# How to build a race which uses photosynthesis? [duplicate]

I've built a species that does eat, but I would also like them to get some of their energy from the sun. Consider the conditions to be identical to earth as far as the sun and atmosphere is concerned.

They are mammal-like, at least, and one of the side effects is that they don't get sun burns like regular humans do.

I haven't seen any creatures like this here on earth--humans might get some supplementary energy and vitamins from the sun (vitamin D) but these creatures, the more time they spend in the sun, the less they need to eat, provided they aren't expending more energy by working than they are gaining by soaking up rays.

Creature is, on average, 30-40 pounds at adulthood.

My question is this, can a system like this work within a hot-blooded human-like creature on a scientific level? If not, what are my stumbling blocks. If so, how should it work?

NOTE: I have seen this question in my search, but despite the title it doesn't seem to answer this question, especially given earth conditions.

## marked as duplicate by James♦, sphennings, Josh King, Frostfyre, JDługosz science-based StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); May 2 '17 at 16:16

• Is it even possible? And here for energy calculations. Ah, here’s the one I was looking for. Try doing a search. – JDługosz May 2 '17 at 4:55
• I feel this topic has been covered...a lot. – James May 2 '17 at 14:10
• @JDługosz yes, the recent question on the social impact inspired this one, because I had already built the race. I did do a search and I must not have put in the correct terms because it didn't give me much. These answers are great for this, I am definitely closing this as a duplicate. – Erin Thursby May 2 '17 at 16:13
• I noted the URL for the search I used. Click that, then reference the search term cheat sheet on the right, to see how it works. – JDługosz May 2 '17 at 16:19
• I also think you deserve recognition for marking your own question as a duplicate, once it was dug up. – JDługosz May 2 '17 at 22:06

They would have to sleep in the sun, with an exsposed black sail for absorbing the sun's rays. they would then have to fold it up into a skin pouch. They would still need certain minerals so i'd either have them eat certain plants, or make them eat rocks. I have heard that a human can live off plants concentrated in 1/2 acre and if you factor in the fact the plants use much of that energy to grow and protect against pestilence, you should be clear with a 20sqrft sail. Hope i could help.

• Welcome to the site, Robert. Numbers and links to references would help support the ideas presented here. Without them, this may be flagged as low-quality or become the target of critique from other users asking for such. Additionally, numbers and references often result in upvotes and reputation. I would suggest taking the tour to get a better understanding of how the site works. – Frostfyre May 2 '17 at 12:03

These are golden jellyfish Rather than catch food and eat it, they host photosynthetic organisms within their tissues and chauffeur them around.

Other animals do the same thing, including sponges and corals.

I do not think this happens with any vertebrate. There is no reason it should not. Vertebrates (especially warm blooded vertebrates) lead a higher energy lifestyle than sponges and so require more energy dense fuel than their surface area can easily generate from the sun. But - some vertebrates lie around a lot more than these jellyfish do. I could imagine a large amphibian or crocodile copying the lifestyle of these jellyfish, shuttling photosynthetic endosymbionts from place to place and acquiring the occasional bite of nitrogen containing food that the symbionts need. Crocodiles already do lie in the sun most of the time.

I could imagine a life stage where environmental circumstances cause an active organism to power down for a sedentary photosynthetic existence. \\

I did math!

• I found average sunlight power at Earth surface in 164 watts / m2 / 24 hours
• I found photosynthetic efficiency (conversion of energy to biomass) of 3 to 6%. Let us use 6%. 6% of 164 is 9.84 watts / 24 hours
• 1 watt = 0.86 kcal. 0.86 * 9.84 = 8.4 kcal.

1 m2 is a little generous: a 40 inch tall child weight 40 lbs has body surface area of 0.72 m2.

8.4 kcal is about 4 chocolate chips. They would have to lie in the sun quite a while. The creatures, not the chips.

Running it backwards to make sure it makes sense: 10 m2 plum tree, 180 growing days. 1800 * 8 kcal = 14400 kcal. 30 kcal / plum. 14400 / 30 = 480. If 1/3 of energy goes to tree and 2/3 goes to make plums that is a crop of 316 plums which sounds right.

Math is not my strong suit: educational corrections welcome.

• +1 for that clarification about the chips and the creatures 😂 – George Willcox May 2 '17 at 15:19