Why does the Platypus and Echidna lay eggs inspite of them being mammals ??

Answer 1

You may offer all kinds of explanation to expound why and how, and go all out to give Lamarckian explanation of how suitable it is for them and what happened to the continental drift and what happened at the end of mesozoic era when mammals were evolving from reptiles. But ultimately you have to address this question to the genes. They have the genes that control vitellogenesis so that they produce yolky eggs. And they don't have the genes that would develop full fledged uterus to hold the embryo. It is all a matter of having some genes and not having others that make them egg-laying mammal.

Answer 2

The egg laying is the ancestral condition for all mammals - inherited from our reptilian forebears.

At one time, all of the mammal species laid eggs, but the development of other, internal systems like those seen in the marsupials (like kangaroos) and the placental mammals (like us) out competed them in most regions.

Critters like the platypus were able to survive because they had carved out successful niches for themselves, and were successful with the egg laying system.

In the case of the platypus, the lack of placental mammals in Australia meant that the egg-laying system was actually an advantage in it's ecological niche. Marsupials have a hard time with aquatic environment. It's hard to go swimming when you have little babies in a pouch - the babies tend to drown. So the egg laying adaptation allowed the platypus to exploit a habitat that marsupials couldn't effectively compete in.

With echidnas, they were specialized enough in the myrmecophagous activity of termite eating that they were able to thrive despite the 'handicap' of still laying eggs. In certain situations, the egg laying can still be an advantage too. Laying eggs means that the female isn't pumping metabolic energy into the young over the entire development period. This is very useful in situations where food is seasonally abundant, but scarce at other times. The energy for baby making can all be expended in one swell foop, rather than draining the mother slowly over time.

So the ancestral condition of egg laying remains a viable option for these species, and the characteristic is retained.

Answer 3

Ok, the problem here is the way that you're presenting your question.
Viviparity (=giving birth to live young) is not one of the characteristics required to define a mammal.
Thus, egg-laying mammals are just one subgroup of mammals that happen to have maintained the ancestral reproductive strategy, while other groups of mammals evolved into marsupials (with a brief placentation, and youngs kept in pouches) and placentals (the more "typical" mammals like us).

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EDIT: Haysoos' answer is fantastic, as usual.
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Daniel W: producing milk to feed the young is indeed one of the characteristics that define mammals, but breathing oxygen is something that pretty much all eukaryotes do ;-)
Here's how to diagnose a mammal by 4 easy traits:
1) Production of milk as primary food for offspring
2) Hair
3) Lower jaw formed by a single bone, the dentary
4) Middle ear with 3 ossicles
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km43dragon: Mammals are not particularly close relatives of birds and reptiles. They evolved within the Synapsida, a group of amniotes that split very early (during the Paleozoic) from the lineage that would eventually lead to the sauropsids (turtles, lizards&snakes, crocodiles and birds).

Answer 4

Platypus And Echidna

Answer 5

The duckbilled platypus and spiny anteater are regarded as one of the most ancient mammals, monotremes, since it has characteristics of both reptiles and mammals...though it does not have a placenta, like most modern mammals, it lays eggs,and when hatched are nursed by the mother, at a very immature state, and they suck milk that secretes through the glands of the mother.

Answer 6

Both the Platypus and the four species of Echidna are monotremes; which are mammals that lay eggs. It is believed that they are the "branch" of the evolutionary tree that lead to marsupial and placental groups. It is also theorized that they are closely related to birds and reptiles.

Answer 7

under the class mammalia, the are 3 main groups:
1)protheria- egg laying mammals, who show gynacomastism, i.e, the male of the species also produces milk.
2)metatheria- marsupials and pouched mammals
3)eutheria- placental mammals.
also, the mammals are defined as viviparous with a few exceptions..

Answer 8

Probably evolution just skipped them by. I think Australia is the only continent that has mammals that lay eggs, and since Australia is an island and a continent, there's not much interaction from species from other continents, so animals that are native to Australia tend to evolve differently than other species.

Answer 9

It is not a rule that all the animals that are grouped by Homo sapiens as mammals not to lay eggs.

Answer 10

The method of birth is not what defines mammals, the only two defining characteristics of all mammals i.e. Egg laying(Monotremes), pouched (marsupials) and all other mammals is that they produce milk to feed their young and they breathe oxygen.