# What is the "core" body format that virtually all mammals, birds and reptiles adhere to? **URL:** \ **Category:** Factual Questions\ **Created:** [August 6, 2015, 5:16am UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133 "2015-08-06T05:16:29Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Stoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/stoid/32/272_2.png) [@Stoid](https://boards.straightdope.com/u/Stoid)\ **Post date:** [August 6, 2015, 5:16am UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/1 "2015-08-06T05:16:29Z")
By which I mean: 4 limbs 5 digits (do birds have vestigial digits?) 2 eyes 1 brain stomach/intestines liver kidneys/bladder uterus/ovaries/testes I know reptiles (all or most, not sure) have cloacas that function as the exits for both waste and eggs, so I left out vaginas…although I think all the males in these groups have penises? In many animals these things are extremely modified, but evidence of the basics is still in place, such as horses: their “middle” toe has become the hoof, and there is some evidence I believe for at least two other toes, but the other two are only evident in their DNA. But it IS in their DNA: > [@](#): > > In other words, the horse’s genetic code still instructs the embryo to create a total of 20 toes (five in each foot) in the early stages of embryonic development. So that’s what I mean, do all members of these groups have the basic coding for all of the above, or is there some member of these groups that has so radically departed from this format that there’s no evidence of (whatever part) at all? (the question occurred to me because of something I saw about bees having five eyes, which made me ponder how insects are so wildly different from larger animals, and then I thought about how larger animals living on the land (for the most part) all share this basic blueprint, even though its sometimes radically modified in the final form) ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [August 6, 2015, 5:55am UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/2 "2015-08-06T05:55:49Z")
Tetrapods all share the same ancestral body plan because all tetrapods are descended from one species of shallow water fish that hauled up on land 360 million years ago. And so they make legs out of fins, and wings and fins out of legs, and so on. Insects are descended from a completely different lineage of multicellular organisms that arose 650 million years ago. But we do share certain things with insects, because all life on Earth has a common origin. So our genetic code is the same, our basic metabolism is the same, the building blocks of life are the same. Now, the important part, about the penises. Our tetrapod ancestors didn’t have penises, and many birds, reptiles and amphibians don’t either. They just spray sperm directly onto eggs as in the case of many amphibians, or touch cloacas. But penises have evolved independently in many different lineages. Take ducks, for instance. The horrifying truth about duck penises is out there, let me get you started here: [Everything About Duck Penises Is Interesting](http://jezebel.com/5992668/everything-about-duck-penises-is-interesting) You can see the advantage of having some extensible portion of the cloaca that facilitates the deposition of sperm into the cloaca of the female. Lots of reptiles have two penises, one on each side. Yes they do. [Life is short, but snakes are long: Why do snakes have two penises?](http://snakesarelong.blogspot.com/2014/03/why-do-snakes-have-two-penises.html) Placental and marsupial mammals have separate ducts for the excretion of urine and feces, and so don’t have a cloaca for both functions like our early reptile-like tetrapod ancestors. But male mammals still use an extension of the excretory system to excrete urine and deposit sperm in the female mammal vagina. And while female mammals have separate urinary tracts and vaginas, this is a newly evolved condition, birds and reptiles don’t. This condition of having one duct to expel both wastes and gametes was probably present waaay back in vertebrate evolution, when we were just worms with a bit of cartilage to stiffen and protect the dorsal nerve cord. Note that arthropods have a ventral nerve cord–their “spinal cord” is along their stomach and the digestive tract is above. Completely different body plan. ---
