# Did the definition of "mammal" change in the last ~30 years? **URL:** \ **Category:** Factual Questions\ **Created:** [August 15, 2010, 6:29pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342 "2010-08-15T18:29:58Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 15, 2010, 10:32pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/21 "2010-08-15T22:32:37Z")
When I was in grade school, we were taught that live birth was one of the traits of mammals in general, but that the monotremes were just really weird mammals. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [August 15, 2010, 10:40pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/22 "2010-08-15T22:40:40Z")
> [@Chronos](#): > > When I was in grade school, we were taught that live birth was one of the traits of mammals in general, but that the monotremes were just really weird mammals. Me, too. I was in elementary school in the 60s, at which time cladistics had not taken hold. The emphasis was on physical descriptions. Mammals = hair + milk + live birth + warm blooded. No need to let those messy monotremes ruin such a tidy definition-- just include them as an exception. Especially when you’re teaching kids. ---
**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 15, 2010, 11:02pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/23 "2010-08-15T23:02:06Z")
> [@John\_Mace](#): > > Mammals = hair + milk + live birth + warm blooded. Actually, as an operational definition for living forms, either hair (with the structure found in mammals; other organisms have hairlike features with different structure) or the production of milk from mammary glands by themselves are sufficient to define a mammal. Warm blood and live birth (except for monotremes), while found in all mammals, are also found in other vertebrates. > [@qazwart](#): > > The answer is sort of “Yes and No”. In a traditional sense, Mammals still are defined as producing milk via lactation and being warm blooded, but as **Colibri** pointed out, the ear bone definition has been picked up mainly because it helps us classify extinct mammals and is much more of an evolutionarily oriented answer than the mere picking of few particular traits. Likewise, as an operational definition, feathers are sufficient to define modern birds, since no other organism has them. However, the discovery that many dinosaurs (that is, non-avian dinosaurs) also had feathers makes it necessary to use a different defining feature for the clade Aves. ---
**Author:** ![Markxxx](https://avatars.discourse-cdn.com/v4/letter/m/5daacb/32.png) [@Markxxx](https://boards.straightdope.com/u/Markxxx)\ **Post date:** [August 15, 2010, 11:28pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/24 "2010-08-15T23:28:30Z")
I would think the OP is just not remember what was taught. I went to school in the 70s and we were always taught mamals inculded the placentals, egg laying and marsupials. I remember being a little shaver and wanting to pet a duck billed platypus ---
**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 15, 2010, 11:48pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/25 "2010-08-15T23:48:42Z")
> [@Markxxx](#): > > I would think the OP is just not remember what was taught. I went to school in the 70s and we were always taught mamals inculded the placentals, egg laying and marsupials. I remember being a little shaver and wanting to pet a duck billed platypus Given his vagueness about other aspects of what he was taught, I think the “bad student” hypothesis is at least as plausible as the “bad teacher” one. 😉 ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [August 16, 2010, 2:06am UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/26 "2010-08-16T02:06:14Z")
> [@Colibri](#): > > True. Actually, I should … Thanks for the clarification. ---
**Author:** ![Zsofia](https://avatars.discourse-cdn.com/v4/letter/z/7bcc69/32.png) [@Zsofia](https://boards.straightdope.com/u/Zsofia)\ **Post date:** [August 16, 2010, 1:42pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/27 "2010-08-16T13:42:32Z")
So nobody cares about “four chambered heart” anymore? That was always one of the characteristics I was taught as a child. (And that, duh, monotremes and marsupials are mammals.) ---
**Author:** ![GHO57](https://avatars.discourse-cdn.com/v4/letter/g/df788c/32.png) [@GHO57](https://boards.straightdope.com/u/GHO57)\ **Post date:** [August 16, 2010, 1:48pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/28 "2010-08-16T13:48:48Z")
> [@Markxxx](#): > > I would think the OP is just not remember what was taught. I went to school in the 70s and we were always taught mamals inculded the placentals, egg laying and marsupials. I remember being a little shaver and wanting to pet a duck billed platypus Aren’t those suckers poisonous? ---
