This article never answered a very basic question: so evolution exiled man's balls from the body because sperm are temperature sensitive - but why didn't sperm just evolve to be less vulnerable to high temperatures? Anyone have a clue on this one?
Well, evolution doesn't have a "why" reasoning. It's not like there's a committee looking for the most efficient design. It may have been more complicated to evolve sperm to be more heat-resistant. Doing so may have even necessitated some bad effects (e.g., low mobility or less chance of fertilization). The dangling testicles may, through random chance, have evolved first, and once that happened, any changes in sperm's heat-resistance were pointless, so they weren't selected for.
With evolution, there's no "why", unless you count "it was evolutionarily beneficial, so it was selected for". But even that doesn't explain everything, like genetic drift.
The "why" is a whole world of debate in and of itself, and is in fact a problem with the language used to convey scientific understanding.
From Wikipedia: Teleology (Greek: telos: end, purpose) is the philosophical study of design and purpose.
What happens is that ideas are often conveyed using teliological phrases, such as
"the sparrows migrate to africa in the winter month so as to find abundant food sources and rear the next generation..." or
"testicles reside outside of the body major in order to benefit from a reduced temperature...".
The problem is that these make sense to us, although they are _very_ incorrect. The teleological concepts of "so as" or "in order to" are the whys and they are very tricky beasts. So, for utmost correctness one would prefer to say:
"the sparrows migrate to africa in the winter month finding the abundant food sources needed to rear the next generation..." or
"testicles reside outside of the body major benefitting from a reduced temperature..."
It's a very subtle issue, and one that is compounded by the popularity and ease with which we talk about biology and evolution in particular. Ernst Mayr wrote some spectacular essays on the subject, he's one of my heros so forgive me for the blatent plug! He claims that a formal understanding of the errors of telology must permeate the sciences, and most especially biology (he was a philosopher of biology), so as to clear up the ambiguities that can lead one to think there is an end or purpos (telos) to evolution. Even now it is still quite common for people to think of it in terms of progress and constant betterment instead of just simple and blind adaptation.
Hey nice comment! I was thinking something similar.I believe that it should be possible to effectively use both mechanistic and teleological language. In mechanics you have newtonian (mechanistic) and lagrangian (teleological) approaches. I often wonder how well determined the shapes and forms of animals are by evolutionary dynamics. I would think that evolution is very sensitive to small accidents and fluctuations in populations, i.e. you could rewind the clock and run evolution forward many times and get many different answers. Against this view are all those examples of convergent evolution.
Another reason that people may react harshly when you use teleological language is that biology (unlike physics) is constantly under attack by people with all sorts of crazy theories about a supernatural designer - (talk about a teleology!) so I can understand why such language would be somewhat taboo.
There may be several reasons why sperm didn't evolve to be temperature tolerant. The first is likely that they would have suffered reduced motility just when it's needed, reducing fertility rates.
Another reason might be that maintaining the testicles at body temperature increases the risk of infection. There are very few purely sexually transmitted diseases, likely due to this temperature difference. Those same sexually transmitted diseases rarely cause harm beyond the genitals, and in fact these diseases are far more harmful to the female reproductive system than the male (a woman with undiagnosed gonorrhoea or chlamydia is essentially guaranteed to become sterile, however in men it either takes longer, or they may never become sterile - note that a male can lose a testicle to injury or disease without becoming sterile or losing fertility).
It's likely that there were more advantages to any male with dangling gonads over retracted. The reason why our testicles require constant temperature control (and independent control) is probably a more intriguing question, which IMO likely has a lot to do with our method of motility. A dog's testicles are unlikely to be affronted by the sun, however any naked male human has a serious problem when walking toward the sun. This likely wasn't a problem when we were semi-erect (in the evolutionary sense, not the other sense) and had hairy gonads, however why did modern humans develop wholly unprotected gonads? Think about it, our testicles don't even have protection against the sun! Dense hair provides protection against heat stroke, so did humans already have protection for our genitals before this became a problem? IE did our invention of clothing happen early enough that it actually changed the development of our sexual organs?
If the sperm are to be activated by the higher temperature inside the female body then they would have to be cooler initially. They evolved to be triggered into action by the normal internal body temperature.
Actually, it's probably a mixture of things, not just heat. Heat is probably just one of the many important pieces.
From what I know about biology/evolution, this answer is just as likely to have a single clean reason as it is that you killed the one and only cockroach living in your apartment.
Zck's answer points out the fallacy of the "why" question, but in gneneral I would surmise that co-evolution of sex-linked traits like this are a harder hill to climb than simple evolutionary adaption of the sperm alone. Developing a "program" that is not much more than "when you hit body-temp it is 'go time'" is less susceptible to external forces or attempts at subversion by either other sperm or the female reproductive tract.
@kurtosis: You should rephrase your question as, "why haven't cells evolved to be more heat insensitive".
One thing the article did get me wondering, and if any hacker here has gone through this and be willing to admit it, i'd love to know:
If you have to ask a doctor for help get pregnant... i.e. if your little fella isn't up to the job- i wonder if there's a course of therapeutic treatment simply around better managing the temperature of the scrotum? I'd imagine it'd be hanging around in loose fitting shorts in a temperature controlled setting until such time as your partner would be ovulating? :)
Yes, a friend was told to avoid spending a lot of time in his hot tub due to heat issues. He also had to do other things, but that's the one that I really remember. I also wonder if the decline in wealthy countries fertility relates in part to long hot showers.
IANA evolutionary biologist, so this is only a just-so story, but...
With ~4 million sperm cells per... egg, there is enormous selective pressure that gets exerted by competition among individual spermatozoa. So the sperm cells that are the most temperature-sensitive (hence best able to take advantage of the metabolic boost due to increased ambient temperature, hence on balance faster-moving) would be heavily favored.
I feel like there needs to be a Your Mom joke in here somewhere...
I am an evolutionary biology student, though this depends on details I don't know...
The sperm are all produced by the same individual, and so you might expect them to be essentially identical aside from the genes they carry, in which case there's no Darwinian selection between them.
However, if genes in the sperm are actually expressed (which would be normal for most other cells) then due to the process of meiosis, different sperm will have different genes and so selection between sperm from the same man could indeed occur.
Sadly I don't know the details of gene expression in sperm... the question really is whether they get their proteins from the parent cells (all of which are genetically identical to the man) or whether they make some of their own proteins.
(In any case, there could of course be selection at the level of individual human males, with those producing better sperm having higher reproductive success.)
My impression is that the sperm are optimized for a brief period of super activity at normal human body temperature. This enables them to reach the egg once inside a woman's body. If they become adapted to this higher temperature then what would kick them into a brief overdrive and burnout?
Perhaps some chemical signal could be secreted to triger them, but that means two beneficial mutations have to happen at the same time. This makes it far less likely and perhaps only strong selective pressure, as in seals, could bring out this double adaptation.
I would imagine it makes more sense for them to dangle out because it gives the body more ability to adjust the temperature as needed. If your body is too hot then they dangle low, if it's too cold outside then they huddle up close. If they were inside the body then this temp control wouldn't be available!
I don't have any clue. But it does make me think about how gender is determined by temperature of the eggs for some species (like crocs and gators and turtles). Sperm is what determines gender in humans. Connection? Maybe. Maybe not.
It's because it was harder for the sperms to become less vulnerable to temperature than it was for the testicles to move apart. When you think the evolution way, you get the answer ;)
