Why Are Marine Mammals So Big?

Marine mammals are famously large, but why is that? And is there a polar bear-sized sea otter in our future?

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Sources:

Aquatic mammals need to be big, but not too big


http://www.pnas.org/cgi/doi/10.1073/pnas.1712629115
https://www.nationalgeographic.com/animals/mammals/b/blue-whale/
https://www.smithsonianmag.com/science-nature/how-did-whales-evolve-73276956/
http://www.uvm.edu/~dstratto/bcor102/readings/01_phylogenetics.pdf
https://www.nature.com/articles/ng.3198
http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/thrcn.html
http://www.jstor.org/stable/10.1086/303400
Media Sources:
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https://commons.wikimedia.org/wiki/File:Mustela_nivalis_-British_Wildlife_Centre-4.jpg
https://commons.wikimedia.org/wiki/File:Humpback_whale_by_Christopher_Michel.jpg
https://commons.wikimedia.org/wiki/File:Manatee_and_fishes_underwater_photo.jpg
https://commons.wikimedia.org/wiki/File:Lev-Beh-Ziz.jpg
https://commons.wikimedia.org/wiki/File:Dark_Dolphin.jpg
https://commons.wikimedia.org/wiki/File:London_MMB_%C2%BB189_Natural_History_Museum.jpg
https://commons.wikimedia.org/wiki/File:Sea_otters_holding_hands,_cropped.jpg

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