Why pregnancy lasts for so long?

[Moreno]

Shell not evolution work in direction to make pregnancy as short as possible? In large mammals such as humans and elephants fertility lasts 9-18 month. In some smaller mammals it lasts much shorter.

There are some live creatures which could grow extremely fast. For example bamboo could grow 1 meter in 24 hr. In humans both pregnancy and period of infant growth takes too much time. Evolutionary reasons?

Edited January 10, 2016 by Moreno

[tantalus]

Usually the question is why not longer, as our offspring are so vulnerable when born.

 

One advantage is the longer an animal gestates, the more developed the offspring is, and the better its chance of survival. Species vary in their strategy, and for simplicity can be represented as a dichotomy of 1.produce lots of offspring, don't help them much, and play the numbers game 2. produce few, and look after them very carefully so as to insure they make it.

 

Once a species heads down a road, evolutionary speaking it is easy to imagine it leading to ever more extreme care to offspring, leading to longer gestation periods.

 

With humans, the baby needs to develop its brain and this takes time and places a serious constraint on development, it may not be possible for our gestation period to be longer due to brain/head size, the width of the female pelvis, and limits linked to the biomechanics of efficient bipedal walking. Womens walk is already less efficient than a man's.

[swansont]

Shell not evolution work in direction to make pregnancy as short as possible? In large mammals such as humans and elephants fertility lasts 9-18 month. In some smaller mammals it lasts much shorter.

There are some live creatures which could grow extremely fast. For example bamboo could grow 1 meter in 24 hr. In humans both pregnancy and period of infant growth takes too much time. Evolutionary reasons?

Just a guess, but: Bamboo doesn't have to develop a nervous system, or internal organs, like a brain. That peobably help with the growth speed

[Moreno]

Just a guess, but: Bamboo doesn't have to develop a nervous system, or internal organs, like a brain. That peobably help with the growth speed

So, what? Are you sure that bamboo cellular structure is simpler that of a human? Could you prove human brain and nervous system cannot "develop" sufficiently during a few weeks or even days (if sufficient amount of nutrients provided)?

[Phi for All]

(if sufficient amount of nutrients provided)?

 

This sounds like magic. A "sufficient" amount allows you to assume adequate development of a brain in a few days. Nice trick.

 

Are you suggesting the only limitation to an organism's development is nutrients? This sounds like I could fill a water balloon with a fire hose efficiently, but I'm sure I couldn't. Or rather that I could increase the growth of bamboo to a meter per hour just by using "sufficient amounts" of nutrients.

[swansont]

So, what? Are you sure that bamboo cellular structure is simpler that of a human? Could you prove human brain and nervous system cannot "develop" sufficiently during a few weeks or even days (if sufficient amount of nutrients provided)?

That's moving the goalposts, though. We need to compare with the typical supply of nutrients. Do humans at any point in their development grow a meter per day? Or a week? A month? Why should a fetus?

 

Another thing to consider is that the bamboo growth is mainly up, i.e. In one dimension. Human growth is three dimensional.

[John Cuthber]

Bamboo is under very strong evolutionary pressure to grow quickly.

We are not.

[Strange]

Shell not evolution work in direction to make pregnancy as short as possible?

 

But no shorter.

 

No doubt it is a mult-variable trade-off between how mature the offspring are, how much care they need, how quickly they can grow, how being pregnant affects the health and vulnerability of the parent, and many other factors.

So, what? Are you sure that bamboo cellular structure is simpler that of a human? Could you prove human brain and nervous system cannot "develop" sufficiently during a few weeks or even days (if sufficient amount of nutrients provided)?

 

The fact it doesn't happen seems like proof enough.

[rangerx]

Shell not evolution work in direction to make pregnancy as short as possible? In large mammals such as humans and elephants fertility lasts 9-18 month. In some smaller mammals it lasts much shorter.

There are some live creatures which could grow extremely fast. For example bamboo could grow 1 meter in 24 hr. In humans both pregnancy and period of infant growth takes too much time. Evolutionary reasons?

 

Perhaps the answer you seek is in mother's milk. Milk is basically water, sugar, fat and protein.

 

Most animals need all these things, but some need more of one than others.

 

Whales and seals live in the water and need fat layers to protect them. This is why whale and seal milk is rich in fat.

 

Cows grow fast especially in muscle mass. This is why cow's milk is rich in protein.

 

Humans are highly intelligent. This is why human milk is high in sugar, to feed our developing brains.

 

As you probably already know, too much of one or another is not better for us, it's worse. If we tried to be like cows, our joints would crystallize with uric acid. Too much sugar, our teeth rot, pancreas overworks and we become obese. Too much fat, we stress our liver, gain weight and gather cholesterol.

 

Nine months of pregnancy in humans is not long. It's normal. Six weeks gestation in dogs and cats is not short. It's normal.

[Endy0816]

Red Kangaroos are a good example of an alternative that is comparable with us in terms of adult size. Has a gestation period of only 33 days, but less mature offspring are placed at greater risk as a result.

[StringJunky]

Red Kangaroos are a good example of an alternative that is comparable with us in terms of adult size. Has a gestation period of only 33 days, but less mature offspring are placed at greater risk as a result.

Aren't they carried in the pouches after they are born?

[Phi for All]

Aren't they carried in the pouches after they are born?

 

They have to wriggle their way into the pouches right after birth, no help from mom iirc.

[CharonY]

Nutrition or cellular growth is not the issue here. Rather there is the amount of differentiation and complexity of physiological structures. In order to form organs or similar you need at minimum signal gradients that sufficiently guide differentiation of cells and to shape morphology. While similar things apply to plants their structure is much simple and allows for many more errors. Also there is the evolutionary history. Organs were evolved in various stages and the development cycle cannot just jump to the final form. That is why certain structures have to be formed first, even it seems to be an unnecessary path from a structural view point.

[Moreno]

Golden hamster pregnancy lasts 15-17 days. Elephant pregnancy 510-730 days. Does it mean elephants or humans are dozens of times more complex than hamsters?

[John Cuthber]

Golden hamster pregnancy lasts 15-17 days. Elephant pregnancy 510-730 days. Does it mean elephants or humans are dozens of times more complex than hamsters?

No, mainly it means they are bigger. Had you not spotted that?

[Moreno]

No, mainly it means they are bigger. Had you not spotted that?

Yes. But I thought that BOTH parents and fetuses are bigger.

[John Cuthber]

Perhaps you would prefer to talk to a dozen hamsters.

It's complicated. Size is obviously a factor, so's complexity, so's maternal nutrition etc.