Why aren't animals solar-powered?

March 12, 2004 7:49 AM

Why aren't animals solar-powered? [more inside]

There are two geese who live on the grounds where I work. As I was watching them this morning, I realized that they expend an awful lot of energy eating. That's all they do is walk around pecking until it's time to sleep. It occurred ot me that they would be a lot more efficient if they had evolved solar panels on their backs, so they could be consuming energy passively, which would give them a lot more time to worry about predators and make little goslings.

Nearly all the energy we encounter on a daily basis comes (indirectly, at least) from the sun. Plants consume solar energy, animals eat plants then die and become hamburgers or crude oil. Why not cut out the middle man? Why haven't animals evolved ways to take at least some of their energy from the sun directly?

These are the things I think about.

posted by jpoulos to grab bag (30 comments total)

Nature seems to have its own solar power working out for it--photosynthesis. But plants don't really do much, so my idea is that nature can't make an efficient enough system for getting solar power.

Just a though, and I Am Not A Scientist.
posted by angry modem at 7:55 AM on March 12, 2004

But take something like a butterfly. Lots of surface area, consumes very little energy. Strap a big mirror on that bad boy, and I bet you could make 'er go.
posted by jpoulos at 7:58 AM on March 12, 2004

I bet BigOil has something to do with it.
posted by bondcliff at 8:04 AM on March 12, 2004

i wonder if it's to do with how much air and water you need to process during photosynthesis? maybe a butterfly would get enough light, but not enough CO2 and H2O? interesting question.
posted by andrew cooke at 8:04 AM on March 12, 2004

I guess if you look at studies about Vitamin D, we are partly solar powered.

Big solar plates on our skulls and shoulders would be cool, too. Let's get to work on that.
posted by pomegranate at 8:16 AM on March 12, 2004

well we need more then just the calories right? We need the vitamins and amino acids that we then use the calories to combine into the enzymes and bio-junk (tm) that the body needs to function. So eating isn't just about the energy.
posted by dirtylittlemonkey at 8:27 AM on March 12, 2004

New line:
...and a solar powered pony.
posted by anathema at 8:28 AM on March 12, 2004

To generate any significant amount of power, solar requires a large surface area. For an animal to be powered by the sun, it would have to increase its surface area (grow leaves) and reduce its energy requirements (become sessile) -- become a tree, in other words. Read that somewhere, but can't recall where.
posted by mcwetboy at 8:35 AM on March 12, 2004

My biology is rusty and I'm having problems googling today but I think it comes down to efficiency. The solar related cycles net less ATP than the non-solar related cycles.

The photosynthesis process doesn't provide as much NET energy as the aerobic process. If plants had to move like animals do, they'd probably starve.

On the other hand, if animals used energy and resources to create systems that create ATP based energy from sunlight, they would just be taking away from more efficient non-solar processes. It'd work but me less effective.

Some animals are solar powered though in that they use thermal energy from the sun to help regulate body temperature. In a way, they ARE solar-powered just not in the way that we would normally think. Solar assisted perhaps.
posted by yangwar at 8:37 AM on March 12, 2004

The short answer is that there isn't enough energy in sunlight to do the job.

A human burns energy at about 3-4 kW.

The nominal (noon) value for sunlight is 137 mW/cm².

Assuming we have a surface area of about 1 m², that's about 1370 kw. Seems possible?

But now the kicker: photosynthesis is not anywhere near 100% efficient, more like 1%. That's the best system nature has been able to come up with.

So, we need about 3 kW, through photosynthesis we could get maybe 0.01 kW. There's at least 100 times less power in sunlight than we'd need to survive.

If we ate sunlight, we'd need to drop our metabolic rate 100 times. In short, we'd be plants.

(on preview mcwetboy rounds the bases first.)
posted by bonehead at 8:38 AM on March 12, 2004

On a side, but related note: I've been researching the potential of solar power panels to power a house, making it completely off-grid. What I've found is that using the most up-to-date solar power panels to source our power needs would run in the ballpark of $20-30k for the whole shebang.

