According to all those old Disney nature films, one nearly universal trait of the young of other species is that they instinctively know to hide. Infant animals, even infant carnivores, are silent/hide the grass/etc. Basically, being an infant is a very dangerous time, and it makes sense that evolution would select for infants who are better able to avoid being detected by the many other critters who would like to eat them.
But human infants break all the rules. They wail, cry, scream to their hearts content, and spew their urine and feces all around themselves. They are pratically a billboard for other animals, screaming “yummy defenseless flesh here!”
The apparent answer is that these traits of human infants didn’t develop until humans were the dominant species, able to (relatively) easily defend their children against all other critters. But this presents a chicken-and-egg problem: if the defenselessness of human infants is related to the development of intelligence, etc., which is what made humans the dominant species, the infants of some relatively unintelligent, non-dominant precursor hominid species must have been wailing, screaming, etc.
Does that make any sense? And if it does, why did human infants evolve the way they did?
Because of the exaggerated brain size of Homo sapiens, their infants, if they underwent a gestation period proportional to that of related species, would be born with heads so big that the mother would almost invariably be injured in giving birth. Thus humans evolved an earlier birth. The tradeoff is intelligence for relative helplessness as infants.
To anthropomorphosize and imply intent where there is none, allow me to say metaphorically that evolution “assumed” that the intelligence afforded by finding a way to get a larger brain out of the mother would also allow an intelligent enough mother to realize that she has to keep her kid quiet.
(The above is VASTLY oversimplified and is not meant as a complete or accurate summary of the actual scientific process involved.)
Primates can carry their young around. So the danger equation changes. Probably, if you’re an infant, you’re usu. better off sticking with mom. It’s just that, with deer and such, you’ve got no choice–you’re a lot slower than mom, and you’re liable to get both of you killed if you always tag along. So you’re selected to sit down and shut up. With primates, your odds of survival are likely better if you cry to convince mom to pick you up than if you stay quiet and let her drop you off somewhere.
Let me point out another thing–most people immediately assume that all adaptations are the result of helping the creature survive in a harsh world, fight off predators, etc. In fact–esp. w/ humans–many adaptations are more the result of trying to out-compete your fellow species members. Thus, the kid who cries more is likely to have mom pay more attention to him, feed him more to shut him up, etc., thereby giving him a better shot at dominance than his quiet, stringy, pushover companions.
Fact is, none of the great apes are really prey to much of anything … they’re generally too big to worry about idly pissing off, and don’t live out on the savannah in big herds, where big cats, etc., can run in at a fast clip and separate and take down a single member. So our babies have to worry less about predators killing us and more about winding up underfed, or left alone for a minute to be killed by a rival male.
I think toadspittle is on the right track. Human babies scream and cry and wail when you put them down. If you hold them 24 hours a day 7 days a week, and pop a nipple in their mouth every time they get fussy, I expect you wouldn’t see much crying. This is pretty much what chimp and gorilla and orangutan mothers do, so I expect that your default human baby expects the same treatment and only cries when it doesn’t get it.
In the case of older infants, I also expect there is some learning going on. They learn that screaming and pitching a fit is not a dangerous behavior, because their parents don’t give them cues to the effect that they need to stop crying right now or the leopard is going to hear. I think babies can pick up some fairly sophisticated non-verbal communication, they somehow just know that you aren’t frightened that their crying will attract carnivores and you just want to sleep.
As far as pooping…well, ancient people certainly didn’t have diapers. If you carry your baby around 24/7, you just learn to read the signs and you hold the baby away from your body when it does its business. But you have to expect a certain degree of failure. Humans and other primates are notoriously hard to potty train. I think it goes back to the arboreal nature of primates…when a monkey poops the poop falls out of the tree and the monkey doesn’t have to deal with it. Whereas a carnivore with a home has a strong selective pressure to learn not to poop in its den. We humans just haven’t been non-arboreal denning animals long enough for selective pressure to provide us with a strong potty training instinct.
A while back, the New York Times ran a very interesting article that talked about these issues. I can’t find it on the NYT website, but a (possibly copyright-violating) copy of it is on this page. Scroll down towards the bottom and look for the title “Facing Backward, Helpless and Chubby”.
There’s a lot more to that article than what’s excerpted above. When I first read it, I sent a copy to my Mom and told her to giver her childbirth horror stories a rest and just be glad she wasn’t a tarsier (“tarsier birth is so difficult and improbable, Roberts said, that once the baby is out, ‘you can’t imagine how the mother ever gave birth to such a huge thing.’”) or a hyena (“the mother [hyena] must give birth…through an extraordinary organ the size and shape of a penis… First births are often deadly, as the penis-like organ rips open as best it can yet nonetheless leaves many fetuses lethally trapped within the tube”).
I’m sure I saw documentaries in which a lost baby chimp, and a lost kitten, wailed for all they were worth. I always imagined it was an instinctive behavior that evolved to help the mother or other adults in the group to find a lost offspring.
Toadspittle beat me to the comment on the motivation for being so attention-getting.
I also want to mention the way David Brin put the answer to the other part of the question in one of his essays (paraphrasing since I can’t recall the exact wording) ‘baby can cry and wail all s/he wants because daddy and his pals are the biggest, baddest animals in the forest and every other animal knows not to even toy with the idea of taking them on.’
A period of relatively little predatory pressure, (for reasons other than the benefits associated with our need to finish forming outside the womb), long enough to get “over the hump?”
As toadspittle suggests - an organism doesn’t have to be perfect (or even nearly so) at evading predators to have a statistical survivial advantage, it only has to be better than the worst of its peers. In other words, as a gazelle, you don’t have to outrun the cheetah, you don’t even have to outrun all the other gazelles - you just have to outrun the slowest gazelle.
