I was at the State Fair, where certain gonads were in plain view, and it raised some evolutionary questions which are thoroughly addressed in this article. I like how he doesn't too quickly dismiss the handicap principle before moving on to the activation hypothesis.
"descended scrotal testicles: that they serve as a sort of “ cold storage” and production unit for sperm, which keep best at lower body temperatures...heat of a woman's vagina radically jumpstarts sperm that have been hibernating in the cool, airy scrotal sack...why in all of evolution would nature have designed a body part with such obviously enormous reproductive importance to hang off the body so defenseless and vulnerable?...male gonads in some phylogenetic lineages went in completely different directions, evolutionary speaking. For example, modern elephants’ testicles remain undescended and are deeply embedded in the body cavity (a trait referred to as “testicond”), whereas other mammals, such as seals, have descended testicles but are ascrotal, with the gonads simply being subcutaneous....the arteries that supply blood to the scrotum are positioned adjacent to the veins taking blood away from the scrotum and function as an additional cooling/heating exchange mechanism...When the ambient temperature rises to body temperature levels, there is a temporary increase in sperm motility (that is to say, they become more lively), but only for a period of time before fizzing out. To be more exact, sperm thrive at body temperature for 50 minutes to four hours, the average length of time it takes for them to journey through the female reproductive tract...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 temperatures...other ancillary hypotheses connected to the activation hypothesis...humans’ well-documented preference--and one rather unique in the animal kingdom--for nighttime sex can be at least partially explained by temperature-sensitive testicles...Additionally, after nighttime sex the woman is likely to sleep, thus remaining in a stationary, often supine position that also maximizes the odds of fertilization...it may still seem odd to you that nature would have invested so heavily in such a precipitously placed genetic bank. After all, we’re still left with the curious fact that our precious gametes are literally hanging in the balance in a completely unprotected vessel...Enter pain. Not just any pain, but the unusually acute, excruciating pain accompanying testicular injury...If you’re male, the reason that you probably wince when you hear the word “squash” or “rupture” paired with “testicle” but not with, say, “arm” or “spleen” is because testicles are disproportionately more vital to your reproductive success than these other body parts are. I, for one, had to pause to cover myself just by typing those former words together. It’s not that those other body parts aren’t adaptively important, but variation in pain sensitivity across different bodily regions, according to this view, reflects the vulnerability and importance different adaptations play in your reproductive success. Male ancestors who learned to protect their gonads would have left more descendants...
No comments:
Post a Comment