It looks like the solar powered pony needs to be making some solar powered pony money.
posted by jazzkat11 at 8:43 AM on March 12, 2004

A human burns energy at about 3-4 kW.

where do you get that from? i just googled and found a basal rate of 1 calorie per kg per hour. a calorie is about 4000 J, so that comes out at 4000*65/3600 = about 100W (which is the number i had in my head - i thought we burn about the same as a lightbulb). light exercise is about the same again and heavy exercise 10 times greater. so by my numbers you're a factor of 10 too high. still doesn't make the numbers match up though, unfortunately.
posted by andrew cooke at 8:50 AM on March 12, 2004

jazzkat11 - so what's the expected lifetime of the panels and your annual power bill? if they last over 30 years and you're paying $1000 a year it's not so bad. but still probably better to wait a few years for technology to improve, i guess.
posted by andrew cooke at 8:53 AM on March 12, 2004

It's pretty simple, really: The diff. between animals and plants is that animals move around on their own volition. This means a) they're not gonna be staying in the same place at the same angle long enough to optimize solar energy, and b) they need more energy to do the moving.
posted by soyjoy at 8:56 AM on March 12, 2004

andrew cooke - The panels themselves last quite a long time, and are very durable. I don't recall the specific numbers, but the system does eventually pay for itself over time, albeit a slow repayment. Plus, depending on the sunniness of your locale, you may generate more energy than you consume and can sell the excess back to the power company for a meager price.

However, from what I've read, it seems that the technology for the panels is ever increasing, and the panels of tomorrow will be cheaper and more efficient than the ones of today.
posted by jazzkat11 at 9:10 AM on March 12, 2004

I'd argue that cold-blooded reptiles are the closest thing we've got to solor-powered animals. They seem at an obvious disadvantage given that it's nighttime for half their life, and cloudy for some of the days.
posted by jeffhoward at 9:17 AM on March 12, 2004

Quite apart from energy concerns, photosynthesis is really very difficult to do. The exact efficiency depends on whether you're talking about light falling on the leaves or on chloroplasts themselves, but either way it is very difficult to achieve - it requires the exact and optimal placement of pores for the entrance of air and carbon dioxide into the leaves, the correct shape, size and orientation of leaf and so on. Trying to achieve that in an animal that moves around and is trying to avoid being eaten is not evolutionarily wise.

Incidentally, I suspect that butterflies use up a rather large amount of energy - far more than could be gained by, say, wings that photosynthesized. Flying is a very tiring affair.
posted by adrianhon at 9:19 AM on March 12, 2004

jeffhoward: Right on. Frogs are much easier to stalk and catch than, say, squirrels, for that very reason.
posted by DrJohnEvans at 9:23 AM on March 12, 2004

andrew cooke - The panels themselves last quite a long time, and are very durable. I don't recall the specific numbers, but the system does eventually pay for itself over time, albeit a slow repayment.

The panels can last up to 30 years if properly maintained. The panels alone are also not the only source of cost, there's also metering, fitting etc. The payback period will depend on a number of factors, including incident light at your location, prevailing weather conditions, local levels of pollution (and how often you clean the panels, which in turn is an added cost), etc, and of course the price of the electricity you will no longer be buying from your local grid. Much will also depend on how connection to the grid is regulated in your neck of the woods. Rules which make it easy and provide you with a decent price for any electricity you put into the grid obviously result in shorter payback periods than if the local utility can screw you for connection (and this is pretty common currently) and your electricity is worthless.
posted by biffa at 9:29 AM on March 12, 2004

It has only been touched on tangentially above, but another advantage to not depending on sunlight is that animals can be active at night, in deep shade, in caves, underwater, and so on where there is not much sunlight, thus freeing up many more niches in the global ecosystem for animals to occupy
posted by TedW at 9:42 AM on March 12, 2004

this is why i love ask metafilter. great question, great answers.
posted by whatnot at 9:56 AM on March 12, 2004

Also of note on the solar powered home sidebar - some states, like California, have programs to subsidize the cost of a system.

