Biology experts, biologists, laymen...help

[IndianaJones]

What determines gestation period in mammals? I appreciate any insight.

[kestrel91316]

1. Time it takes for fetus to develop to the point where it can survive.......

outside the womb.......not sure what you are getting at. What is the REAL question here?

[IndianaJones]

3. the real question is...what factors contribute to length of gestation period. nt.

[Xithras]

4. Natural biological growth rates, nutritional levels, even stress can play a part.

I fall under the laymen category, by the way.

Most species have a "normal" growth rate that is primarily driven by the speed of cell division. It's the same reason you're toddler isn't 4 feet tall on his second birthday...the body can only grow so fast without screwing up. The larger the organism is, the longer the gestation period is going to be (all animals start out as a single cell, so the smaller the creature is, the faster it reaches its birth weight).

Stress, nutrition, and other factors can impact this growth rate, but any successful birth is still going to be within a fairly narrow window surrounding a "normal" gestation period.

[kestrel91316]

7. It's something that is very species-specific and has developed over

millennia of evolution.

What are you actually trying to get at? Maybe it would help if I knew that.

[IndianaJones]

8. I am having a discussion with someone that claims the following regarding human gestation period:

"The answer is bipedalism. The upright stance of humans and the semi-upright stance of other hominids and primates dictates a smaller birth channel through the pelvis."

[kestrel91316]

9. Hm. I'm out of my league with this issue.

[HiFructosePronSyrup]

10. There's an argument...

that because the human hip isn't very good already due to bipedalism, and the female human hip is further weakened due to the birth canal, that humans are born with their skull, and presumably brain, underformed to compensate, and that's why we are so weak as infants compared to other mammals.

Is that what you're getting at?

[IndianaJones]

12. no, this person claimed that humans had a shorter gestation period than many mammals..

due to bipedalism. His only stated reason was that the birth canal was narrower due to walking upright, and thus the gestation period was limited. I have not seen evidence that the human gestation period is unusually short.

[mondo joe]

22. But our gestation period is relatively longer than some mammals, and

shotrer than others.

[HiFructosePronSyrup]

14. Given the context...

it sounds like he's arguing the gestation period is shorter than it should be, i.e. infants would be less helpless if it were longer.

[IndianaJones]

18. He is actually making an idiotic argument using science as the foundation.

I was hoping to discredit the claim he made.

[Xithras]

23. Actually, his claim is close, but not quite correct. Brain size, not hip size.

The reality is that humans are born helpless because our brains aren't fully developed at birth. Many portions of the brain aren't properly developed for up to a year after birth. This is only indirectly related to bipedalism.

Our pre-human ancestors developed bipedalism and narrower hips long before we developed big brains, and it's probable that our pre-human infant ancestors were born from those narrower hips just as active and aware as chimpanzees are today. The difference is brain development. As our brain developed into a larger and more complex organ, the newborn babies had to still fit through a pelvic cavity that had been narrowed by bipedalism millions of years before. The evolutionary solution is that the higher parts of our brain don't develop until AFTER birth, which leads to a pretty helpless newborn.

By the way, among primates specifically, humans have the longest gestation period. When body size is factored in we're average for mammals, but those "more developed" chimpanzee babies are birthed in only eight months. For humans to be born in a similar fashion with our larger and more complex brains, the human gestation period would need to be about 18 months.

Indirectly I guess you could concede his point. Bipedalism led to a narrower pelvis, which causes humans to birth offspring less developed than other species. In reality, the two concepts aren't all that related, since the female human body would have an extremely hard time supporting a fetus for 18 to 20 months anyway. Even if the pelvis weren't a factor, I would expect that the human gestation cycle would have ended by the 12th month, giving you a newborn with the developmental equivalent of a 3 month old baby. That's still incredibly helpless when compared to other animals, and is a byproduct of brain size, not hip size.

[IndianaJones]

24. do you have a background in biology or a related field? thanks. nt.

[Xithras]

26. Anthropology studies in college.

My majors were math and computer science, but I'm a science geek (still am) and took several Anthro courses. The mechanisms of human evolution are pretty well understood at this point, and that's what we were taught in school (I'm only 33, so it wasn't THAT long ago that I was in college either).

