Thread: How does evolution explain egg-laying mammals?

How does evolution explain egg-laying mammals; especially the platypus that has so many bizarre things about it? I can't seem to find this information online and would appreciate your insights. Thank you.

Hey there, egg laying mammals are called Monotremes. During the mesozoic period (approx 252 to 66 million years ago) is when monotremes probably split from the lineage that lead to other mammals. They are often placed in a separate subclass from other mammals as Prototheria. They retain many characteristics of their therapsid ancestors such as a complex pectoral girdle, limbs oriented with humerus and femur held lateral to body, and a cloaca. The skulls of monotremes are almost birdlike in appearance, with a long rostrum and smooth external appearance. Modern monotremes lack teeth as adults and lacrimal bones are absent. Monotremes have several important mammalian characters, however, including fur, but they lack vibrissae, a four chambered heart, a single dentary bone, three middle ear bones, and the ability to lactate.

For more information you can check out the Wiki:

Monotreme - Wikipedia, the free encyclopedia

Evolution tries everything that it can, given what it has, and keeps whatever works. Egg laying mammals work. But not as well as live birth mammals. Live birth is generally a prefered reproductive statgey.

Monotremes have several important mammalian characters, however, including fur, but they lack ...... the ability to lactate.

Not so. They do lactate. What they lack are nipples. The mammary glands simply excrete through the skin.

Wouldn't a better question be to ask why/when vivipary (live birth) evolved?

Because humans like scrambled eggs so if eggs never were evolved then we wouldn't be able to have them to eat.

Originally Posted by Strange

Wouldn't a better question be to ask why/when vivipary (live birth) evolved?

true - egg-laying in amniotes (which includes mammals) is the ancestral condition
some mammals have retained that condition (and 100 million years ago egg-laying would have been the more common mammalian trait), and some lineage have taken up viviparous reproduction, which then became the defining condition of most surviving mammals because marsupials and placentals became ridiculously successful and most of the other branches died off

it's not that uncommon for reptiles to take up viviparous reproduction, but because the trait is not overwhelmingly common, we tend to see reptiles as egg-laying (even though for instance ichthyosaurs were all live-bearing)

Originally Posted by ukjojo

How does evolution explain egg-laying mammals; especially the platypus that has so many bizarre things about it? I can't seem to find this information online and would appreciate your insights. Thank you.

Others have provided a lot of detail on this. But surely, an egg-laying mammal with birdlike features is just what one might expect if mammals are indeed descendants of creatures that also gave rise to birds. So in that sense the platypus is a sort of prediction of evolution, isn't it?

Originally Posted by exchemist

But surely, an egg-laying mammal with birdlike features is just what one might expect if mammals are indeed descendants of creatures that also gave rise to birds. So in that sense the platypus is a sort of prediction of evolution, isn't it?

except that in order to find the most recent common ancestor of birds and mammals you'd have to go back to the upper carboniferous at the very least to find the node where synapsids (leading up to mammals) and diapsids (leading up to reptiles, dinosaurs and birds) meet, hence the reason why there are egg-laying mammals has nothing to do with the fact that birds lay eggs, apart from the fact that it represents the ancestral condition of early synapsids and diapsids

presumably the main reason why birds are practically unique amongst amniotes never to have flirted with vivipary is that live young would become too cumbersome and that egg-laying and -brooding suits their lifestyle better (although vivipary doesn't appear to hurt bats, so maybe that's not the whole reason)