# Did the definition of "mammal" change in the last ~30 years?
**URL:** https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342
**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:** 1
### Author:  [@Fenris](https://boards.straightdope.com/u/Fenris)
#### Post date: [August 15, 2010, 6:29pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/1 "2010-08-15T18:29:58Z")
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?
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### Author:  [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)
#### Post date: [August 15, 2010, 6:35pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/2 "2010-08-15T18:35:40Z")
I think you had bad teachers. I went to elementary school in the early 1980’s but our books tended to be a decade or three older than that. I lived in a rural area and everyone knew what a possum looked like up close and they looked like other mammals so there was no sense saying they were something completely different even though they are marsupials because it just wouldn’t seem true. We did get to hear about the platypus though and were told how strange it was. It was listed as a mammal but laid eggs and had the skeleton of a reptile. That sounded cool but there weren’t any around so we didn’t give it much thought.
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### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [August 15, 2010, 6:40pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/3 "2010-08-15T18:40:11Z")
I cannot recall any proposals to place monotremes and/or marsupials outside the Class Mammalia.
> [@Fenris](#):
>
> 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.
Given that you can’t recall the difference between class and genus, which aren’t even close taxonomic categories (there are two levels between them, that is, order and family), there is the distinct possibility that you are not accurately recalling what you were taught. For one thing, both marsupials and monotremes produce milk.
Traditionally, the Mammalia has been subdivided into three subclasses, the Protheria for the monotremes, the Metatheria for the marsupials, and the Eutheria for the placentals. Perhaps it is this subdivision that you are recalling. But they are all within the Mammalia. Any teacher who would teach anything else was teaching their own classification, not one recognized by any taxonomist.
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### Author:  [@Twoflower](https://boards.straightdope.com/u/Twoflower)
#### Post date: [August 15, 2010, 6:51pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/4 "2010-08-15T18:51:56Z")
Marsupials and monotremes _do_ produce milk. I concur with the bad teacher theory.
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### Author:  [@Fenris](https://boards.straightdope.com/u/Fenris)
#### Post date: [August 15, 2010, 6:53pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/5 "2010-08-15T18:53:35Z")
We were taught that to be a mammal you had to have 5 characteristics.
1. Live birth
2. Give milk/mammary glands
3. Warm blooded
4. Have hair.
5. Internal skeleton (I think–not 100% sure this was the final characteristic)
I remember that part clearly and the teacher was pretty adamant about how playpuses (and maybe marsupials) weren’t mammals because they didn’t fit on the list. I’m positive that’s what I was taught. I’m totally open to having been taught wrong, but I clearly remember spending a day or two in elementary school on either marsupials or marsupials and monotremes and being taught that they were one of nature’s GREAT MYSTERIES because they weren’t mammals but were so very close.
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### Author:  [@John\_Mace](https://boards.straightdope.com/u/John_Mace)
#### Post date: [August 15, 2010, 6:58pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/6 "2010-08-15T18:58:52Z")
> [@Fenris](#):
>
> We were taught that to be a mammal you had to have 5 characteristics.
>
> 1. Live birth
> 2. Give milk/mammary glands
> 3. Warm blooded
> 4. Have hair.
> 5. Internal skeleton (I think–not 100% sure this was the final characteristic)
>
> I remember that part clearly and the teacher was pretty adamant about how playpuses (and maybe marsupials) weren’t mammals because they didn’t fit on the list. I’m positive that’s what I was taught. I’m totally open to having been taught wrong, but I clearly remember spending a day or two in elementary school on either marsupials or marsupials and monotremes and being taught that they were one of nature’s GREAT MYSTERIES because they weren’t mammals but were so very close.
I think your #5 is wrong, and you’re thinking of vertebrates.
Also, it should be noted that while monotremes do produce milk, they don’t have nipples, as do marsupials and placentals.
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### Author:  [@Uncertain](https://boards.straightdope.com/u/Uncertain)
#### Post date: [August 15, 2010, 7:01pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/7 "2010-08-15T19:01:21Z")
> [@Fenris](#):
>
> We were taught that to be a mammal you had to have 5 characteristics.
>
> 1. Live birth
> 2. Give milk/mammary glands
> 3. Warm blooded
> 4. Have hair.
