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Photosynthesis does not provide enough energy to allow animal activities on its own (see this post for example). But can we think of a species using photosynthesis as a secondary energy source? Here is my question, as precise as I can write it:

For what purpose could an animal species make use of photosynthesis?

Actually, photosynthesis is so inefficient that it would be very accessory. One answer might be growing bones (like antlers for example) on a large time scale. Is this plausible? And especially, can you come up with other / better features?

Importantly, photosynthesis does not need to be the best way to achieve this purpose (coming back to my example, I know that it would not be better for deers to grow their antlers using photosynthesis). I am only looking for functioning designs (never mind if these designs cannot be evolved).

Moreover, for the moment I don't care about how photosynthesis would be performed by animals. Just assume they can. If needed, assume 30% of their body area contributes to it.

Also, assume we are on Earth, in the environment of your choice.

EDIT : As pointed out by @Renan, this already exists in our world. See this link for examples (including the amazing sea slug). However, I would like to focus on human-size species, although it's less realistic.

Argemione
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  • Hi, welcome to Worldbuilding.SE! Please take the time to read our [Tour] page is you havent yet. This seems like a useful question but can you clarify a few things? Firstly, how big is this photosynthesising creature meant to be? Deer size? Dog size? Elephant sized? Mouse sized? That will help to determine the plausibility of such a system. Also, what environment is this in, is this a temperate place, such as Europe, a desert such as the Sahara or set in the Arctic? – Liam Morris Apr 09 '19 at 15:46
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    It all depends on the ratio between surface area and volume. For big quasi-spherical animals such as people and cattle, photosynthesis is a non-starter, since it could not provide more than a pitifully small fraction of the energy requirements. (Instead of taking the trouble to use photosynthesis, the deer can make the same amount of energy available by sleeping a few more minutes per day.) For small animals such as some coral polyps, photosynthesis (with the help of symbiotic algae) is a way of life. – AlexP Apr 09 '19 at 15:51
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    ...being a sloth. Or a crocodile, or any reptile that sunbathes. – Mephistopheles Apr 10 '19 at 07:53
  • In Old Man's war novels series: "His new body is a younger version of himself, but genetically engineered with enhanced musculature, green skin, and yellow cat-like eyes..." – Rodrigo Menezes Jul 16 '19 at 16:15

7 Answers7

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Animals do it in our world, for sustainance.

Kleptoplasty or kleptoplastidy is a symbiotic phenomenon whereby plastids, notably chloroplasts from algae, are sequestered by host organisms. The word is derived from Kleptes (κλέπτης) which is Greek for thief. The alga is eaten normally and partially digested, leaving the plastid intact. The plastids are maintained within the host, temporarily continuing photosynthesis and benefiting the predator.

The article above mentions some unicellular organisms, but also some sea slugs such as costasiella.

Photosynthesis may be inneficient for animals like us and other larger ones. But due to the square-cube law, the smaller you get, the more surface you have relative to your volume. Therefore being really small (costasiella is 1 cm long), coupled with not being warm-blooded (and actually not blooded at all) and a diet that is not exclusively photosynthetic, can actually make animal photosynthesis feasible.

