Why are platypusi considered mammals even though they lay eggs?

Question

My husband doesn't really understand why it is classified as a mammal if it lays eggs. Any help?

Answer 1

Basically, because live-bearing reproduction is not one of the diagnostic (=exclusive) characters that define mammals. Classification of groups of animals is based on shared derived characteristics (note the "derived" part: all mammals have four limbs, but that's a primitive characteristic and thus not useful for this).

Mammals are defined by having:

- lower jaw formed by a single bone, the dentary (that's how you can tell a mammal from the jaw alone, if you find a skull)
- three ossicles (small bones) in the middle ear (other tetrapods have only one)
- hair (some birds might have hair-like feathers, but they're not homologous; neither are the hairs of insects or other invertebrates, and of course not those of plants)
- mammary glands


As you see, the platypus fulfills all of these characteristics. And no other vertebrates possess them.

In contrast, other characteristics are not exclusive, and thus are not used to define what a mammal is. For example, birds and some fishes are warm blooded too, crocodiles and birds have 4-chambered hearts, and many fishes and lizards/snakes are viviparous.

More about mammals, from the Animal Diversity Web:
http://animaldiversity.ummz.umich.edu/site/accounts/information/Mammalia.html

PS. I would not consider the worldwide consensus of biologists as a "bunch of crusty scientists". Let me emphasize: homeothermy (warm-bloodedness) is NOT a characteristic of mammals, because 1) it occurs in other groups and 2) it's not possible to determine without doubts in fossil mammals (although it may be inferred from other features). Even if it's considered as a derived character, it's not easy to know where/when this characteristic appeared in the lineage from primitive synapsids ("mammal-like reptiles") to the mammals.

Answer 2

A lot of great answers but I'll add my own thoughts as well.

Taxonomy (not Taxidermy which is the preservation of animals by stuffing them) is the science that studies classification of all living organisms by a system that determines placement by similar characteristics and lineage.

Evolution does play a part in understanding this tree-branch like system. There are 3 different species of Monotremes and it is believed that the Platypus and 2 species of Echidna are descendents of the earliest mammal/mammal-like creatures that began to evolve from reptiles millions of years ago (first fossil evidence puts the first monotremes at the Cretaceous period.

To tell you the truth, Taxonomy is not the most precise of sciences dealing with a great deal of lack of genetic information and difficulty piecing together actual time lines. Taxonomists constantly shift the placement of living things on the Classification Tree over time as more gaps get filled by fossil and DNA evidence.

Quite simply, the reason why the platypus is a mammal is because a bunch of old crusty scientists said so. The platypus' great ancestor's were the beginning of the mammal family tree and as a species were still successful enough on the continent of Australia that they didn't die out to the various extinction events.

And like others have stated before, according to scientists, live birth is not one of the set criteria for mammals.

--Having mammary (mammal comes from this word) glands aka breasts
--Being able to thermo-regulate (warm-blooded).
--And having hair/fur

Answer 3

All mammals produce milk, have hair, and a warm blooded. Egg laying mammals form a group of mammals called the monotremes. This is a small group consisting of Echidna and platypus (both from Australia and Papa New Gunie). Humans fall into the group of placental mammals and the 3rd group are the marsupials (eg. wambat) from Australia and South America

Answer 4

Mammal characteristics: 1. Hair 2. Nurse their young 3. Warm-blooded 4. Use lungs to breathe 5. Give birth to live young. The platypus falls under all but one category. Therefore it is a mammal, but in its own special category - monotreme. The platypus and the echidna are the only mammals that fall under that category. It's just a better way to group them together. Just like a kangaroo is a mammal, but it's considered a marsupial because it carries its young in a pouch.

Answer 5

They are among the most primitive mammals... but they are not the only mammals that lay eggs. See link below describing monotremes.

Aloha

Answer 6

Platypus are mammals because they have hair, mammaries, and other physiological features characteristic of mammals. There are three basic types of mammals: monotremes (egg-laying mammals, currently platypus and echidna); marsupials (pouch-rearing mammals such as kangaroos, opossums, wallabies); placentals (internally forming young such as humans, canines, felines, rodents, whales, seals...all the "typical" mammals).

Answer 7

Because they give milk to their young. This is the definition of what a mammal is. Mammal, mammary, breast, milk.

Answer 8

because other than that,they share all their other features with mammals.Platypusses are actually living fossils...all the other egg-laying mammals(except echindnas)went extinct

Answer 9

bcoz the female has mammary glands & feeds its young on it's secretion, & they have fur on their body: like all other mammals.
Platypus are one of the ancient types of mammals.

Answer 10

~Gee, such a poser. Why are penguins considered birds even though they can't fly, but bats are mammals and they can?