> 5. Internal skeleton (I think–not 100% sure this was the final characteristic)
>
> I remember that part clearly and the teacher was pretty adamant about how playpuses (and maybe marsupials) weren’t mammals because they didn’t fit on the list. I’m positive that’s what I was taught. I’m totally open to having been taught wrong, but I clearly remember spending a day or two in elementary school on either marsupials or marsupials and monotremes and being taught that they were one of nature’s GREAT MYSTERIES because they weren’t mammals but were so very close.
The “live birth” part is just wrong, and marsupials would qualify anyway because they do produce milk (as do monotremes, which, however, lack live birth).
Why would any of this be a great mystery, regardless of how we define “mammal”?
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### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [August 15, 2010, 7:01pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/8 "2010-08-15T19:01:56Z")
Marsupials have all those characteristics. Monotremes lack only number 1. The fifth characteristic doesn’t make a lot of sense to include, since it does not differentiate the mammals from any other vertebrate.
The actual defining characteristic of the Class Mammalia is not any of those characteristics, but having three inner ear bones, rather than one as in reptiles and birds.
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### Author:  [@DSeid](https://boards.straightdope.com/u/DSeid)
#### Post date: [August 15, 2010, 7:36pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/9 "2010-08-15T19:36:38Z")
> [@Colibri](#):
>
> Marsupials have all those characteristics. Monotremes lack only number 1. The fifth characteristic doesn’t make a lot of sense to include, since it does not differentiate the mammals from any other vertebrate.
>
> The actual defining characteristic of the Class Mammalia is not any of those characteristics, but having three inner ear bones, rather than one as in reptiles and birds.
Really? This seems like it _should_ be wrong. I can imagine another creature somehow convergently evolving a similar earbone structure but sharing no phylogentic history. I think [wiki](http://en.wikipedia.org/wiki/Mammal) has it right this time:
> [@](#):
>
> Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals).[2]
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [August 15, 2010, 7:48pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/10 "2010-08-15T19:48:14Z")
> [@](#):
>
> The actual defining characteristic of the Class Mammalia is not any of those characteristics, but having three inner ear bones, rather than one as in reptiles and birds.
I presume that’s used as the criterion because ear bones fossilize more easy than mammary glands? Are there any known organisms that give milk but don’t have three ear bones, or vice-versa?
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### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [August 15, 2010, 7:51pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/11 "2010-08-15T19:51:31Z")
> [@DSeid](#):
>
> Really? This seems like it _should_ be wrong. I can imagine another creature somehow convergently evolving a similar earbone structure but sharing no phylogentic history.
It’s correct. The fact of the matter is that none have, and the structure is so unusual that it would be virtually impossible for it to have evolved convergently in any other lineage. (In any case, I’ve simplified the definition a bit; it’s not just that there are three ear ossicles, but which jaw bones they developed from and the kind of jaw hinge that was the result).
> [@](#):
>
> I think [wiki](http://en.wikipedia.org/wiki/Mammal) has it right this time:
>
> > [@](#):
> >
> > Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals).[2]
You’re mistaking the abstract cladistic definition for the synapomorphy (shared derived characteristic) on which the clade is based, which is what I was referring to. The most recent common ancestor of those taxa would have had the synapomorphy of three ear ossicles.
There are other synapomorphies as well, such as hair, milk production, and homeothermy, but since these are not evident in the fossil record the operational one used by paleontologists is that of the ear ossicles/jaw articulation.
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### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [August 15, 2010, 7:53pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/12 "2010-08-15T19:53:57Z")
> [@Chronos](#):
>
> Are there any known organisms that give milk but don’t have three ear bones, or vice-versa?
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.)
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### Author:  [@DSeid](https://boards.straightdope.com/u/DSeid)
#### Post date: [August 15, 2010, 7:59pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/13 "2010-08-15T19:59:22Z")
Yet if genomic evidence somehow showed that an animal that has that ear bone structure had actually come from a line that had split off before that most recent common ancestor, would the fossil record be deferred to, or the genomic evidence? Isn’t using synamorphy is just a proxy for the better cladistic definition; used only because we do not have genomes available to compare?