The Square-Cube Law
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    "inneficient for animals like us and other larger ones" possibly it could be argued as a potentially valuable supplementary food source ~ but I'd hate to have to try & work out the math on how many calories can be plausibly shaved from a humans diet if they had photosynthesis in their skin. – Pelinore Apr 09 '19 at 16:09
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    @Pelinore Randall Munroe already did it for cows, you may extrapolate from there. – The Square-Cube Law Apr 09 '19 at 16:28
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    Nice :) if a human could do as well as a cow (we're upright so catch less sun with our surface area but we're smaller.. so square cube law) not going to extrapolate further (too daunting) then 4% may not sound like a lot for an individual but taken across the entire world population of 7.53 billion (2017) that would be an extra 0.3 billion we could support without increasing our food production. – Pelinore Apr 09 '19 at 16:39
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    An average man needs 2500 calories to maintain their current weight, a reduction in fidgeting can shave 65 calories & starvation mode can shave another 55 calories ~ I'm unclear on exactly how many calories might be saved by remaining completely motionless lying on a beach in the sun all day so I'll go with those figures ~ so, if we assume 4% like the cow that's 100 calories a day so in clinical starvation mode with fidgeting cut out an individual would have a deficit of 2280 calories they'd need to eat normally. – Pelinore Apr 09 '19 at 17:17
  • @Pelinore fidgeting? Seriously? I'll count my fidgeting as workout from now on. – The Square-Cube Law Apr 09 '19 at 17:30
  • One estimation of the resting metabolic rate I've found is 1600 calories a day (I think we can count fidgeting as included in that) ~ so with a 55 calorie reduction for starvation mode & 100 calories from photosynthesis.. you get a deficit of 1445 calories you still need to eat ~ of course stretched out in sunlight all day you might get a few more calories from the photosynthesis. – Pelinore Apr 09 '19 at 17:44
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    ^ So we might imagine a species living in a highly seasonal environment that builds up fat reserves in the plentiful seasons & then lies in the sun the rest of the year subsisting on fat reserves & photosynthesis? – Pelinore Apr 09 '19 at 17:51
  • That looks like me except I don't do photosynthesis and I eat all year long. – The Square-Cube Law Apr 09 '19 at 17:54
  • Hibernation is the kind of creative ideas I was hoping for. @V.Aggarwal just posted something similar. – Argemione Apr 10 '19 at 08:14
  • photosynthesis may work better for cow, since most humans don't run around naked – Sonvar May 06 '19 at 05:24
  • @sonvar remember that humans once did. If we evolved to photosynthetise, we would forego using clothes. – The Square-Cube Law May 06 '19 at 09:55
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Reproduction

We're on Earth, in an area whose weather is cold and dry most of the year, with a short summer and long winter (and brief transitional periods).

We have a mammal, around the size of humans, with a short weaning period.

The species doesn't hibernate but individuals are able to slow down and survive without a lot of calories or fluid intake. Infants, however, are completely dependent on their mothers and need a lot of calories and fluids in order to develop the thick skin/fur they need to survive winter. Their only food/fluid intake is breastmilk and they need it for as long as possible.

Mothers can produce milk briefly as summer transitions to winter, but then their milk dries up. There is little water available to drink (it's frozen) and their food supplies dwindle fast. Their bodies go into survival mode, which means they can not afford the calories/etc to produce milk.

In a land like this, babies conceived at the end of summer and born at the beginning of the next summer have the best chance of survival. Pregnant adults have plenty of food and fresh water available at the start of their pregnancies, which is enough to bring an embryo to the fetal stage before their winter slow down begins.

If the baby is born just after the start of summer, there will be enough food and fresh water for the mother to have the energy to give birth and to nurse her baby. The body grows and develops all summer and, come winter, is old enough to withstand a winter (or at least to have a fighting chance).

How does the fetus grow?

Gestation requires calories too; about as much as breastfeeding in some stages. The fetus may be protected from the cold all winter, it may not need fluids (what it needs is already there and the mother just needs enough for herself), but it still needs to develop and to grow larger.

Some of the caloric needs will come from the mother's fat stores, but this isn't enough. She needs most of those for her own survival.

Photosynthesis

Photosynthesis regulates the reproductive systems. Like with plants and many animals, the systems know the season based on the day length and quality of light. Only mid-summer (and a bit later) light can stimulate gonads. Males produce high quality sperm and females ovulate.

Photosynthesis regulates fetal growth. The energy generated by photosynthesis isn't enough for an adult to survive. It also isn't enough for a child, because their body's surface area is too small. But an adult can generate enough energy for a fetus. The fetus already is in place and has done most of its development (the second half or so of pregnancy is more gaining size than it is outright development, though there is some of that too). Photosynthesis gives the fetus the extra it needs to come to term.

