determining the relative influences of mutation accumulation and life history optimisation. The causes of the well-established inverse relationship between lifespan and body size across dog breeds are used here to test these two classes of theories. The lifespan-body size relationship is confirmed for the first time after controlling for breed phylogeny. The lifespan-body size relationship cannot be explained by evolutionary responses to differences in extrinsic mortality, either of contemporary breeds or of breeds at their establishment. The development of breeds larger and smaller than ancestral grey wolves has occurred through changes in early growth rate. This may explain the increase in the minimum age-dependent mortality rate with breed body size and thus higher age-dependent mortality throughout adult life. The main cause of this mortality is cancer. These patterns are consistent with the optimisation of life history as described by the disposable soma theory of the evolution of ageing. The dog breed lifespan-body size relationship may be the result of the evolution of greater defence against cancer lagging behind the rapid increase in body size during recent breed establishment.

Data are from published sources.

Data were analysed using R.