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Quizzing the Anonymous
Ignoramus et ignorabimus
Why are the testicles located outside of the body? 
18th-Jun-2010 04:14 pm
The usual answer is that the temperature of dangling testes needs be lower than the body temperature to optimize sperm production. But why? During the gestation the testes descend from the chest across the whole body cavity to get outside of the body and this passage weakens the wall structure resulting in a higher propensity to hernias (Neil Shubin likes this example as showing our fish origins). On the other hand, it is unclear why wouldn't evolution be able to develop more heat-tolerant sperm. Indeed, the sperm sensitivity is extreme, clearly affecting our reproductive chances:

...if the testes heat up by as little as 2°C, sperm formation is adversely affected. Sperm count will drop, the number of normal sperm will fall and the number of abnormal sperm will increase. Semen quality is naturally lower in summer compared with winter. Although semen volume does not change significantly, the total sperm counts per ejaculation fell from 320e6 in winter to 250e6 during July and August.

Given the physiological/reproductive penalties, there should be a good reason why it is done this way. Observe that desdendant tests and scrotum are not the universal features even for the mammals (though the majority of mammals do have their testes dangling outside). E.g., elephants wisely keep theirs sizeable testes hidden inside the body. It has been argued that the scrotum is an older primitive adaptation, and more recent testicular evolution among mammals “has proceeded from scrotal to ascrotal but never the reverse,” while the “loss of descensus is relatively rare”. The biologists have been scratching their heads for many decades searching for a plausible rationale accountig for our pendulant ornamentations:

...The traditional interpretation is that the scrotum evolved to provide a cool environment that would ensure for optimal spermatogenesis. It was also argued that the epididymis functions as a cold-storage site for sperm, and therefore it was the epididymis that was the prime mover for the evolution of the scrotum. We now know that both the storage and formation of viable sperm for most mammals requires that the testicles be maintained below body temperature, but why this is the case has never been adequately explained. Some theories of testicular descent include the idea that reduced scrotal temperatures function to minimize gamete mutation rates. Not only are scrotal temperatures typically below body temperature, but because of reduced blood flow to the testicles another hypothesis is that these conditions combine to create a hostile environment that functions to train or test sperm as a means of selecting for a high quality ejaculate. It has even been suggested that the scrotum evolved as a signaling device or form of sexual ornamentation to promote male social/sexual competition. As an extension of this hypothesis, it is interesting to note that scrotal testicles could be related to what is known as the “handicap hypothesis” (like peacock's tail). While this might seem to be a plausible account of scrotal testicles because the handicap they entail carries enormous potential costs, most traits that fit the handicap hypothesis become more exaggerated and more costly over time as a consequence of female choice and competition among males for limited reproductive opportunities. With the possible exception of colored scrota among a few species of primates, there is little evidence that this has been the case.

When the best idea is that we have testes outside of the body precisely to make our lives miserable, you know you are losing it. One very recent idea I've heard (which I like much more) is that the testes are located outside because we rely on a very archaic way of sperm activation, which no one else is using:

...The integrity of the gonads is of paramount importance when it comes to reproduction. Because of vulnerability to damage, insult, and temperature variation, unprotected ovaries located outside the body cavity would be an enormous handicap/serious reproductive disadvantage. The same reasoning applies with equal force to the testicles. But unlike ovaries, descended testicles located outside the body cavity in the scrotum are common among many mammals in spite of all the obvious risks and disadvantages. We suggest that descended scrotal testicles in humans and many other mammals evolved to provide a situation specific means of activating sperm. As a result of consistent temperature differences between the male and female reproductive tracts (i.e., scrotal temperatures are typically maintained at 2-3oC below body temperature), we propose that the rise to body temperature that accompanies insemination into the vagina serves as one of several triggers for the activation of sperm. The rise to body temperature occasioned by depositing sperm into the vagina functions as a primitive but highly appropriate, situation specific trigger that augments sperm activation and functions to further increase the likelihood that a sufficient number of sperm will be able to pass through the cervix and reach the oviducts to fertilize an egg. Descended scrotal testicles evolved to both capitalize on this copulation/insemination contingent temperature enhancement and function to prevent premature activation of sperm by keeping testicular temperatures below the critical value set by body temperature. http://www.epjournal.net/filestore/ep07517526.pdf

It is an interesting hypothesis, as it explains testicond animals, like elephants: during the ovulation, their females rise basal body temperature, so the differential is still present; it is only the differential that matters. Furthermore, it explains odd habits of certain bipeds:

...Humans show a peculiar cross-cultural nocturnal copulation bias. Having sex at night could promote any number of adaptive outcomes. Nocturnal copulation may have functioned to minimize vulnerability to predation and/or detection and interference from rival males, accommodate clandestine copulations, promote incest avoidance, and even enhance the likelihood of sperm retention by postponing the resumption of an upright posture by the female during the post ejaculatory period. Patterns of nocturnal copulation may also be a circadian adaptation to descended scrotal testicles. When daytime temperatures approach or exceed body temperature which is common in equatorial regions, testicular adjustments would fail to function favorably. However, in the evening and at night when ambient temperatures fall back below body temperature, more effective scrotal temperature adjustments are reinstated and the likelihood of optimal sperm viability may increase.

