Resolution of the Paradox 117
of the body, which in turn depends on the trade-off between repro-
duction and maintenance.
If success is equated with the probability of an individualís survival
in a natural environment, then it follows that those species which
have the lowest annual mortality rates will survive longest. So
we see the largest herbivores, higher primates and largest marine
mammals sometimes surviving in the wild for long periods. In these
circumstances, their ageing may influence reproductive success as for
example, when a dominant male Walrus or stag eventually loses his
harem of females to a younger and stronger male. In such situations,
it can be shown that there will be selection for increased longevity.
The selection may be on males, but the females have almost the same
complement of genes. In hominids, selection for longevity operated
primarily on later reproduction in females.
Whatever the longevity of a given species, very similar processes
lead to senescence, but they operate at different rates. One of the main
causes of ageing is the inability of the organism to replace cells in vital
organs, such as the heart or brain. Individual cells die either through
the accumulation of genetic damage in their genes and chromosomes,
or through the inability to get rid of defective proteins, or by breaking
down such proteins to harmful smaller fragments. The relative impor-
tance of genetic or protein changes is a matter for future research. What
we can say now that the ability of each cell to maintain its viability has
defined limits. Human neurons or heart cells are able to survive very
much longer than those of a mouse or rat. From many published exper-
imental investigations we now know that the efficiency of important
maintenance mechanisms is related to the animals maximum lifespan.
We need to know a great deal more about the reasons for the eventual
failureofthemaintenanceofnon-replaceablepartsofthebody.Thiswill
help us understand the reasons for the outset of a whole range of age-
associated diseases. These diseases consume a disproportionate amount
of medical care in developed countries. It is a far better strategy to delay
or prevent onset of these diseases, than to merely treat each one as it
arises.Suchtreatmentcanbeextremelyexpensive,involvingavarietyof
life-supportsystems,anditiscounterproductive,ifsuccessfultreatment
is soon followed by the emergence of a different medical problem in the
verysameindividual.
It is usually taken for granted that the replacement of defective parts
of the body is beneficial. False teeth have been available for decades,
as has skin grafting and plastic surgery. More recently, there have
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