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In a science-fiction story I am writing, there is a species of penguins that has evolved to consume seaweed rather than fish, arthropods, cephalopods, and echinoderms. They are my trolls. Also, they are descended from the world's largest carnivorous bird (in terms of volume and mass): the emperor penguin (Aptenodytes forsteri).

In real life, penguins belong to the Ardae superorder, and all members of this clade are carnivorous (I do not know if they are facultative carnivores or obligate carnivores, but the latter is more plausible). So, I wonder how could a species from the largest bird clade with only carnivorous species evolve to have a diet composed of 99 % seaweed.

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    $\begingroup$ If crocodiles can evolve into herbivores, anything can with enough time and pressure. nationalgeographic.com/science/article/… $\endgroup$
    – DWKraus
    Commented Aug 30, 2021 at 5:09
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    $\begingroup$ Animals in general can certainly evolve from carnivores to herbivores —- the ancestors of horses were carnivores. The specific case of penguins is a bit trickier, though. $\endgroup$
    – Mike Scott
    Commented Aug 30, 2021 at 8:15
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    $\begingroup$ Their eating apparatus would need to be completely redesigned. New jaw & teeth, much longer gut, possibly multiple stomachs. The end result would resemble a Walrus more than a Penguin. But not impossible, just impropable $\endgroup$
    – PcMan
    Commented Aug 30, 2021 at 8:25
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    $\begingroup$ Sure. They could become sentient and decide that eating fish is unethical. $\endgroup$
    – Lawyerson
    Commented Aug 30, 2021 at 14:56
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    $\begingroup$ @PcMan Who would believe in a flightless walrus? $\endgroup$
    – Barmar
    Commented Aug 30, 2021 at 15:04

2 Answers 2

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"So, I wonder how could a species of the largest bird clade with only carnivorous species evolve to have a diet composed of 99 % seaweed"

I believe every single animal on this planet can digest seaweed, but we want it to be effient? Nay?

In that cause the pinguins need longer intestines!

The good thing about herbivores is that they can digest plant and algae matter faster and better and at the same time they never lose the ability to digest meat, cause meat is just plant matter re-arranged.

Hence why humans eat plenty of meat without ever evolving any digestive adaptation for it and having an intestine comparable to a freaking panda.

But hey! What about dem vitamins? And there must be certainly some proteins that don't exist in plants? Right!?

Sure....but seaweed is often covered in small lifeforms and eggs, plently of animal matter right there...and they can always supplement with cannibalism once a month or a fish even.

Even the most extreme herbivorous animals ever need B12, but they eat off the floor! Fecal matter, contaminated water, buggs and worms are plently on the floor and they are all rich sources of B12.

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Doubtful

The nice thing about being at the top of the food chain is that the organisms lower down have done all the hard work concentrating energy into themselves. Consuming a herbivore is a much more energy-efficient activity than gathering, eating and digesting all of the vegetation that the herbivore had to consume, with the added bonus that the carnivore can extract the useful energy with a much shorter digestive tract. This lets lions (actually the lionesses) make a kill occasionally and spend the rest of the time lying in the sun, while the typical prey animal spends all of its time nervously eating and digesting grass except when it is actively running away from lions.

This is really important for penguins, which live in one of the most inhospitable places on Earth and exist right on the borderline of having a viable energy budget. A penguin swimming gracefully through the Antarctic water may look really pretty, but it is desperately trying to find food as quickly as possible so it can get out of the freezing cold water into the freezing cold but less conductive air where it loses heat more slowly. The penguins' insulation is really good, but it is losing heat faster when in the water than when it is out of it.

Switching from meat to seaweed would make the penguin spend longer in the water to gather the same amount of food which would result in it losing more heat (=energy). It would also require it to have a longer digestive tract, which would make it less efficient overall with the consequent additional disadvantage of being more vulnerable to predators.

Putting all this together - the penguins need to be in a much warmer environment with a reduced prey population but plentiful seaweed and less aggressive predators in order to even start evolving in the desired direction. This is a major problem, because flightless birds tend to go extinct rather than migrating successfully when their environment changes unfavourably, as has happened to various subspecies of penguins in the fossil record. I find it really hard to construct a scenario where penguins could evolve into the desired direction, although it might be possible to genetically engineer or selectively breed and train penguins in a controlled environment to achieve the desired changes over a very long period.

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    $\begingroup$ One possibility would be for the penguins to become very much larger, decreasing the surface to volume ratio, and thus heat loss. Whether they could survive on a seaweed diet is debatable, but they could certainly evolve to feed on plankton, as the baleen whales do. $\endgroup$
    – jamesqf
    Commented Aug 30, 2021 at 5:18
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    $\begingroup$ Have a look at Humboldt Penguins (South America) or the African Penguin (South Africa) - They don't live in arctic waters. While penguins are famous for living in Antarctica, they are equally found elsewhere in the southern hemisphere. $\endgroup$
    – DetlevCM
    Commented Aug 30, 2021 at 13:56
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    $\begingroup$ Pandas evolved to become herbivores when all evidence seems to suggest they were carnivorous or omnivorous in the distant past. $\endgroup$
    – J...
    Commented Aug 30, 2021 at 17:59
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    $\begingroup$ @J... Pandas are also trying very hard to go extinct. $\endgroup$
    – Mermaker
    Commented Aug 30, 2021 at 20:02
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    $\begingroup$ @T.Sar Whatever, they haven't yet succeeded at going extinct and the question only asks whether such evolution could happen. $\endgroup$
    – J...
    Commented Aug 30, 2021 at 22:24

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