**Author:** ![mozchron](https://avatars.discourse-cdn.com/v4/letter/m/c2a13f/32.png) [@mozchron](https://boards.straightdope.com/u/mozchron)\ **Post date:** [August 16, 2010, 2:34pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/29 "2010-08-16T14:34:41Z")
> [@GHO57](#): > > > [@Markxxx](#): > > > > I would think the OP is just not remember what was taught. I went to school in the 70s and we were always taught mamals inculded the placentals, egg laying and marsupials. I remember being a little shaver and wanting to pet a duck billed platypus > > Aren’t those suckers poisonous? The males are - they have hollow spurs and poison glands in their hind legs. And a nasty disposition. For the layperson, the main common defining characters of mammals are mammary glands (but not nipples, as mentioned above) and hair during some point in development. Not live birth. As stated, many organisms have live birth, but only mammals have hair and mammary glands. To the taxonomist, there are other defining characters (ear bones, heart structure etc…) that are useful. This is the problem with bad science textbooks being taught by teachers with no science background. They simply teach the book as gospel. ---
**Author:** ![Oldeb](https://avatars.discourse-cdn.com/v4/letter/o/ea666f/32.png) [@Oldeb](https://boards.straightdope.com/u/Oldeb)\ **Post date:** [August 16, 2010, 2:35pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/30 "2010-08-16T14:35:44Z")
The males are. They have a spur on the each of the back legs for injecting venom. In humans it mostly just causes some swelling and pain, lots and lots of pain. It won’t kill you, although you might wish it had over the next few days (weeks or months if you’re really unlucky) as the stuff works it’s way out. Oh, and according to a report from 1992 not even morphine was able to block out the pain. However the glands are only active during the breeding season. So would a neutered platypus produce venom? ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [August 16, 2010, 2:42pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/31 "2010-08-16T14:42:34Z")
> [@Markxxx](#): > > I would think the OP is just not remember what was taught. I went to school in the 70s and we were always taught mamals inculded the placentals, egg laying and marsupials. I remember being a little shaver and wanting to pet a duck billed platypus I’ve not been close enough to a platypus to pet it, but I have been close to echidnas, both in the wild and in captivity – and I have patted one. Those spines look sharp, but in fact they’re pretty blunt, though pretty hard, so it isn’t like patting an animal with normal fur. ---
**Author:** ![WhyNot](https://avatars.discourse-cdn.com/v4/letter/w/c77e96/32.png) [@WhyNot](https://boards.straightdope.com/u/WhyNot)\ **Post date:** [August 16, 2010, 2:50pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/32 "2010-08-16T14:50:09Z")
Pedantic nitpick: animals that can kill you with a toxin by getting that toxin into you via injecting it through your skin are venomous, not poisonous. Poisonous is a plant or animal with a chemical that sickens or kills you when you ingest it or touch it. So a male platypus is venomous, not poisonous. Carry on. 😉 ---
**Author:** ![Captain\_Amazing](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@Captain\_Amazing](https://boards.straightdope.com/u/Captain_Amazing)\ **Post date:** [August 16, 2010, 2:58pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/33 "2010-08-16T14:58:58Z")
> [@WhyNot](#): > > So a male platypus is venomous, not poisonous. What if you eat their venom gland? ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [August 16, 2010, 3:03pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/34 "2010-08-16T15:03:56Z")
> [@Captain\_Amazing](#): > > > [@WhyNot](#): > > > > So a male platypus is venomous, not poisonous. > > What if you eat their venom gland? Then you become eligible for a Darwin Award. 😉 ---
**Author:** ![digs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/digs/32/14889_2.png) [@digs](https://boards.straightdope.com/u/digs)\ **Post date:** [August 16, 2010, 3:07pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/35 "2010-08-16T15:07:57Z")
> [@John\_Mace](#): > > > [@Colibri](#): > > > > No, at least not from mammary glands. (Pigeons produce a kind of “milk” from their crops to feed their young, but obviously it is very different from mammalian milk.) > > So, are saying I won’t be ordering a Pigeon Milk Latte at Starbucks anytime soon? Don’t even try. The girl just looks at you funny. ---