If you really want to know for sure, I'm a college professor and I'm teaching a logic class tonight in the same building as our current anthro and biology courses. I could probably ask one of the instructors tonight and come up with something a bit more definitive (maybe even a cite or two), but I wouldn't be able to post back until tomorrow.

Other than that, you just have my take on it as an educated layman.

[IndianaJones]

27. I was asking because I would like to cite you as a source and quote your post.

Is that ok?

[sharp_stick]

11. I think I know what this is about

The idea they are trying to get across is that because humans are bipedal they have developed smaller birth canals so they can walk upright. This smaller birth canal may be a reason that human babies are born at a much lower self sufficiency stage. By this I mean that baby cows, goats etc, can walk almost as soon as they are born but people are really totally helpless for months and not self sufficient for years.

IMO the small birth canal has something to do with it but it has more to do with the fact that humans are able to take care of their young for the years it takes them to develop, we have virtually no predators and our brains are so complex that it would take a long long gestation just to get the newborn to the point of being able to move. Chimpanzees also have helpless young and long periods of adult care of the young.

Does that make any sense?

[IndianaJones]

13. But is the human gestation period unusually short compared to other mammals? nt.

[HiFructosePronSyrup]

15. You can't compare.

Most mammals gestation periods are shorter, but then again they're mostly smaller.

There aren't any other mammals that are the same size and bipedal, so there's no way to compare.

[IndianaJones]

16. I was really hoping to discredit his argument. thanks. nt.

[gatorboy]

17. Get some hamsters.

Then you'll see just how short a gestation period can be (plus, you'll have lots of hamsters)

[sharp_stick]

19. No, not really

when based on size it's pretty much on par with other mammals. For humans (266 days) about the same length of time as a chimpanzee (227 days), gorilla (257 days) and orangutang (260 days). I have heard that the gestation period is more related to the ability of the umbilical cord to provide nutrition than anything else. When the mother can no longer support the growing baby it's time for it to come out.

I may have been misleading in my previous reply. I don't think there is any evolutionary advantage to have a shorter gestation period. I feel that the reasons Humans survive having such weak and helpless young is because of our ability to protect the young, this is where our evolutionary advantage lies.

[ChickMagic]

20. That's kinda it

There are at least 5 different forms the placenta can take in different species of mammals. Though each species always takes the same form. The differences involve changes in degree of contact and the number of layers of tissue between the maternal blood supply and the embryo's. On the maternal side the possible layers are called the endometrial epithelium, connective tissues and the endothelium of the blood vessels. On the embryo's side there is also the endothelium of the blood vessels, the mesenchymal tissues and the chorion. The differences are related to the different lifestyles of the various mammal species.

[Xithras]

21. Biologically that is correct.

But it has little to do with the human gestation period. It DID lead to evolutionary changes in infants including soft skulls, but didn't greatly impact our gestation period.

Among mammals, the human pelvis is unusually narrow. This was an evolutionary change to permit efficient bipedalism (without it, we'd be bowlegged). A side effect of reduced pelvis size is a reduced birth canal width.

If I remember my college anthropology course correctly, the human pelvis is as small as it could possibly get and still allow childbirth. Any narrower and the mothers would all die in childbirth. Any wider and they wouldn't be able to run from predators as well as other humans (remember the old joke...you don't have to outrun the lion, you just have to outrun the OTHER guy running away from the lion.) It's an evolutionary compromise.

[Ian David]

2. Doesn't answer your question, but here's an interesting chart...

Average Animal Gestation Periods and Incubation Times
http://encarta.msn.com/media_701500881_761572784_-1_1/Average_Animal_Gestation_Periods_and_Incubation_Times.html

[HiFructosePronSyrup]

5. Why are you asking?

Need help with homework?

[HereSince1628]

6. The same stuff that determines which species is which.

In a very general sense its in the genes.


on edit If I could type accurately it would be nice.

[Warren DeMontague]

25. Jesus.

Sorry, I thought you were asking what the official Bush Administration answer to this question was.