Re: Why do different animal have different length of life span?

   Natalie, this is a very good question and one that has been discussed by 
gerontologists (people who study aging) for the past 100 years.  While 
there are some general rules that can be applied to maximum lifespan, they 
are not concrete and there is much variance from theoretically predicted 
lifespan to observed lifespan among species.  We usually think of aging and 
senescence when we think of maximum lifespan.  But the concept of aging and 
what constitutes aging has been hotly debated over the years.  There is no 
agreement as to why we age in the first place.  You have to remember that 
most individual organisms never reach a point in the wild when they start 
to show signs of senescence (I will define senescence as the decline of 
physiological function over time).  However, almost all organisms show 
signs of aging, with the possible exception of some single-celled 
organisms.  
   One of the first global generalities we can assign to maximum lifespan 
is the correlation between fecundity and longevity.  Basically, the more 
offspring an individual of a species is capable of producing, the shorter 
the maximum lifespan of that species.  Evolutionarily speaking, two basic 
reproductive methods have evolved to increase the likelihood that an 
individual will contribute their genes to the next generation.  The first 
is called prodigal (or r-selected, where r is the mathematical symbol for 
the rate of population increase) and is associated with high fecundity and 
high mortality.  The second is called prudent (or K-selected, where K is 
the mathematical symbol for the carrying capacity of the environment) and 
is associated with small numbers of offspring coupled with lower mortality. 
 These differences are obvious when one compares flies to humans.  The 
common housefly only lives for a few days but on average lays 120 eggs.  In 
a given year there will be an average of 7 generations of houseflies born. 
 If you had a fly in your house in January that laid its eggs and if all 
the subsequent generations survived at the end of the year there would be 
5,598,720,000,000 flies in your home!  A fly has a lot of offspring but 
doesn't stick around too long to make sure they live.  The vast number of 
flies born will ensure some will live.  Humans on the other hand give birth 
to relatively few babies during their lifetime but spend a lot of energy 
making sure each offspring survives.  This correlation between reproduction 
and lifespan is obvious when we discuss organisms as different as flies and 
humans but it generally holds true for more related animals.  If we look at 
only mammals this correlation between fecundity and lifespan also holds 
true (dogs and cats can have several litters of multiple offspring and they 
rarely live more than 2 decades) there is a correlation between the age of 
the animal when it is first capable of reproducing (reaches sexual 
maturity) and its maximum lifespan.  The longer the developmental period 
(that time between birth and sexual maturity) the longer lived the animal 
is.  Of all animals, humans have the longest developmental period and are 
among the longest lived of all animals.
   Another correlation with lifespan is body size.  Generally speaking the 
larger the animal and the larger its brain, the longer lived.  This is 
generally believed to be the result of the metabolic rate and is best 
exemplified in mammals.  Small mammals have a large surface area compared 
to their weight and lose body heat at a greater rate than a large animal.  
In order to maintain their body temperature they have a higher metabolic 
rate (i.e. they burn food faster).  This tends to "burn out" their bodies 
quicker.  Indecently, if warm-blooded animals are raised in cold 
environments they have a shorter lifespan because their metabolic rate is 
higher; cold-blooded animals show the opposite affect.  One of the major 
exceptions to the body size rule are humans.  Humans have a much longer 
lifespan than their body size should allow (we should live about as long as 
most pigs).  This lead to the idea that brain size must be just as 
important as body size.  Generally, the greater the brain/body size ratio, 
the longer lived the animal.  Humans have the biggest brain to body size 
ratio and live a long time (accepted maximum is about 122 years).  
  Lastly, there appears to be a correlation with lifespan and the number of 
hazards in an animal's normal environmental niche; the more hazardous the 
environment the shorter the maximal lifespan.  This may be due to the 
necessity of reaching sexual maturity quicker in these animals to ensure 
their reproduction.
  I hoped this answered your question.  If you have any other questions 
please feel free to contact me.