In humans we know that physiological aging starts at different times in different individuals, and that it can occur at different rates even within the same person, depending on the tissue or physiological system examined ( Sehl and Yates, 2001 Belsky et al., 2015). Research to date has shown age-specific changes in the immunity of wild animals ( Cichon et al., 2003) and in certain other components of their physiology such as haematocrit (blood oxygen stores), hormones and metabolism ( Massot et al., 2011 Elliott et al., 2014). In contrast, functional senescence, at least in wild animals, is less well studied. ![]() Most work focusses on demographic senescence and we now know that there is a great diversity in trajectories of demographic senescence, not only between different species but also between individuals of a given species ( Jones et al., 2008, 2014). ![]() Aging in animals can be considered in terms of the decline in the body's performance (functional senescence) or the decline in fertility rate coupled with an increased risk of mortality (demographic senescence). In many species, individuals continue living well beyond their functional peaks ( Froy et al., 2018), and often carry on reproducing albeit at a declining rate ( Nussey et al., 2013 Jones et al., 2014). More recently, however, focussed-studies on multiple taxa have shown otherwise ( Nussey et al., 2013 Jones et al., 2014). For many years, wild animals were thought not to grow old, dying before senescence took hold ( Medawar, 1952). Once mature, long-lived individuals of all species experience aging-a progressive decline in the functional capacity of their bodies, a loss of integration of homeostatic systems and an increased vulnerability to death ( López-Otín et al., 2013 Li et al., 2015). “ Comparative biology teaches us that reproduction is life's solution to the inevitability of death in the hostile environments of Earth” ( Carnes, 2007). To understand the role of activity in functional senescence, studies must measure the detailed activity of free-ranging animals, possibly describing it in terms of intensity, frequency and duration, coupled with records of resultant physiological and DNA damage. These observations raise a question that has not yet been addressed: while in humans, at least, activity is considered beneficial to health, could high intensity activity play a part in the rate that wild animals age? Studies to date suggest that increased “effort” can lead to reduced survival in free-living animals, but “effort” refers to different processes in different studies, and is rarely clarified or quantified. Physical activity typically increases levels of oxidative stress, and many animals exhibit long and intense periods of active behavior. Oxidative damage is probably an important component of the aging process in many species. However, in animals, aging in terms of survival (demographic senescence) is understood in considerably more detail than in terms of declines in the body's functional capacity (functional senescence). There is considerable interest in why the process of aging varies between individuals, both in humans and other animals. ![]()
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