"Peter Webb"  wrote in
news:c32ufs$ghh$1{at}darwin.ediacara.org: 

 
> Still unconvinced. My wide doesn't seem to miss playing with her
> ovaries. The only reason we have bodies at all is to provide an
> environment for testes and ovaries to do their thing - there has to be
> some underlying reason for the most valuable part of the male body (in
> an evolutionary sense) occupying the most exposed real estate ...

1. It isn't the most valuable. Devore noted that baboons care most about 
food, politics, and sex, in that order. So the head is the most valuable 
part of the male body - yet it is also the most exposed. The testicles 
are in fact located in a position that is generally well protected in 
quadrupeds.

2. Tim has cited a website discussing thermal concerns, which is probably 
the best general explanation. Ovaries might benefit by being external in 
terms of potential mutation rate, but too many of their other functions 
require them to be internal. Reptiles - which are cold blooded - do not 
have external testicles. The question then is why birds - which are warm 
blooded - also do not. The answer may be because they need to be 
streamlined. Many birds lower their body temperature at night, and I have 
seen the suggestion that they produce sperm at night - I haven't seen any 
studies on this. Bats - which also need to be streamlined -  keep their 
testicles inside except during breeding season. AFAIK dolphins and whales 
have internal testicles. Seals do also, but apparently sea lions do not. 
There is also an element of chance - the "frozen accident" - involved.    


 

>> > 2. Why do we die, indeed, why do we age at all?

> Interesting, but in animals which don't nurture their young,
> maintenance programs have no point unless they can sustain
> reproductive processes. I still see no advantage to the individual in
> menopause or death, but a considerable advantage to the species. Do
> you believe that death occurs because biological machines with
> indefinite lifetimes cannot in principle be built, or because of some
> evolutionary advantage to death? If the latter, is this an advantage
> to the species or the individual? These are not rhetoric questions; I
> am genuinely trying to reconcile death with Dawkins. 

Again Tim has given a more detailed response, and you would do well to 
follow up his leads. I can add the following, if it helps:

Menopause is different from senescence - it occurs a considerable time 
before senescence, among females, and only in a few social species. The 
general explanation is that at a certain age a female will improve her 
overall reproductive success more by helping grandchildren, especially by 
making accumulated knowledge available, than by risking death in having 
more children. 

Most of the theories about senescence do not ultimately rely on benefit 
to the species, nor on the inability to create long lifetimes, but only 
on simple statistics. An animal has a certain chance each year of dying 
(due to disease, predation, starvation, sever storm, or accident). The 
chance varies depending on the animal's niche, but after enough years the 
odds of an individual surviving become close to nil ( a quick check on my 
calculator indicates that a 5% chance of dying per year translates to 
only a 2.75% chance of surviving to age 70). So genes which prolong the 
age past that "nil" point have no possible selective advantage - the 
individuals which carry them will no longer be alive to have children.  
Meanwhile all the other mechanisms which Tim outlined continue to act. We 
tend to think that we die because we age.That is only true of humans in 
very recent (from an evolutionary standpoint) history  In fact we age 
because we die.  

Yours,

Bill Morse
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