Why don't any animals use photosynthesis?

Why aren’t there any large animals that use photosynthesis for energy? I know the easy answer is that photosynthesis wouldn’t provide enough energy for an elephant to move, but that doesn’t mean that an animal couldn’t supplement their food energy with photosynthesis.

Why has evolution decided that elephants can’t have leaves on their backs?

Just my WAG: Animals can move around to find food, but plants have nowhere to go. Getting energy from the sun is prety much their only option.

Most animals do use photosynthesis, either by eating plants that use photosynthesis, or by eating other animals that in turn have eaten plants. They just don’t do it themselves. Similarly, termites don’t digest wood that they have eaten: they don’t need to, because they have bacteria in their gut which digest the wood for them. Why evolve a complex metabolic process when there is another organism that will do it for you?

If you wish to consider a Protist as an animal (as opposed to a plant), then I give you the euglena.

There are a number of sessile or very slow moving invertebrates which use photosynthesis as a major source of caloric intake - most species of coral, a few species of clams and jellyfish, and probably some others I don’t know about. AFAIK all of them use symbiotic microorganisms to do the actual photosynthesis.

The short answer is that the ancestors of the earliest animals didn’t include photosynthetic metabolic pathways. It is likely that photosynthesis evolved only once, and all photosynthetic organisms are descended from the first photosynthesizers or commensial with those descendants.

So animals likely would never independently rediscover photosynthesis. An alterative would be for photosynthetic organisms to become more animal-like, or for animals to have symbiotic plants living in or on them. The most animal-like plants are the carnivorous plants that eat insects. But there are many animals that have photosynthetic algae in their tissues, especially coelenterates (coral, anemonoes, and jellyfish). Giant clams also have symbiotic algae. Most of these organisms are sessile filter-feeders that approximate the plant lifestyle anyway. Oh, and sloths have algae growing on their fur, but they don’t get any nutrients from that algae.

So why don’t more animals have symbiotic algae? Probably because the benefits are so low that only sessile organisms can get an appreciable fraction of their metabolic needs from symbiotic algae.

But wouldn’t any energy derived from photosynthesis be essentially free? I guess I see very little drawback to having a ‘green’ monkey… might even help camoulflage (sp?).

Sounds like a job for recombinant DNA. You think with a little gene splicing we could do ourselves up in green without significantly screwing up the rest of what makes us us?

“It’s not eee-zy being green…”

It’s not free for plants. They have to grow leaves, and that costs energy. Then those leaves are eaten by animals, so the plant might need to evolve other energy-using devices to discourage those animals (like thorns). They also have to compete against other plants in getting those leaves exposed to the sun, so trees grow tall enough not be shaded by other trees: that meansgy for the tree trunk, and energy getting water from the roots up to the leaves.

But carrying around photosynthetic algae is NOT cost-free. At base there is simply the weight of the algal tissue. Any tissue that you have to carry around is a metabolic cost. The energy output of the photosynthetic tissue would have to be greater than the metabolic cost of moving it around every time the animal moved. The photosynthetic tissues will also need to be provided with oxygen, CO2, nutrients and water. Plant cells require nutrients to grow, if the animal is going to be harvesting the excess production of the plant cells the plant cells will have to be supported. If you just find and eat plants how they get their nutrients and how fast they grow are not your concern.

And there are also non-metabolic costs. The animal has to evolve to where the algal cells are recognized as benign by the immune system and not destroyed. So that could compromise the host animal’s immune response. Also most symbiotic algae grows in organisms with transparent tissue. But most land animals have opaque skin to protect them against the sun. You’d have to evolve transparent skin that the algae live in, and a secondary opaque skin layer underneath.

Isn’t this also the case with plants (or at least their earliest ancestors? - Aren’t chloroplasts the remnants of captured, separate organisms?

Well, animals are, er… animated, mainly due to the need to access energy sources. If you can access an easily available and vastly abundant energy srouce without having to move… a kind of evolutionary lethargy sets in. Why bother hunting up new sources of food?

On the other hand, as already noted by Lemur866, if you’ve already progressed to being animated, your energy demands far outweigh what photosynthesis can deliver. It’s a marginal benefit, with unknown associated costs with maintaining photosynthetic ability.

However, we might be able to make use of purple membrane. It uses bacteriorhodopsin to make what is basically a light driven proton pump. None of this middleman business of cytochrome cascades and hypercyclic chemical roundabouts to make a sugar and then rip it apart… one step to a chemiosmotic gradient… two steps to ATP synthesis. Wonderfully efficient. And in an ideal example of “Why bother?” purple bacteria don’t seem to have the ability to do anything…

Yes, chloroplasts are derived from cyanobacterial (“blue-green algae”) endosymbionts. There are even some organisms that have “captured” chloroplast-containing intracellular symbionts, yielding former endosymbionts within former endosymbionts.

My high school biology answer is as follows,
Photosynthesis while being super neat doesn’t produce a whole lot of energy. It can fuel organisms just sit and grow and reproduce, but more complex organisms that think and move constantly need a lot more power.
So why hasn’t any organism evolved p/s to give it an extra energy boost? Mostly because it wouldn’t be effective. A good steak would probably produce more energy than a weeks worth of sunlight.
Plus, who wants to be green?

Hijak, but it’s my thread, so hey, its all good.

What other ways do organisms derive energy? I know that there are living things on the ocean floors that do something with either the heat or the sulfer, right?

So you’ve got photosynthesis, herbivores, carnivores, and sulfer munchers? Anything else?

Of course you have things like fungus, but in a strange way, that’s just another type of herbivore - as in an organism that gets it’s energy from plant material. What about those crazy lichens, on the rocks? Photosynthesis, or something else?

There are basically just two sources of energy for life on Earth. One is the sun. Most organisms derive their energy from the sun, either directly (as plants do) or indirectly (such as herbivores that eat plants, or carnivores that eat the herbivores). A very few derive their energy from geothermal sources–basically energy from tidal forces and nuclear decay. These organisms can be found on the sea floor clustered around underwater geysers called “smokers”. There are bacteria there that metabolize the sulphur compounds that are belched forth, and a variety of creatures that eat the bacteria and others that eat them, in turn. Here is one article one these hydrothermal vent ecosystems.

The technical term for the method the “sulphur munchers” use to fix carbon is chemosynthesis, as opposed to photosynthesis.

The technical term for an organism like a fungus, that gets its energy from formerly living organisms, is a detritivore.

Lichens are a symbiotic combination of a photosynthetic alga and a fungus. The alga provides food to the fungus, while the fungus provides nutrients to the alga and makes the structure of the combined organism.

I recollect that some chemosythetic organisms are also found in ‘terrestrial’ hot springs and geysers. (As opposed to ‘bottom of the sea’ hot springs). Though there are not enough to support the complex ecosystems which are found at some of the undersea vents.