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There are currently only five species of monotremes remaining, and all of them are found in Australia. Additionally, I read a science article claiming that competition from marsupials essentially forced monotremes into the water, where marsupials couldn't follow without drowning their young.

Why are monotremes so rare? Are the traits that define a monotreme (such as egg-laying) inherently less competitive, or is their relative scarcity more of a quirk of fate?

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  • $\begingroup$ Can you please link to the article you're referring to? $\endgroup$ – Remi.b Feb 18 '18 at 2:58
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    $\begingroup$ @Remi.b Sure, there you go. $\endgroup$ – Era Feb 18 '18 at 3:00
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When you stop to think about it there is one big reason live birth has an advantage. Lets assume with habe organism A and organism B, A and B both have offspring of exactly the same size and needing the exact same amount of resources (nutrients, etc.) A lays and egg and B gives live birth. A needs to have all of the nutrients its offspring will need all together at once because once it forms hte egg it cant add anything while organism B does not need it all, they can keep eating and supplying the nutrients over an extended period of time, as they gain it, so they need less "on hand" as it were. A and B may be putting the same amount of resources into their offspring but A does it all at once but B can spread it out over a long period of time meaning it does not need to build up huge reserves before breeding. As an added benefit if times are bad B can dump the offspring and try again later, saving all that would be wasted material for itself, so it can survive famine and drought better.

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Its hard to definitively say "why" some groups are more successful than others. Without replication, answers can simply be "Just-so stories". But there is some replication here. Monotremes have been eliminated everywhere they once existed, except for Australia, and New Guinea. There is a similar replicate of what happened to the marsupials in S. America when the Panama isthmus connected the eutherians of Central America to the marsupials in S. America; extinction of many of the marsupials in S. America. The same thing happened in Australia when eutherians were introduced, but on a lesser scale (so far). These are two examples of eutherians outcompeting marsupials. I know, not the same as monotremes. That monotremes found refuge in Australia may not be a coincidence in that they did not need to compete with eutherians. Marsupials not laying eggs may have been an advantage that allowed them to outcompete monotremes. But there is not similar replication to move beyond "Just so".

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