Understanding the reasons for variation in demographic rates – births, deaths and migration – is fundamental to predicting population size and evolutionary changes in natural populations. One of the best studied factors influencing these rates is age and, in many organisms, distinct phases of life in which demographic parameters differ markedly are readily identified. In birds and mammals, juveniles typically have high mortality and dispersal rates and young adults perform poorly until they are fully grown and have gained sufficient breeding experience. However, a period of ‘prime’ adulthood is often identifiable, when individuals have highest survival and reproductive prospects, and this is frequently followed by declining performance in older age. The red deer on Rum have provided important support for this pattern. In their 1982 book, Red deer: Behaviour and ecology of two sexes, Tim Clutton-Brock, Steve Albon and Fiona Guinness showed that adult female deer in our study area on Rum peak in survival and reproductive rates between the ages of around 6 and 9 years old, showing declines either side of this ‘prime of life’. Individual animals differ markedly both in the way they develop and improve through early life, their performance during prime age, and the onset and pattern of deterioration in later adulthood. It is this later decline that has been the focus of recent research on Rum.


Senescence refers to the physiological deterioration that we all experience as we grow old. The process is ubiquitous, fabulously complex and astonishingly variable among individuals. But what drives this variation? How much of this it is genetically based? How important are early-life environmental conditions? And, how well supported are evolutionary theories that could explain this variation? These are questions that had rarely been tackled outside of a handful of very short-lived laboratory-bred organisms. Although senescence has been widely documented in wild birds and mammals, it had rarely been the subject of in depth study. In 2005, Tim Clutton-Brock and Loeske Kruuk were awarded a NERC grant to address these questions using the uniquely detailed information available from the long-term study on Rum. Shortly before this, Ted Catchpole and colleagues had undertaken meticulous study of the factors influencing survival in red deer on Rum. Their results, shown in the figure below, beautifully illustrate declining survival probability from around nine years of age, and clearly show that this decline is more rapid in males (blue) than females (red).



A few years later, Dan Nussey and colleagues explored the patterns of ageing across traits associated with reproduction in both male and female red deer, documenting further differences. As the figure below shows, annual fecundity (the number of offspring an individual can expect to produce on average at each age) shows a much later decline in females (dashed red line) than in males (solid blue line). However, the pattern of decline depends very much on the trait in question – some female traits, such as the weight of their offspring at birth (solid red line) decline steadily from 9 years onwards and some male traits, notably antler size (dashed blue line) do not decline at all.


The remarkably detailed records collected over four decades in the Kilmory study area, charting the fate of thousands of individual all the way from birth to death on Rum, allowed researchers to delve deeper into the factors influencing individual differences in senescence. Work led by Alastair Wilson in 2007 used the deer population’s pedigree to show that variation in ageing rates among female deer on Rum had a genetic basis. This was, and remains, the best evidence of its kind from any wild mammal population. In the same year, analyses led by Dan Nussey showed that females experiencing high population density – and presumably high competition for food – in early life showed more rapid declines in fecundity and survival probability in later adulthood than females experiencing low densities. Linking early-life conditions and senescence has proved challenging in long-lived animals, but among hinds on Rum it seems environmental conditions experienced during the first year of life can influence the way they grow old many years later. Evolutionary theory predicts links between early-life and senescence, specifically that increased investment in growth and reproduction should come at the costs of earlier or more rapid senescence. In support of this hypothesis we found that female deer that bred more often in early adulthood (before 9 years) showed more rapid declines in reproductive performance after that point.

Many outstanding questions and challenges remain regarding senescence in wild animals, that the Rum red deer study and others like it will be able to address in coming years. These include, explaining why male and female mammals differ in their pattern of senescence, and why reproductive traits seem to differ too? Determining whether and how age-related declines in survival and reproduction are underpinned by changes in foraging behaviour or dominance, and how they are linked to physiological processes such as tooth wear and cellular damage? Linking the processes of early life improvement and later life deterioration, and understanding the consequences of variation in ageing for fitness and population growth, are important if we are to understand why individuals vary so much in the way they age. We probably know more about senescence in the red deer on Rum than in any other wild mammal population on earth – but we’re still a very long from understanding the causes and consequences of individual variation in the ageing process.