Oh, interesting. I should have clarified that when I spoke of body holes as “vulnerable”, I wasn’t thinking of them as being more vulnerable or more targeted specifically with reference to large predator attack, or intraspecies fighting, than other anatomical regions. You are right that large predators and combatants tend to go for areas that will produce most bang for the buck, so to speak, massive-trauma-wise.
What I meant by calling body holes especially “vulnerable” is that they tend to have delicate tissues that are more susceptible to minor trauma (also infection) than other regions. A male human can shrug off a punch to the shoulder that would be temporarily incapacitating if delivered to the testicles, for example. A human forearm can easily recover from a poke with a sharp stick that would permanently destroy a human eye. That sort of thing.
So anatomical hole-consolidation is surely not primarily a defense against large predators, which as you note can get plenty of mileage out of attacking other body parts. What it helps to protect against (in practice, at least: I have no idea to what extent this effect actually influenced vertebrate anatomy evolution over the multi-millennia) is exposure of fragile tissues to comparatively minor and frequent injuries that other body parts are tough enough to stand.
Thanks for the clarification. But saying that the fragile tissues are vulnerable in this way leads to the question, how does their rearmost placement reduce that vulnerability? I don’t have an answer for that. Can you fill me in?
Not an evo bio or anything like it, so all you’re getting here is my speculation! But my speculation is more about the consolidation of the holes than their particular location.
That is, I can see why it “makes sense” for reproduction and elimination holes to be close together to keep those especially fragile tissues more or less in one place. And I can see why it kind of “makes sense” for that hole-cluster to be tucked into the crotch area of the torso, rather than flappin’ in the breeze out on the calf of the leg, for example.
But is there any reason it “makes sense” to have that hole-cluster specifically in the crotch rather than, say, in the center of the abdomen? Why don’t we have our genitalia and anal/urinary vents in our navel areas, where we could easily shield them by crouching, for instance? I don’t have a flipping clue.
Neither eyes nor testicles are “holes”, but sweat glands are.
My guess is that it’s just how the first worms were arranged, and with no major disadvantages, most of their descendants have kept the same basic plan.
Sure, if you don’t care about answering the question. Answering “evolution” doesn’t say much more than answering “Because”. Why did they evolve that way?
Mmmmokay, but that seems a bit over-literal? (And not totally accurate, given that the pupil of the eye is indeed a hole.) There’s a pretty clear sense in which the sweat glands, distributed throughout most of the human skin, don’t fall into the “particularly fragile tissues” category that for convenience I was labeling “holes”.
Huh, i thought you were going to go with, “the eyeballs sit in holes in the skull”, which is true. But the pupils aren’t “holes” in any meaningful sense, as it’s entirely internal, and the lens may be tougher than the the muscle around it.
And the pores are definitely holes in the sense that they are a vulnerability in our defenses against the outside world. Stuff gets stuck in pores and can be absorbed that way.
Testicles are, indeed fragile, but i would never refer to one as a hole.
I guess if you wanted to say that it’s convenient to group fragile bits together, i can see where that’s coming from. But the whole belly of a mammal (below the ribs and above the hips) is pretty fragile.
If we start with ancient proto-worms, then it makes even more sense. Essentially they are a tube, food in front, ex-food out back. With the cloaca model, therefore the reproductive functions come from either the front or back hole. Pushing reproductive products out the front right at the mouth, where the offspring might immediately become a meal, seems to be counter to good survival logic, so presumably the species that used the tradesman’s exit at the back would be more survival-prone.
Therefore the reproductive organs located themselves in the body nearest the rear exit to miniimize travel distance, and Mother Nature has stuck to that plan, generally, since then.
IIRC, sweat glands tend to be all over. And hair and milk glands are simply evolution repurposing sweat glands. (As are fingernails or claws or rhino horns, I presume?) Not in the same category of “major pathway into the body’s interior” that the mouth/nostrils and waste elimination openings are.