X-Message-Number: 29041
Date: Sun, 28 Jan 2007 10:29:12 -0500
From: Francois <>
Subject: Why do different animals have different rates of aging?

As Basie often reminds us, there seems to be no good reason, on a purely 
biological and genetic level, for different animals to have different life 
expectancies. Yet, when one examines this problem is some detail, such a 
reason does emerge.

In order for a species to succeed, it must at least replenish itself from 
one generation to the next. In order to do that, individuals must invest 
resources in two different domains. First, they must invest resources in 
offsprings. Second, they must invest resources in keeping themselves alive 
long enough to produce enough offsprings to prepetuate the species. Any 
resource invested in one domain cannot be invested in the other.

During their lifetime, animals are subjected to many dangers, such as 
predation, disease, accidents, toxins, etc. All of these tend to lower their 
life expectancy. They must therefore invest resources in defending 
themselves from them. Even assuming that an animal doesn't age, the dangers 
it faces ensure that it will die sooner or later. If it tries to live as 
long as possible in order to reproduce for as long as possible and have as 
many offsprings as possible, it will reach a point where the resources 
diverted from reproduction will lower its overall reproductive success. 
Since different animals have different survival strategies, that point will 
be different for each of them and they will therefore have different life 
expectancies.

For example, take mice and the common brown bat. Both are about the same 
size and have about the same metabolic rate. Mice are the favorite prey of 
many predators, and many of them die very young as a result. It doesn't pay 
for them to invest in keeping themselves young and free of tumors, chances 
are they won't live long enough to reap the benefits. Therefore, their 
strategy is to produce as many offsprings as possible as fast as possible. 
Enough of them will have time to reach sexual maturity and reproduce to keep 
the species going. Bats, on the other hand, can evade predation much more 
easily since they can fly. This allows them to invest more in living longer 
and not have to reproduce at the same frantic pace as mice. As a result, a 
brown bat will easily outlive a mouse by a factor of three or four times.

Based on this hypothesis, it should be possible to create a model that can 
predict a specie's life expectancy based on the dangers it faces and the 
degree of ease with which it can evade them. Has such a model ever been made 
or attempted?

Francois

Good health is merely the slowest
possible rate at which one can die. 

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