**Author:** ![blue\_infinity](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_infinity/32/2817_2.png) [@blue\_infinity](https://boards.straightdope.com/u/blue_infinity)\ **Post date:** [August 6, 2015, 8:37am UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/3 "2015-08-06T08:37:20Z")
[Here’s](http://www.timetree.org/home) a cool site which gives an estimate of when lineages diverged. E.g.: [horses & bees](http://www.timetree.org/search/pairwise/9796/7460) ---
**Author:** ![Iggy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iggy/32/3364_2.png) [@Iggy](https://boards.straightdope.com/u/Iggy)\ **Post date:** [August 6, 2015, 11:35am UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/4 "2015-08-06T11:35:43Z")
[Hox genes](https://en.wikipedia.org/wiki/Homeobox#Hox_genes) are highly conserved groups of genes that lay out an overall body plan. They are present in fruit flies and humans, and many evolutionary steps in between. ---
**Author:** ![Nefario](https://avatars.discourse-cdn.com/v4/letter/n/59ef9b/32.png) [@Nefario](https://boards.straightdope.com/u/Nefario)\ **Post date:** [August 6, 2015, 2:12pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/5 "2015-08-06T14:12:18Z")
Are there any left/right asymmetrical “soft” animals? Including invertebrates. I know that asymmetry exists in crab, crayfish, lobsters with exoskeletons but I can’t think of any soft invertebrates that are not symmetrical. Hmmm…just thought of flounder…but I believe the young are symmetrical. A long time ago I read _The Mote in God’s Eye_ about aliens that became asymmetrical as they evolved to build and repair machines. 🆒 ---
**Author:** ![Hari\_Seldon](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hari_seldon/32/5173_2.png) [@Hari\_Seldon](https://boards.straightdope.com/u/Hari_Seldon)\ **Post date:** [August 6, 2015, 3:11pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/6 "2015-08-06T15:11:24Z")
Apparently, the earliest tetrapods had 8 digits on each limb, but this was quickly reduced to five. And of course, in many mammals the number of toes is further reduced. A horse has just one toe and one vestigial digit on each leg. A recent thread made the point that we are more closely related to a salmon than the salmon is to a shark or skate. For a fascinating read on all this, look at “Your Inner Fish” by Jerry Coyne. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [August 6, 2015, 3:23pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/7 "2015-08-06T15:23:59Z")
> [@Nefario](#): > > Are there any left/right asymmetrical “soft” animals? Including invertebrates. Oh yes, tons of them. All the echinoderms (sea stars, sea urchins, sand dollars, etc) have radial symmetry. Same with cnidaria (jellyfish). Porifera (sponges) usually have no symmetry at all. –Mark ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [August 6, 2015, 10:48pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/8 "2015-08-06T22:48:58Z")
> [@Stoid](#): > > By which I mean: > 4 limbs > 5 digits (do birds have vestigial digits?) > 2 eyes > 1 brain > stomach/intestines > liver > kidneys/bladder > uterus/ovaries/testes The more general characteristic is called _bilateralism_, as I recall. It’s basically that our bodies are divided by a center line, and we hav 2 of most items, one on each side. 1 brain (but 2 halves of it) 2 eyes 2 ears 2 nostrils 2 breasts 2 lungs 1 heart (but again, with 2 sides) 2 kidneys 2 testes/ovaries 2 legs There are only a few organs that we have only 1 of; liver, digestive organs, and some of the endocrine system. Plus the heart & brain – they have 2 halves, but generally need both to function. For most of these, we can survive with only one of them working; this provides an evolutionary advantage to the species. This is common to all the mammal species, and a lot of others, like reptiles, birds, dinosaurs, etc. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [August 6, 2015, 11:05pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/9 "2015-08-06T23:05:14Z")