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### Author:  [@John\_Mace](https://boards.straightdope.com/u/John_Mace)
#### Post date: [August 15, 2010, 8:02pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/14 "2010-08-15T20:02:36Z")
> [@Colibri](#):
>
> > [@Chronos](#):
> >
> > Are there any known organisms that give milk but don’t have three ear bones, or vice-versa?
>
> 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?
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### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [August 15, 2010, 8:51pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/15 "2010-08-15T20:51:48Z")
> [@DSeid](#):
>
> Yet if genomic evidence somehow showed that an animal that has that ear bone structure had actually come from a line that had split off before that most recent common ancestor, would the fossil record be deferred to, or the genomic evidence? Isn’t using synamorphy is just a proxy for the better cladistic definition; used only because we do not have genomes available to compare?
True. Actually, I should further clarify that the definition used by Wiki is narrower than the traditional one used by taxonomists:
> [@](#):
>
> Phylogenetically, Mammalia is defined as all descendants of the most recent common ancestor of monotremes (e.g., echidnas and platypuses) and therian mammals (marsupials and placentals).[2] This means that some extinct groups of “mammals” are not members of the crown group Mammalia, even though most of them have all the characteristics that traditionally would have classified them as mammals.[3] These “mammals” are now usually placed in the unranked clade Mammaliaformes.
That’s simply one way to define Mammalia; it is not the traditional one, and would not be universally accepted. In this case, they are defining the Class by working backward from extant groups. As the article mentions, the synapomorphies traditionally used to define the Linnean Class Mammalia predate the common ancestor of monotremes, marsupials, and placentals. The Linnean Class Mammalia as traditionally defined is essentially congruent with the clade Mammaliaformes mentioned by Wiki.
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### Author:  [@hajario](https://boards.straightdope.com/u/hajario)
#### Post date: [August 15, 2010, 8:54pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/16 "2010-08-15T20:54:20Z")
I remember being taught that there were three kinds of mammals:marsupials, egg laying and, um, placential (?). This would have been in the 1970s.
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### Author:  [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)
#### Post date: [August 15, 2010, 9:08pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/17 "2010-08-15T21:08:49Z")
While the marsupials and monotremes produce milk, they don’t use lactose as the sugar in milk (except for tiny quantities in certain species). That’s another reason why they are considered ancestral. From what I’ve been taught.
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### Author:  [@Colibri](https://boards.straightdope.com/u/Colibri)
#### Post date: [August 15, 2010, 9:20pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/18 "2010-08-15T21:20:09Z")
> [@Exapno\_Mapcase](#):
>
> While the marsupials and monotremes produce milk, they don’t use lactose as the sugar in milk (except for tiny quantities in certain species). That’s another reason why they are considered ancestral. From what I’ve been taught.
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.
Just how monotremes are related to other extant mammals is still controversial. While the ancestors of marsupials and placentals undoubtedly had egg-laying ancestors, it’s possible that they diverged from earlier groups separately from the monotremes and are not descended from them.
Lactose sugar in milk would be an example of a synapomorphy uniting the placentals.
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### Author:  [@qazwart](https://boards.straightdope.com/u/qazwart)
#### Post date: [August 15, 2010, 9:33pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/19 "2010-08-15T21:33:12Z")
> [@](#):
>
> The actual defining characteristic of the Class Mammalia is not any of those characteristics, but having three inner ear bones, rather than one as in reptiles and birds.
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.
By the way, Monotremes have always been considered mammals even though they don’t give live birth. If you learned differently when you were a kid, your teacher mislead you.
Don’t worry, that’s typical for elementary education. I remember seeing the Disney film on lemmings and how they jump off cliffs when the population gets too high. I believed that for years until i learned that Disney had faked the whole thing. The truth is that lemmings don’t jump off cliffs when the population gets too high. They just starve and get eaten.
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### Author:  [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)
#### Post date: [August 15, 2010, 9:39pm UTC](https://boards.straightdope.com/t/did-the-definition-of-mammal-change-in-the-last-30-years/550342/20 "2010-08-15T21:39:13Z")
> [@Colibri](#):
>
> Lactose sugar in milk would be an example of a synapomorphy uniting the placentals.
A synapomorphy? Oh, sy-nap! 🙂
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