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Competently Mobile Photosynthetic Animal?

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I have an idea for a land based mobile organism that moves and uses photosynthesis for its primary means of energy generation. Can a motile organism conceivably get enough energy from photosynthetic processes as its primary energy source to move at a competent speed?

Said animal doesn't have to be elephant sized, something as small as a rabbit would be fine, but said animal would have to be as fast as average animals of its mass. For example, venus flytrap wouldn't count, it can't really move anywhere. Elysia Chlorotica doesn't count because of its speed, its heterotrophic tendencies, size, and ocean dwelling habitat.

According to this answer plants have low energy production rates. However it seems due to the square cube law, it would seem that for smaller animals even current efficiencies of photo synthesis should be more effective, especially for a cold blooded animal.

Additionally lots of plants are known to be primarily CO2 limited1, not limited by light, and indeed too much light can harm plants, causing plants to change in response, such as turning red. This leads me to believe that if an organism could make use of that extra energy, the efficiency of photosynthesis may be able to be increased to compensate beyond the levels seen by modern plants.

  1. Ecology of the Planted Aquarium, Walstad

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No matter what mechanism your animal uses for running its systems from sunlight, you still have the fundamental limitation of sunlight power per area (insolation).

Solar insolation at earth's distance is 1367 W/m2. About 30% of that gets reflected back into space by the upper atmosphere. That leaves 957 W/m2 hitting the ground near noon in the tropics on clear sunny days.

Actual insolation goes down from there rapidly with cloud cover, in shade, and of course at night. Look up some charts of insolation worldwide intended for estimating solar panel output. Then consider those assume a constant clear view of the sky, so they only derate for climate and latitude.

I just looked, and the desert outside of Phoenix Arizona is rated for 5.5 hours equivalent insolation per day. That is saying that due to sun angle, night, and average weather, you get about the equivalent of 5.5 hours of full sun each 24 hour period. That's about as good as it gets in the US, and brings the average incident sunlight power down to 219 W/m2 continuous equivalent.

This 219 W/m2 figure is assuming panels optimally pointed but at a fixed orientation. This is also not derated for being in the shade. To get this amount, you have to be in the open desert not under shade whenever the sun is shining.

And that is just incident power. There will be some inevitable inefficiency in converting the incident power to some kind of storage, distributing that throughout the organism, and then using it in each cell. Put in terms of typical earth animals, there will be loss converting sunlight to glucose, then more loss using the glucose to do work at each point of use.

How inefficient that process is, is hard to say. Plants are actually pretty inefficient, and nature has had a long time to tweak the process. Artificial solar cells are better, but even the best ones only convert 20% of sunlight to electrical output.

Let's be very generous and say you can hand wave 3.5 billion years of evolution having found a way to use 25% of incident solar radiation as internally available power. So given all this, your animal gets to run from 55 W/m2 on average in the Arizona desert, while always being in the sun whenever the sun is out.

Animals like this would evolve to be flat and thin. This maximizes the collection area relative to the mass that needs to be powered. They would also probably evolve to be able to point their collectors towards the sun. Such an animal would be cold-blooded, in current earth terms.

Imagine something turtle-like, flat and low to the ground, but with the ability to point its "shell" towards the sun. Let's say you have roughly a ½ meter by ½ meter such "solar turtle". It would need to run on 14 W average continuous. Is that enough? I'm not sure what a cold blooded animal that size would need, but it's at least in within the range of plausibility.

Some interesting characteristics that follow from above:

  1. It would spend most of the day in place following the sun by pointing its collectors appropriately.
  2. Point #1 means that it's a "sitting duck" to more active predators, so needs significant passive defenses. Turtle-like makes sense here again.
  3. It would move around during the night. There is no insolation to be had, so it doesn't matter if it's under something, and it doesn't have to worry about keeping the collectors aimed. It doesn't need to eat, but it does need water. This is when it goes around looking for water, or food sources with enough water in them so that it doesn't take too much energy to extract. Remember, removing salts and other dissolved materials from water takes energy.
  4. This is one of the few animals that have evolved to eat some of the local cacti, despite those cacti being poisonous to most other animals. However, it still prefers fresh water, or even puddles. Those take a lot less energy to purify.
  5. It's going to be slow. With 55 W average budget for a 1 x 1 m animal, there just isn't power available for running around. Fortunately, it doesn't have to run away from predators, or catch prey. It only needs to move around to good open locations in the day, and water sources at night.

So in conclusion, an "active" or "fast" animal is out. But something cold blooded and slow with passive defenses is at least plausible enough if accompanied with sufficient hand waving.

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