I heard once that part of the reason that human children, out of all the mammals, are the least equipped to survive on their own is that our heads are so large (to accommodate our brains), it was more evolutionarily stable to give birth early and let development continue outside the womb than to develop wider vaginal cavities.
Any truth to this claim?
Early you say? 9 months is a pretty far cry from anything I would describe as ‘early.’ Other animals, like rats, with pretty large brains (and bigger skulls perportionally) only take 21 days to reach term. If you look at gestation times of animals you will see a trend, but it has nothing to do with relative head sizes and everything to do with the size of the fetus. Larger animals, like cows, horses and elephants all have longer gestation periods while smaller animals, like mice, rats and dogs all have shorter gestational periods.
Early considering the condition of the baby - all those other animals you mentioned can stand up when born.
I’m not sure exactly what you’re asking. It’s true that our babies are born developmentally immature (I’ve heard the first 3 months of life referred to as “the fourth trimester”) and have a long period of dependency on adults, and it’s true that we have obnoxiously large heads compared to our pelvic outlet, and it’s true that we have ridiculously large brains for our body weight inside those heads…but is the large brain “why” we give birth to immature young? Who knows? Why do kangaroos have very immature babies? Why don’t elephants?
Remember that “evolution” doesn’t do anything on purpose nor does it choose what would seem to be the best solution to animals’ problems. It just takes something that works accidentally and runs with that. “Why” isn’t really answerable.
But I also don’t think that “human children, out of all the mammals, are the least equipped to survive on their own” is actually true. Certainly our culture has a great deal to say about that - if we were in the jungle eating grubs, my 3 year old would have a pretty good handle on her life skills at this point. A 10 year old urchin in Dickensian London may have been surviving on his own for years. The fact that we’ve decided kids also needs to learn to read and write and get a job and discuss politics (all of which take time to learn how to do) are not biologically certain. Infants of most non-prey mammals are pretty helpless early on. Certainly this little guy doesn’t look ready to leave Mom’s pad and get a job!
Actually, most other mammalian babbies are just as helpless when first born. My breeder rats and mice are not capable of anything for a few days other than barely and weakly crawling a couple of cms from their mother. Horses are born ready to run, but that’s not the norm of many mammals. Plus, the question deals with our large head size, which A) has nothing to do with gestation time and B) is not really that large.
As a developmental biologist there is zero evidence to suggest gestation time relates to anything aside from embryo size.
Humans being bipeds is a significant factor. The female pelvis is a compromise between efficient walking/running on her hind legs (favors short, narrow pelvis) and being able to pop out a kid without dying in labor. (favors tall, wide pelvis) Quadrapeds need not make such a compromise.
