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Are dinosaurs better divers than mammals?

December 11, 2012
Great Auk taxidermy. Pinguinus impennis holds the reccord for the second deepest dives among non-mammalian tetrapods.

Great Auk taxidermy. Pinguinus impennis holds the reccord for the second deepest dives among non-mammalian tetrapods.

We all know that modern marine mammals can dive deeply. Elephant seals and sperm whales are among the few mammals that make a living in the abyssal zone, feeding miles beneath the ocean surface. Beaked whales are speculated to also dive as deeply. However, all these marine mammals have one thing in common: they’re considerably large animals, weighting more than 500 kilos. Their bodies are large and robust enough to withstand the water pressure, and can store oxygen more efficiently than any other non-avian animal.

Seabirds, by contrast, are much smaller animals. Since the Oligocene/early Miocene, no diving bird has weighted more than 45 kilos, and even the giant penguins and plotopterids of the Palaeogene might not have weighted significantly above 80 kilos, being relatively lithe animals. The hollow bones and airsacs of a bird could also be detrimental in an environment with several tons of sea water creating immense pressure on the body.

Yet, diving seabirds seem to be more suited for deep dives than mammals. The champion of deep dives are the Aptenodytes penguins, which manage to reach depths of 500 meters beneath the surface. After them comes the now extinct Pinguinus, which is thought to have been able to dive as much as 200 meters.

Compared to sea mammal reccords, this doesn’t seem much. However, aside from sperm whales, beaked whales and sperm seals, very few sea mammals reach depths of even half of the avian reccords. More importantly, both penguins and great auks don’t exceed 45 kilos, the latter in particular thought to weight about as much as just 5 kilos. If human weighted dolphins can’t dive anywhere as deep, to see a bird lighter than a domestic goose is very impressive, to say the least.

How exactly these birds can do this is not certain. Perhaps the very adaptations thought to detriment the dives, like airsacs, allow this to happen, as birds obviously can store oxygen more efficiently than mammals thanks to the pulmonary airsacs. Avian ribcages are also immensely robust, especially thanks to the evolution of the notarium.

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