Q:   Some mammals evolved from land-living to water-living because of the supply of food, as you suggest. But other mammals went extinct because of an inadequate supply of food. Do you know why some chose to return to the sea, and others did not?
— D. Tong
A:   A lot depends on the food available, the degree of specialization of the different species, the competition, and the selection pressure that would drive animals to seek meals from the sea. Many animals are highly specialized in their food habits. Koalas are specialized for eating the leaves of gum trees. If the gum trees were to go extinct, the koala would follow, unless a new mutation arose that allowed the koala to diversify its diet. Australian marsupials, with all of their great diversity, have not produced a semi-aquatic species. It may have been that when marsupials were forming new species in Australia, the platypus already was in the water and out-competed any potential marsupial swimmer. Much later a rat was able to colonize the water, but it occupied a slightly different niche than the platypus. The only semi-aquatic marsupial is found in South and Central America.

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