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As far as I understand, all the energy that living beings rely on comes from the sun. It's processed by plants in photosynthesis. These plants are consumed by herbivores, which in turn are consumed by carnivores. Energy trickles down that way, with photosynthesis being the crucial basis for all the other layers.

A couple of days ago I was wondering, how come animals don't do photosynthesis? This story talks about a few exotic animals that can do photosynthesis, but they seem to be an edge case.

It seems like there's a rule of thumb: Iff a creature can do photosynthesis, it can't move. Why is this so? Wouldn't it be beneficial for a herbivore-like mammal to be able to create sugars directly from sunlight? They wouldn't have to graze at all, just find water and sunlight. It's possible they'd have an easier time avoiding predators and expanding into more territory.

Can you think of a good reason why such creatures never became mainstream?

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It is all about energy.

It is estimated to take 5000 square meters of plants to feed the average person in the developed world, while the same source gives the lowest possible for a human under ideal conditions at 700 square meters. In comparison, the average person covers less than 1 square meter. While there is a lot of waste (animals are able to eat only a few percent of the energy the plant produces), the total energy available from photosynthesis is still orders of magnitude smaller than daily requirements for a typical animal.

Even in the case of the slug you linked (which you would expect would need about as little energy as possible for a mobile animal), it seems they obtain little or none of their actual energy from photosynthesis.

Hence you see photosynthesis restricted to life that has extraordinarily low energy requirements, which typically means being stationary (or moving very, very slowly as in the case some plants). As soon as you adapt to a mobile, higher energy niche, you need access to a lot more energy than photosynthesis can provide.

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  • $\begingroup$ I don’t think that completely answers the question though. Yes, photosynthesis alone is definitely not enough to sustain any animal with a non-trivial energy requirement, but what’s stopping animals from evolving photosynthesis anyway? Wouldn’t it be advantageous to have photosynthesis as a complement to energy from food? Besides, assuming photosynthesis in animals would also be chlorophyll-based, wouldn’t that give animals better camouflage in places like rain forests? $\endgroup$
    – Kal
    Commented Dec 1, 2020 at 10:13
  • $\begingroup$ @Kal Nothing is stopping animals from using photosynthesis (some do), but even in the case of the low energy slug, the amount of energy they can get from photosynthesis is so small it cannot complement food intake (see link). $\endgroup$
    – user1850479
    Commented Dec 1, 2020 at 13:31
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The amount of energy coming from the Sun is limited (about 750 W/m^2, perpendicular to the incoming light). It takes a comparatively large amount of energy to move around, so a photosynthetic animal wouldn't be able to move much or very often. (And in fact the known examples don't.)

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  • $\begingroup$ The key issue isn't the lack of incident solar energy, but the inefficiency of photosynthesis. The average human power output is only about 100W, but photosynthesis can only harness about 5% of the Sun's power, so it's still not enough. $\endgroup$
    – Nuclear Hoagie
    Commented Nov 30, 2020 at 16:09
  • $\begingroup$ @Nuclear Hoagie: I thought the inefficiency was a given. But note that even with much more efficient PV cells (over 20% for the commodity sort you'd put on your roof), electric motors, and nothing used for unrelated metabolic functions, it still requires wheels & very efficient aerodynamics to move things with solar power. $\endgroup$
    – jamesqf
    Commented Nov 30, 2020 at 17:14

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