Cyn
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  • Unavoidably, photosynthesis could be replaced by something simpler here. But this is definitely the kind of cool and plausible story I was looking for. Thanks – Argemione Apr 11 '19 at 10:17
  • @Argemione Yeah I took your statement "I am only looking for functioning designs (never mind if these designs cannot be evolved)" literally. I'm glad it works for you. – Cyn Apr 11 '19 at 13:59
  • Many temperate climate mammals already have a system like this without photosynthesis. but if the evolution took this path early, it may be hard to shift paths to a season regulated hormone cycle. – Sonvar May 06 '19 at 05:16
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Vitamin D is produced when UV light from the sun interacts with chemicals in the skin. There could be critical chemical in the body that only form in sunlight. There are certain kinds of resins that harden in sunlight, an animal's skin could excrete UV activated resin forming a protective outer covering or to quickly seal wounds. There are plenty of different reactions that could occur in the body because of sunlight.

GiruŠatuku
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    The question is specifically about photosynthesis & you seem to have skipped any mention of that? – Pelinore Apr 09 '19 at 16:27
  • @Pelinore vitamin D production is technically photosynthesis. But I agree this is a rather bare bones answer. – John May 06 '19 at 00:59
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Photosynthesis does not provide enough energy for an animal to perform its daily tasks, but what if the animal does not require that much energy, for example during Hibernation.

For Sleeping, much energy is not required, just enough to keep the body alive.

So, Imagine a species that goes to hibernation but cannot store much food in its body, as a bear can, or there is just a shortage of food to start with. This particular animal eats as much as it can and then goes to hibernation relying on the Photosynthesis to provide additional energy.

Also, it does not need to sleep under direct sunlight, just someplace that is bright enough (no dark caves), like some plants that can survive indoors.

Now, we come to the time when this animal is not sleeping. At these times, this Photosynthesis energy can be used for individual tasks like Digestion.

Yes, your body needs energy to digest food itself, so this animal will rely on photo-energy to digest its food, because there is not enough food to waste energy, right ?? (I am not an expert on animal anatomy, so these are just my assumptions.)

Just make sure that the photo-cells are near the belly and back region.

V.Aggarwal
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    There's a technical term for an animal that would hibernate in a place sufficiently exposed to allow enough light in that would make photosynthesis work. That term is "food". – Keith Morrison Apr 10 '19 at 15:39
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    @KeithMorrison "That term is "food"" ;D one would presume this is a niche adaption somewhere safe from predators, somewhere isolated like the Galapagos for example, like the Dodo of course any "adjustment" to the environment from the arrival of foreign species & such could prove "unfortunate" :) – Pelinore Apr 10 '19 at 16:53
  • @Pelinore when animals become slow they are eaten. Coconut crabs eat birds that are too old or too ill to fly. A dodo that slept in the open for days would be esten by the local fauna, no need for an alien species to be introduced. – The Square-Cube Law Apr 10 '19 at 17:51
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    @Renan imagine a beach full of massive photosynthetic seals dozing in the sun as they wait for the annual migration of tuna that passes by their beach for two months of each year, they're carnivorous & eat anything smaller than them that comes close enough, the island is empty of all other animal life, because they ate it, there was nothing on the island originally that could prove a threat to them, like the Dodo they'll be fine, until something bigger & badder than them arrives on a floating log. – Pelinore Apr 10 '19 at 18:06
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Trace chemicals AKA vitamins.

Photosynthesis does not just mean generating calories. Multiple animals use photosynthesis to produce important trace chemicals. Humans use it to produce vitamin D although we can get it from food as well. But I could easily see an animal that uses it to produce something it can't get from food.

Howard Tayler's schlock mercenary comic had an alien species that used photosynthesis to make an important neural transmitter, and they actually become mentally unbalanced if denied sunlight for several days.

Oriental Hornet - Vespa orientalis use sunlight to generate an electric current, although no one is certain why.

John
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Possibilities would be to use the small energy to synthesize vitamins/nutrients not available raw in the environment (i.e. humans and vitamin D from sunlight), or to power protection against irritants like fleas or bacteria with a microvolt electrical field.

Patrick Hughes
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Trickle of energy from photosynthesis could, in theory, help sustain migratory birds or insects in flight. But on Earth there's always some food available on the sea/ground, and insects spread better by eggs.

Juraj
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