> [@Lemur866](#): > > Placental and marsupial mammals have separate ducts for the excretion of urine and feces, and so don’t have a cloaca for both functions like our early reptile-like tetrapod ancestors. But male mammals still use an extension of the excretory system to excrete urine and deposit sperm in the female mammal vagina. And while female mammals have separate urinary tracts and vaginas, this is a newly evolved condition, birds and reptiles don’t. And the penises and vaginas of marsupials and placentals differ quite a bit. For one thing, male marsupials have a [bifurcate penis](http://i.imgur.com/IvO7aZH.jpg) (“two pronged dong,” as we learned in vertebrate anatomy), while female marsupials have not one, not two, but three, count 'em, [three vaginas.](http://static02.mediaite.com/themarysue/uploads/2012/04/Kangaroo-vaginas-chart.jpg) Not to be outdone, the echidna, a monotreme, has a [four-headed prehensile penis,](https://www.youtube.com/watch?v=96jdqDSDE2E)even more remarkable because the female doesn’t have even one vagina. (She does have a cloaca, though.) ---
**Author:** ![Shagnasty](https://avatars.discourse-cdn.com/v4/letter/s/9dc877/32.png) [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)\ **Post date:** [August 6, 2015, 11:19pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/10 "2015-08-06T23:19:02Z")
> [@Lemur866](#): > > Take ducks, for instance. The horrifying truth about duck penises is out there, let me get you started here: [http://jezebel.com/5992668/everything-about-duck-penises-is-interesting](http://jezebel.com/5992668/everything-about-duck-penises-is-interesting) From the link: “The length of the duck penis, as mentioned in the tweets, grows to 8 or 9 inches during the summer mating season. In winter, it shrinks to less than an inch.” Same here (more or less). I am going to start telling everyone that I am “hung like a duck”. ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [August 6, 2015, 11:37pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/11 "2015-08-06T23:37:53Z")
And don’t forget the spikes and barbs that many penises are encrusted with. You hu-mon females have it easy. ---
**Author:** ![marshmallow](https://avatars.discourse-cdn.com/v4/letter/m/df705f/32.png) [@marshmallow](https://boards.straightdope.com/u/marshmallow)\ **Post date:** [August 7, 2015, 12:10am UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/12 "2015-08-07T00:10:38Z")
During embryonic development, all deuterostomes develop an anus before any other opening. This means we all start out as assholes. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [August 7, 2015, 1:38am UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/13 "2015-08-07T01:38:38Z")
Many male mammals have a bone in their penis, known as a baculum or _os penis_. Dogs, bears and gorillas are some of the best known examples of animals with penis bones. As the wiki article says, with what strikes me as a considerable understatement: > [@](#): > > A bone in the penis allows a male to mate for a long time with a female, which can be a distinct advantage in some mating strategies. > **[Baculum](https://en.wikipedia.org/wiki/Baculum)** > > The baculum (pl.: bacula), also known as the penis bone, penile bone, os penis, os genitale, or os priapi, is a bone in the penis of many placental mammals. It is absent from the human penis, but present in the penises of some primates, such as the gorilla and the chimpanzee. The baculum arises from primordial cells in soft tissues of the penis, and its formation is largely influenced by androgens. The bone lies above the urethra, and it aids sexual reproduction by maintaining s The word baculu... ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [August 7, 2015, 3:00am UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/14 "2015-08-07T03:00:07Z")
> [@Lemur866](#): > > And don’t forget the spikes and barbs that many penises are encrusted with. You hu-mon females have it easy. Yeah, much better than being, for example, a female bedbug. The male bedbug practices the aptly named “traumatic insemination”. The male doesn’t bother finding any orifice, but jabs his penis directly through the body wall of the female and injects semen into her body cavity. The process is said, presumably with understatement, to be “detrimental to the female’s health”. –Mark ---
**Author:** ![Pleonast](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pleonast/32/1183_2.png) [@Pleonast](https://boards.straightdope.com/u/Pleonast)\ **Post date:** [August 7, 2015, 4:18am UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/15 "2015-08-07T04:18:24Z")
> [@Lemur866](#): > > Note that arthropods have a ventral nerve cord–their “spinal cord” is along their stomach and the digestive tract is above. Completely different body plan. It’s been a long while since I’ve read about this stuff, but I recall that arthropods and vertebrates do have the same body plan, but vertebrates flipped over soon after branching off from the arthropods. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [August 7, 2015, 2:24pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/16 "2015-08-07T14:24:26Z")