I wish all states had similar programs - like mine for example.
posted by jazzkat11 at 11:03 AM on March 12, 2004

Andrew Cooke: you're correct, and worse, I knew that you're correct. The kilowatt figure came out of the dim recesses of my faulty memory.

In fact, Lance Armstrong estimates that his maximum power output is something like 600W.

An alternative estimate: if we consume 2500 (k)Cal/day, and energy in is equal to energy out, that's a power usage of about 120 W.

So, yeah, I was off by an order of magnitude. Still way too much for photosynthesis though.
posted by bonehead at 11:08 AM on March 12, 2004

Another key point is that animals look like we do and plants look like they do for a certain reason...The theory goes that we have evolved into the structure we are due to how we consume food, not the other way around. Plants usually have a relatively high SA/volume ratio, while humans is much lower. With most animals having such low SA/volume ratio's, simply put, we couldn't produce energy to sustain out high volume. As organism's digestive systems have become more evolved over the eons, the size (namely volume) of animals have typically grown in stride. Also, it is important to note an animal's real source of energy, production of ATP via the mighty mitochondria, did not evolve in animals...Rather, the leading theory goes that we gained our mitochondria via ingesting a bacteria (or some kinda' organism) that produced its own engery source, and over the years, the organism just stayed in the animal's system. I forget about plants (I THINK they evolved photosynthesis, but i forget exactly), but same idea with algae...Their ancestors ingested an archebacteria (or something like that) and devolped the power to photosynthesize, much like animals w/ mitochondria. This all has to do with symbiosis.
posted by jmd82 at 1:22 PM on March 12, 2004

On re-reading my post, I want to make clear that the archebacteria are thought to have actually evolved the ability to photosynthesize (supposedly also oxygenated our atmosphere), and it is the algae that ingested the archebacteria and devoleped the ability to perform photosynthesis though symbiosis.
posted by jmd82 at 1:42 PM on March 12, 2004

(me x solar power) + xbox = dead me
posted by Frasermoo at 2:19 PM on March 12, 2004

Its all about niches. You could also ask "why do complex animals exist when simple one-celled ones eat and reproduce just fine." Well, they don't, they're always competing so the blind hand of evolution pushes things into different niches.

Advantages of being an animal are that you can eat smaller animals, fight off parasites, and even eat plants who sit around all day and soak up energy. Even better most plants don't even have defenses against animals.

If you're really interested in this topic, Dawkins addresses it in a very easy to read way in his The Selfish Gene.

Also, its worth noting that you can't have complexity with such a weak energy source. You get a lot more energy eating plants and other animals than from the sun, especially if you can eat a few lbs a week.
posted by skallas at 8:57 PM on March 12, 2004

On a tangential note - simple passive solar solutions can be far more cost effective than solar panels. For example, a hyper-insulated box filled with jugs of water (to collect the energy), which has one insulated window facing south. It sucks up heat during the day and radiates the heat, with a simple fan system, at night into the main house.

But - to address jpoulos question directly - there hasn't been any need, apparently. The status quo has been sufficiently efficient. Plants accumulate solar energy, in the form of bound carbon, and then animals (ruminants) eat the plants. Then, predators eat those ruminants.

Or - the evolutionary lines which might have pointed in that direction were terminated by random catastrophe. Think "Wonderful Life" (Stephen J. Gould).
posted by troutfishing at 12:01 AM on March 13, 2004

some thoughts while walking home last night:

- arguing that animals need to be active at night is misleading, since energy can be stored in the body (they don't have to eat at night).

- it would be interesting to do the energy balance calculation for a butterfly, since it's not at all clear to me that the argument scales well to such extremes (i'm supposed to be working, or i'd do it myself...).
posted by andrew cooke at 7:23 AM on March 13, 2004

This question raises a fascinating corollary - would it be possible, through genetic engineering, to introduce photosynthetic cells into a animal, to produce a succesful hybrid?

It wouldn't be all that hard to be green if everyone was.
posted by troutfishing at 3:03 PM on March 13, 2004