Senescence, the increased risk of mortality or decrease in fecundity with age, has historically been described as a strictly genetic process. Empirical evidence suggests the environment also has a strong influence on life history evolution. Density in particular has been shown to alter life history across the tree of life: in amphibians, birds, mammals, plants, and even microbes. Rapid evolution is common and often driven by density dependent processes. Senescence has important implications for conservation and human health, but density’s effects on senescence are largely unknown and typically assumed negligible. Our findings show this assumption is false. We have found evidence of classic Hamiltonian, age-specific senescence that varies according to the density of the microsite within our population. Using 23 years of mark-recapture data, we conclude that senescence is influenced by genetics and the environment and call for future investigation of other environmental factors’ influence on the evolution of senescence.