Senescence, or biological ageing, refers to the general deterioration of an organism’s physiological functions, leading to increased susceptibility to diseases and ultimately death. It is a complex process that involves many genes. Lifespans vary greatly across different animal species. Thus, for example, flies live for four weeks, horses for thirty years, whereas some hedgehogs may live for up to two centuries. Why is the range of lifespans in nature so broad? This is one of the basic and most intriguing questions faced by biologists.