Senescence is the deterioration in physiological function with age, resulting in declines in survival probablity and reproductive performance. It is amongst the most complex and variable of biological processes, and one we are a long way from fully understanding. Evolutionary biologists view this process as the result of a weakening of the strength of natural selection with age. In the natural world, individuals die from a variety of environmental causes that may have nothing to do with senescence, such a predation, disease or accidents. This means selection is stronger on genes expressed in early life than in later life. So genes associated with senescence can accumulate, despite their fitness costs in later adulthood, because selection is simply too weak to remove them or because they have beneficial fitness effects in early life which outweith those costs.
So why do longevity and ageing rates vary so much between species, populations and individuals in nature? Research into ageing in short-lived model laboratory study organisms - principally yeast, nematode worms, fruit flies and mice - point to conserved genetic and physiological pathways regulating senescence. They also suggest that the lifespan-extending effects of these manipulations often come at a cost to growth or reproductive function in earlier life, consistent with evolutionary predictions. The importance of early life is also emphasised by medical and epidemiological resesarch on humans which suggests stress during development and growth can have profound, long-standing effects on subsequent adult health.
For a long time, researchers interested in ageing dismissed the possibility that wild animals experience senescence - assuming that too few individuals would survive to see old age under natural conditions. There's now abundant evidence that this idea is wrong - hundreds of studies of wild birds and mammals demonstrate declines in survival, reproduction or physiological function with age during adulthood. Studying ageing in the wild - although overlooked until fairly recently - can help us understand how natural selection and environment interact to generate individual variation in patterns of senescence. Importantly, the fact that many of these studies are longitudinal - tracking known indivduals from birth to death - means that the links between early-life conditions and adult heath and senescence can also be examined.
We use information and biological samples collected as part of long-term, individual-based studies of wild animals to address questions including:
- How do early-life conditions and life history decisions influence the process of senescence in later life? 
- Do well do cellular biomarkers of ageing (immune markers, oxidative stress, telomere length) predict subsequent survival and reproduction? And how do early-life conditions and genetic factors influence variation in these biomarkers? 
- To what extent is lifespan and senescence rate genetically determined in wild animal populations?
- How and why do the sexes differ in senescence patterns?
- How and why do the fitness costs of senescence vary among individuals, populations and species?
Recent publications:
Moorad, J & Nussey, DH (2016) Evolution of maternal effects senescence. PNAS. 113:362-367. [Download here]
Hayward AD, Moorad J, Regan CE, Berenos C, Pilkington JG, Pemberton JM & Nussey DH (2015). Asynchrony of senescence among phenotypic traits in a wild mammal population. Experimental Gerontology, 71: 56-68. [Download here]