> [@Shagnasty](#): > > From the link: > “The length of the duck penis, as mentioned in the tweets, grows to 8 or 9 inches during the summer mating season. In winter, it shrinks to less than an inch.” > > Same here (more or less). I am going to start telling everyone that I am “hung like a duck”. Yeah, but you live in the southern hemisphere. 😃 ---
**Author:** ![Smeghead](https://avatars.discourse-cdn.com/v4/letter/s/f1d935/32.png) [@Smeghead](https://boards.straightdope.com/u/Smeghead)\ **Post date:** [August 7, 2015, 5:18pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/17 "2015-08-07T17:18:39Z")
> [@Nefario](#): > > Are there any left/right asymmetrical “soft” animals? Including invertebrates. Sponges are completely asymmetrical. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [August 7, 2015, 6:32pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/18 "2015-08-07T18:32:34Z")
> [@Nefario](#): > > Are there any left/right asymmetrical “soft” animals? Including invertebrates. > > \ > > Hmmm…just thought of flounder…but I believe the young are symmetrical. Not just flounder. All (or nearly all) flatfish: sole, turbot, halibut, plaice, and so on. But if you think about it, most vertebrates are assymetrical. Humans and other primates have their hearts offset to one side, and a smaller lung on that side. ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [August 7, 2015, 11:54pm UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/19 "2015-08-07T23:54:57Z")
> [@markn\_1](#): > > Yeah, much better than being, for example, a female bedbug. The male bedbug practices the aptly named “traumatic insemination”. The male doesn’t bother finding any orifice, but jabs his penis directly through the body wall of the female and injects semen into her body cavity. The process is said, presumably with understatement, to be “detrimental to the female’s health”. > > –Mark Cry me a river, bedbugs! ---
**Author:** ![Blue\_Blistering\_Barnacle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/blue_blistering_barnacle/32/3386_2.png) [@Blue\_Blistering\_Barnacle](https://boards.straightdope.com/u/Blue_Blistering_Barnacle)\ **Post date:** [August 8, 2015, 12:09am UTC](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133/20 "2015-08-08T00:09:24Z")
> [@t-bonham@scc.net](#): > > The more general characteristic is called _bilateralism_, as I recall. > It’s basically that our bodies are divided by a center line, and we hav 2 of most items, one on each side. > 1 brain (but 2 halves of it) > 2 eyes > 2 ears > 2 nostrils > 2 breasts > 2 lungs > 1 heart (but again, with 2 sides) > 2 kidneys > 2 testes/ovaries > 2 legs > > There are only a few organs that we have only 1 of; liver, digestive organs, and some of the endocrine system. Plus the heart & brain – they have 2 halves, but generally need both to function. > > For most of these, we can survive with only one of them working; this provides an evolutionary advantage to the species. > > This is common to all the mammal species, and a lot of others, like reptiles, birds, dinosaurs, etc. The brain is quite symmetric. The heart, not so much. (In humans, the brain \*looks \*very symmetric, but functions tend to preferentially lateralize to one hemisphere or another. This is an advanced “Motie”-like thing. Less cerebral mammals have far more symmetry of function, just as “watchmakers” have more symmetry than “Moties”.) The digestive system is quite asymmetric. This is probably related to the rotation the gut undergoes (to help us stuff a grater length of tubing in there). I can’t recall if the liver and spleen start out as midline unpaired organs and rotate to the right and left respectively (or if they develop as asymmetric size/function structures from a more symmetric paired structure). As someone else pointed out upthread, the heart and lungs are not (very) symmetric. The lungs are a derivative of the digestive tract, BTW. [Next page](https://boards.straightdope.com/t/what-is-the-core-body-format-that-virtually-all-mammals-birds-and-reptiles-adhere-to/727133.md?page=2)