The measurement of time is based on external and internal phenomena. These phenomena have something to do with periods, cycles, and oscillations. External phenomena are the daily periods of sunlight and darkness, the weekly periods, the four-week periods related to the appearance of the moon. Also the year-period and certain multiples of this period have to be added to this list – like the 11-year cycles of sunspots. In biological systems such external cycles have been internalised by the development of biological clocks which synchronise the functions of the system. In addition, biological systems have internal time-related functions which have interesting correlations to the size of the system. Here we will discuss the scaling functions particularly in mammals. One can say: the smaller an animal, the shorter are its internal time periods, including, e.g., heart period and lifetime. Therefore, ageing is a size-dependent process. Several theories try to explain the process of ageing by genetic determinants, by genetic or metabolic defects, by immunological failure, or by other reasons. It is noteworthy that the duration of life is correlated to the size of the body of different animals. This fact indicates a close relation to metabolism. It can be assumed that under optimal conditions of life, defects in function or structure are repaired by special maintenance functions. During the process of ageing these functions lose effectivity. It seems that the organism tries to achieve an optimisation of these maintenance functions. Optimisation is defined as a process by which the organism tries to minimise biological cost functionals. Loss of optimality contributes to instability in the most sensitive periods of life: early development and old age. The concept of optimality and competence includes the need to stabilise vital functions by external aid. In addition to biological aspects of ageing, social and psychological factors have to be considered.

Institut für Physiologie der Karl Franzens Universität Graz Graz

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