Isn't it exciting how one thing leads to another and then still another? Our mammalian lineage are the only animals keeping their testes outside of the body because our synapsid ancestor had a primitive and peculiar way of activating sperm during the ovulation. It was simpler to redesign the body than finding a proper biochemical solution to this seemingly trivial problem. Millions of years of nocturnal romance later, some mammals found a way of fooling this mechanism, which is also rather unimaginative - it is amazing that it took that long. The testicles promptly got back inside the body. Now the dangling testicles are on the way out. But for now we have them where we have them, and it is good.

Why are the mammalian testicles located outside of the body?

PS: another take on the evolutionary history of scrotum http://www.ias.ac.in/jbiosci/mar2010/27.pdf
...We conclude that: (1) testicondy represents the plesiomorphic character state for Mammalia, because of the lack of testicular descent in all Monotremata and almost all Afrotheria; (2) the scrotum evolved twice during the evolutionary history of mammals within Marsupialia and Boreoeutheria; (3) the distribution of mammals with a fast mode of locomotion (gallop) is aligned with the distribution of the scrotal life forms on the phylogenetic tree of mammals; at the same time, repeated loss of the scrotum in many taxa within Laurasiatheria occurs in groups that do not gallop. ("The evolutionary history of testicular externalization and the origin of the scrotum")

18th-Jun-2010 10:19 pm (UTC)
Easier to scratch.
19th-Jun-2010 12:27 am (UTC)
Biologists scratch the opposite:
shkrobius> The biologists have been scratching their heads for many decades
19th-Jun-2010 12:44 am (UTC)
I've seen them scratching everything...
18th-Jun-2010 10:22 pm (UTC)
I've had the same idea (temperature activation) for many years, but never bothered to publish it.
Interestingly, I've heard scrotum mentioned as an argument against intelligent design theory a few times - once by Steven Colbert on Comedy Central :-)
18th-Jun-2010 10:28 pm (UTC)
Oh, you should've published it - it's a beautiful idea; I did not know that female elephants rise the body temperature, which makes it rather plausible. Mammalian design is so weird in general that dangling testicles do not even seem to be particularly out of place...
18th-Jun-2010 10:49 pm (UTC)
Publishing an idea out of the blue is difficult...
Anyway, I wonder how marine mammals get around this problem.
18th-Jun-2010 11:36 pm (UTC)
They tell how: theirs are in the subc. fat, so it is water cooled - clearly, this not an option for land animals. The whales, actually, have descensis, just no scrotum.
19th-Jun-2010 01:28 pm (UTC)
Well, they have to solve two problems, - cooling and drag.
20th-Jun-2010 05:17 pm (UTC)
...the ‘galloping hypothesis’ states that the scrotum originated
in mammals which frequently gallop, leap or jump. During
such a bumpy lifestyle, the process of spermatogenesis in
an abdominal testis or, alternatively, the sperm stored in the
epididymis, would be endangered by the fluctuations in intra-abdominal
pressure; the evolutionary origin of a scrotum is
a solution which accommodates these crucial reproductive
organs in a relatively non-destructive surrounding.
19th-Jun-2010 08:48 am (UTC)
Don't all the female mammals rise their BBT in ovulation? The human females certainly do.
20th-Jun-2010 05:08 pm (UTC)
They do, but it is < 0.2F. The claim is that in testicond animals it is significantly higher (observe that they ovulate rarely, so the penalty is low). I do not know what it is, the authors imply it is significant.

Their take on evolution of scrotum may well be incorrect. Here is another one, which reaches the opposite conclusions (e.g., the ancestral testicondy, etc)

I ended this post in a question because the answer is not known; this is just a guess. That paper lists five other explanations (e.g., the gallop hypothesis, regulated mutation hypothesis, etc.)
19th-Jun-2010 05:48 am (UTC)
What exactly is "activation of sperm"?
20th-Jun-2010 05:12 pm (UTC)
Look at their paper (I give another one in the PS). Here is what they mean
Most sperm are kept inactive by different factors while stored in the epididymis and activation of sperm is initiated as a consequence of exposure to glandular secretions during ejaculation (e.g., Okamura, Tajima, Soejima, Masuda, and Sugita, 1985).

