Wikipedia:Reference desk/Archives/Science/2019 January 2

= January 2 =

Animals with head first delivery preferred?
What animals other than humans have the preferred delivery being head first (as opposed to for example a cow, for which the front hooves is the preferred first part of the animal to deliver.Naraht (talk) 01:18, 2 January 2019 (UTC)


 * Well, the other great apes and indeed a majority of simians. This is actually an interesting evolutionary matter, as live birth is believed to have evolved independently in various parts of the tree of life on at least a couple hundred occasions, and the variations on the physiological morphology are remarkable as a result.  There are a number of different factors which are believed to have influence which species evolved head-first birthing (meaning superior portion of the body first, which a strong but by no means absolute majority of live birth species use), but by far the most significant is cephalo-pelvic ratio: basically, the higher this ratio the more likely the species is to require this form of birthing, and then at the high end of that spectrum (especially where mammals are concerned) are mostly clustered the cranial-first species.  Of course we must also account for aquatic live-birth species, of which there are quite a few, most of whom have no legs to compete with the head. Snow let's rap 12:06, 2 January 2019 (UTC)
 * Thank you very much. I am excluding the aquatic in this. What is the split within the simians?Naraht (talk) 14:59, 2 January 2019 (UTC)
 * You're very much welcome. As to your further inquiry, I'm unaware of an obvious single source that aggregates that data, and could not immediately find one, but it's likely to be out there somewhere. Here are a few sources which at least provide detailed consideration of the relative physiological proportions in various simian species, though a majority of them are behind paywalls which may restrict your access depending on your research portal resources:, , , , ,.


 * My suspicion, based upon what I do know about parturition across mammalian species, is that the vast majority of all simians birth cranium first. Of course, you should take that uncited statement with a grain of salt, but there's actually a generally reliable test for figuring out which species are going to birth which extremities in which order: Because childbirth is an exceptionally risky process for both parent and offspring in most all species, there is a substantial selective pressure to ease and expedite the process to the extent the morphology of the child allows. As such, in mammals there is a pronounced, almost universal preference for births to proceed in the order which minimizes the maximal circumference to which the vagina will be stretched, thus minimizing potential trauma to the mother and child.  So obviously with most bipeds, allowing the hands and arms to come out extended alongside the cranium would increase the radius and also put dangerous pressures upon parts of the body already tightly constrained by pressure that is already at the high end of all animal parturition.  Whereas with most livestock species, to borrow your example from above, the most complicated and problematic method would be too have the legs pressed downwards against the trunk of the body, which is why most all avoid this by having the forelegs precede the head. Now this is not an absolute rule, because there are other considerations and many species actually have room to spare in the vagina (well, at least compared against humans, who are at an extreme end of the spectrum), including some simians.  But as a general rule, you can look at a species, consider it's biomechanics (which way the extremities most naturally flex and which profile would allow for a minimal demand upon increase in the diameter of the birthing canal) and make a rough reliable guess for whether they are likely to come out cranium first or superior limbs first.  Sno<b style="color: #b2dffe;">w</b> <b style="color: #d4143a">let's rap</b> 23:16, 2 January 2019 (UTC)


 * FYI: Apparently, hyenas have a very high mother-and-infant mortality rate during childbirth, but a low post-partum chld mortality rate, becasue the fetal hyena develops farther and larger in utero than most other mammals. The latter is alluded to in the wiki hyena article.  (I read about this someplace years ago (perhaps Tuesday NYTimes Science section?), but don't now have a reference.) Acwilson9 (talk) 21:27, 7 January 2019 (UTC)