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Wild Animals/Reg: Human Inability



Why don't human infant swim or walk at the moment of birth?       but it can be done by others like wild/domestic animals����

I didn't see any survival of the fittest for current or ancient cave human on it. pls clarify...


Dear Dinesh,

Humans (and their infants) have been under different selective pressure than other species, and our helplessness at birth reflects that.  Any individual is born with heritable/genetic traits that can be either...

adaptive - increases its chances of reproduction
maladaptive - decreases its chances of reproduction
neutral - does not affect its chances of reproduction

In general (and in the right organismal "package"), adaptive traits will be passed to the next generation, and maladaptive ones will die out with the organisms exhibiting them.  But the crux is:  What is adaptive?  The answer?  It depends on environmental context and natural selection pressure.

To give a silly example:
Sharks hatched out with feathers will not likely have an adaptive advantage over scaled sharks because feathers don't work very well in an aquatic environment.  But a primitive, reptilian animal living on land might well have an advantage if the genes controlling its scale development mutated enough to make them form feathery structures.  In a marine environment, a mutation promoting feathers would be maladaptive (or neutral, at best).  On land, it might be more likely to be adaptive.

In humans, natural selection has practically run away with the size of our brains.  Our brain size (relative to our bodies) and the intelligence and behavioral flexibility it confers is probably the single most adaptive trait we have.  But it comes at a price:  to accomodate large brain size, our cranium at birth is very large, compared to that of other animals.  That means that--compared to other animals--we are quite altricial at birth.

Altricial animals are helpless at birth, and need (sometimes extensive) parental care.  All primates are somewhat altricial compared to something like an ungulate (hoofed animal) that can run a few hours after birth.  But humans are the most altricial of all, unable to grasp thing, focus, or do much of anything except be carried around by the parent at birth.  But being born at a relatively early stage of development allows the cranium to expand to large proportions as the child grows. This is something that even our closest relatives, the chimpanzees, cannot match.

In short, brain size has been a more adaptive trait than precocial young (i.e., young who can fend for themselves much more than a relatively altricial animal) in humans.  So we just wait to learn other skills such as swimming and running.

Hope that helps.


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Dana Krempels, Ph.D.


I'm an evolutionary biologist with a passion for animals. Ask about natural history, behavior, ecology, evolution. PLEASE NOTE:

If you have found an "orphaned" or injured wild animal or bird:
Please don't waste time asking questions on the internet, as the answers may come too late. DO NOT FEED THE ANIMAL, and DO NOT HANDLE IT unless it is in imminent danger. (Many wild "orphans" are not orphans at all!) If you are absolutely sure it is orphaned, keep it warm and quiet, and find a LICENSED WILDLIFE REHABILITATOR HERE. Don't try to raise a baby yourself, or rehabilitate an injured anmal. Many a well-intentioned rescuer will do more harm than good, especially with baby birds and baby rabbits.

Without geographic location, time of day and habitat, I can't help. A clear picture is always best.

It's impossible for me to I.D. an animal call without hearing it myself.

I'm not an expert on comparative strengths of different animals (more complicated than you might think!) nor bite forces.

I refuse to answer "Which of these two animals--X or X--would win in a fight?".

These hypothetical matchups range from impossible (Grizzly Bears and Gorillas don't even occupy the same continent.) to ridiculous (Someone asked me "Who would win a fight between a Great White Shark and a tiger?").

The vast majority of animals--even the fierce and powerful--are not as warlike as Homo sapiens, and it's childish to project our aggressiveness onto them.


I have been the fortunate caregiver to a group of Black-tailed Jackrabbits rescued from the Miami International Airport, and not releasable in this area because they are not native. I also have rehabbed and released Eastern Cottontails, and am in contact with many very experienced wildlife rescuers who regularly handle injured or orphaned rabbits and hares.

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I have a Ph.D. in Biology, with main areas of expertise in evolutionary biology, genetics, botany, and ecology.

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