Among mammals with descended scrotal testicles one of the key features that distinguish the male reproductive system from the female is that the female reproductive tract is maintained at body temperature. Although chemical features of the vagina exert an influence, it has been shown that a rise in temperature is sufficient to produce activation of sperm under laboratory conditions. Marin-Briggiler, Tezon, Miranda, and Vazquez-Levin (2002) demonstrated that capacitation of human sperm maintained in vitro at 20oC could be produced by raising the temperature to body temperature (37oC). It has also been discovered that capacitated human sperm are chemotactically responsive to follicular fluid. However, both chemotaxis and capacitation are short-lived and only last from 50 min to 4 hrs (Eisenbach, 1999). Raising the temperature of sperm to body temperature also produces a time-bound increase in sperm motility (Makler et al., 1981; Si, 1999).

Why should raising sperm temperature to body temperature be so critical in preparing sperm not only to achieve their fertilization potential, but in enabling sperm to acquire the necessary motility to penetrate the cervix and reach the fallopian tubes? As a primitive adaptation, perhaps descended scrotal testicles evolved as a consequence of an equally primitive algorithm. The chance for sperm to travel up through the female reproductive tract and fertilize an egg is represented by a relatively short, time-bound
opportunity. Once activation and capacitation are achieved, sperm viability is short lived (Eisenbach, 1999).

Recall that sperm motility increases as temperature rises to body temperature. Among mammals with descended scrotal testicles a consistent and universal feature of ejaculation into the female reproductive tract is that it raises the ambient temperature of the ejaculate to body temperature. Thus, the rise to body temperature occasioned by depositing sperm into the vagina functions as a primitive but highly appropriate, situation specific trigger that augments sperm activation and functions to further increase the likelihood that a sufficient number of sperm will be able to pass through the cervix and reach the oviducts to fertilize an egg. In our view, descended scrotal testicles evolved to both capitalize on this copulation/insemination contingent temperature enhancement and function to prevent premature activation of sperm by keeping testicular temperatures below the critical value set by body temperature.
20th-Jun-2010 05:12 pm (UTC) - cont.

After sperm penetrate the cervix and reach the oviduct they appear to be trapped in a reservoir, where capacitation and hyperactivation occur and only a few sperm are released to fertilize an egg when ovulation occurs (Suarez, 2001). Capacitation and hyperactivation are required for sperm to achieve their fertilization potential and penetrate the zona pellucida, and occur as a result of complex biochemical interactions (involving cAMP/PKA and tyrosine kinase/phosphatase signaling pathways and a Ca2+ signaling pathway) that come into play during the time sperm are trapped in the oviduct (Marquez and Suarez, 2004).

Capacitation involves a series of structural changes in the membrane of the spermatozoa, culminating in the acrosome reaction, which allows the sperm to bind to the oocyte. As sperm capacitate, there is also a concomitant change in sperm motility resulting in hyperactivation. This is characterized by a dramatic increase in flagellar bend amplitude and beat asymmetry and a curved and highly convoluted pattern of movement. Hyperactivation is an important part of capacitation, as it is thought to be the mechanism by which the sperm are able to detach from the thick mucus of the oviduct and propel themselves vigorously towards the egg (Suarez and Ho, 2003). Thus, there appear to be several phases of activation: glandular activation during ejaculation, temperature induced activation upon the release of semen into the vagina, and subsequent capacitation and hyperactivation that occur after sperm reach the oviduct.

The timing of hyperactivation and the acrosome reaction are critical to successful fertilization. A premature acrosome reaction is believed to be a cause of male infertility (de Lamirande, Leclerc, and Gagnon, 1997). The effect of scrotal temperature on the timing of this reaction may be important. Si (1999) has shown that hyperactivity can be induced in sperm incubated at body temperature, and effectively stopped when the temperature drops. It is suggested that as temperature increases, so does lipid diffusibility in the sperm plasma membrane, which may alter membrane permeability and subsequent membrane-bound enzyme reactions. These reactions include signaling pathways for essential ions such as calcium bicarbonate, which are responsible for hyperactivation and the acrosome reaction.
Due to the effect of temperature, sperm created and maintained at body temperature would be in a constant state of activation. By depleting metabolic stores this would create significant obstacles to successful fertilization. There is evidence, for example, that sperm that become hyperactivated prematurely cannot penetrate the cervix (Zinaman et al., 1989).

According to Suarez and Pacey (2006) the female reproductive tract plays a role in ensuring that only sperm with normal morphology and the capacity for vigorous movement reach the oviduct. Of the several hundred million sperm contained in a single human ejaculate only a few thousand penetrate the cervix and have a chance to reach the fallopian tubes. Sperm that achieve advanced stages of capacitation before reaching the cervix lack the necessary energy reserves to successfully navigate the remainder of the female reproductive tract, and may even be prevented from advancing towards the oviduct by the female’s own reproductive defenses.
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