**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 16, 2010, 3:08pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/36 "2010-08-16T15:08:45Z")
> [@Zsofia](#): > > So nobody cares about “four chambered heart” anymore? That was always one of the characteristics I was taught as a child. (And that, duh, monotremes and marsupials are mammals.) While all mammals have a four-chambered heart, again it is not a character unique to them. Birds also have a four-chambered heart, as do crocodilians also (although crocodilians have a special shunt between the arterial and venous systems). > [@WhyNot](#): > > Pedantic nitpick: animals that can kill you with a toxin by getting that toxin into you via injecting it through your skin are venomous, not poisonous. Poisonous is a plant or animal with a chemical that sickens or kills you when you ingest it or touch it. > > So a male platypus is venomous, not poisonous. This is not really correct. In common usage, it is perfectly correct to refer to animals that inject toxins as poisonous. From [Merriam-Webster:](http://www.merriam-webster.com/dictionary/poisonous) > [@](#): > > Main Entry: poi·son·ous > Pronunciation: \ˈpȯiz-nəs, ˈpȯi-zən-əs > Function: adjective > Date: 1565 > 1 : destructive, harmful > 2 : having the properties or effects of poison : venomous > 3 : spiteful, malicious The distinction between venomous and poisonous you mention is made in some technical usages; it is not necessary in informal usage. It is perfectly correct to call a rattlesnake or a platypus poisonous. ---
**Author:** ![Thudlow\_Boink](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/thudlow_boink/32/320_2.png) [@Thudlow\_Boink](https://boards.straightdope.com/u/Thudlow_Boink)\ **Post date:** [August 16, 2010, 3:29pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/37 "2010-08-16T15:29:12Z")
I (as a kid in the 70s) remember learning that the mammals certainly included marsupials, and that platypuses were weird precisely because they’re mammals that lay eggs, and that all vertebrates are either mammals, birds, reptiles, amphibians, or fish. > [@Colibri](#): > > Neither monotremes nor marsupials are ancestral to placentals. Marsupials and placentals as groups are apparently of approximately the same antiquity; neither gave rise to the other. They Might Be Giants agree. [QUOTE=They Might Be Giants] Placental the sister of her brother Marsupial Their cousin called Monotreme Dead uncle Allotheria Mammal, mammal Their names are called They raise a paw The bat, the cat Dolphin and dog Koala bear and hog The fox, the ox Giraffe and shrew Echidna, caribou [/QUOTE] ---
**Author:** ![matt\_mcl](https://avatars.discourse-cdn.com/v4/letter/m/7ab992/32.png) [@matt\_mcl](https://boards.straightdope.com/u/matt_mcl)\ **Post date:** [August 16, 2010, 3:37pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/38 "2010-08-16T15:37:53Z")
> [@Zsofia](#): > > So nobody cares about “four chambered heart” anymore? That was always one of the characteristics I was taught as a child. (And that, duh, monotremes and marsupials are mammals.) It’s a _large_ four-chambered heart, maintaining the very high metabolism rate they have. (On preview: Hi, **Thudlow!** ) ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [August 16, 2010, 4:40pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/39 "2010-08-16T16:40:28Z")
Expanding on the four-chambered heart point: the one dinosaur fossil which _may_ have preserved thoracic soft parts (whether they are or not is hotly debated) is a Heterodontosaurus, a human-sized ornithopod which is exhibited about 20 miles west of where I sit typing, at the North Carolina Museum of Science. There is a concretion in the thoracic cavity that may or may not be the heart – and it appears to have four-chambered structure. (For anyone wondering why this is important, having four chambers allows for separate pulmonary and systemic circulation: deoxygenated blood is brought into the right atrium, pumped to the lungs by the right ventricle to be reoxygenated, then returned to the left atrium, then pumped out to the body by the left ventricle. Fish, amphibians, turtles, and lepidosaurs (lizards, snakes, tuarara) mix oxygenated and deoxygenated blood in a single ventricle. Birds, mammals, and crocodilians on land maintain the double circulation, but crocodilians have adapted to reuse the (partially) deoxygenated blood while submerged and not actively breathing.) ---
**Author:** ![Siam\_Sam](https://avatars.discourse-cdn.com/v4/letter/s/d78d45/32.png) [@Siam\_Sam](https://boards.straightdope.com/u/Siam_Sam)\ **Post date:** [August 16, 2010, 4:54pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/40 "2010-08-16T16:54:33Z")
> [@Fenris](#): > > Or did I just have some rotten teachers? > > In the early-to-mid '70s, I was taught that marsupials were their own class(? genus?–whichever the higher one is)-they were separate from mammals as they didn’t produce milk and that…monotremes (platypuses) were _certainly not_ mammals because they laid eggs and didn’t produce milk. > > Looking at wikipedia and other sites, that’s not the case now and nothing I’ve found has said that monotremes were ever classified as anything but mammals. > > Were monotremes (and/or marsupials) folded into the mammal class fairly recently (like Pluto being de-planeted) or did I just have bad teachers? Ya know, this sounds awfully familiar. I may have been taught that myself back in Texas, say slightly earlier than that time period. [Previous page](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342.md?page=1) [Next page](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342.md?page=3)