Ageing


    Ageing occurs in almost all animals, and can be said to be absent in only a few simple forms. Why does it occur? We now know the answer to that question, and it is summarised here. Its existence is strongly related to sexual reproduction. Animals depend on energy from food, and it can be said that this energy is used in three ways. First, there is regular metabolism: respiration and the conversion of food to energy, the synthesis of all the components of living cells, movement, excretion, in fact, everything that is necessary for the normal function of a living animal. Second, there is sex and reproduction, which in mammals includes gestation, lactation, care of offspring, and development to the adult. Third, the adult has to survive if it is to reproduce. So what does survival depend on? There are specific mechanisms that maintain adults in a normal state during which they can reproduce. Only a few can be mentioned here: there is ongoing repair of defects in DNA, there is an immune system, there is turnover of protein molecules, there are defences against harmful oxygen free radicals, there are hormones and regulatory mechanisms, and many others. All these can be collectively referred to as maintenance mechanisms. They depend on a continual investment of energy.
     We also have to understand that in their natural environment animals are exposed to a number of hazards, which include starvation, drought, predators and disease. This means that animals very rarely survive to their maximum lifespan. That is only seen under protected conditions, such as a zoo, under domestication, and during the last few thousand years of human existence. Now consider the survival of individual species in their natural environments. It is well known that small ground living mammals such as rats and mice, live in a very hazardous environments, with a high rate of mortality. The best strategy for survival is to reproduce quickly and efficiently. Thus, energy is invested in reproduction, which means there is less available for maintenance. As a result the adult body does not last very long: the lifespan of a rat or mouse is only about three years. Other animals have a very different strategy for survival.     Larger animals produce fewer offspring, invest more resources in maintenance and have a much longer lifespan. The longest lived mammalian species are elephants, whales, the higher apes and humans. These have the slowest rate of development and the lowest reproductive rate. There is a strong inverse relationship across mammalian species between reproductive potential and maximum lifespan. Also, many experimental studies have now been done which demonstrate beyond any doubt that the efficiency of maintenance mechanisms is directly correlated to maximum lifespan.

References:

Holliday, R. Understanding Ageing. Cambridge University Press (1995).

Holliday, R. Aging: the Paradox of Life. Springer, Dordrecht, The Netherlands (2007).