You are right, some animals get energy not just
from their food, but also from the sun. The most
common examples I can think of are reptiles. They
are cold blooded, which means that they heat
themselves using heat from outside. It's not so
much that these animals need to do it this way,
but that it is advantageous for them to do it that
way. Most animals that do that live in hot places,
and so the heat they get from the sun is pretty
much free. On the other hand, if you needed to use
food energy to heat your body, it would mean that
you'd have to eat a lot more food. So, in
environments where heat is easier to find than
food, it makes sense for animals to save their
efforts and rely on the sun to heat themselves up.
That's what crocodiles and large snakes do. They
can go weeks and months between meals and part of
that is because they don't need to waste food
energy to heat themselves up.
Animals don't need energy from the sun, not
directly anyway. Animals get energy from the food
they eat. However, that food either got its energy
by eating other food, or by photosynthesis, and
sooner or later almost all energy on Earth comes
from the sun. There are some ecosystems in the
deep ocean that use geothermal energy, but we use
sunlight, because we eat plants, or we eat things
that themselves eat plants, etc.
You are right that animals get their energy
from the food that they eat. Our bodies do not
get energy directly from the sun (other than
getting warmed up by standing in sunlight). But
without the sun, we would have no food. Plants
need the sun for energy and we eat those plants or
the animals that ate the plants.
At each level, a lot of energy is lost. Only a
tiny bit of all of the sunlight falling on Earth
is captured by things doing photosynthesis.
Plants, algae, and other photosynthesizers use a
lot of that energy for things like fighting
diseases, making chemicals, and other things that
don’t produce food. So if we eat corn, for
example, we are only getting a fraction of the
energy that was in light that hit the corn plant.
If we feed corn to chickens, then eat the
chickens, we get even less of the energy in the
light that hit the corn plant. The chickens use
energy eating and digesting, walking around,
fighting disease, and all of their other body
functions. A rough estimate is that about 10% of
the energy at one level of the food chain makes it
into the next link up the chain.
What happens to the rest of the energy? It
can’t just disappear. It becomes heat. The heat
spreads out. If we tried to gather that energy up
to do work, it would take more energy than we
would get, so the energy is “lost” to the
ecosystem, even though it doesn’t disappear.
We also use energy from the sun for solar
power. When we burn fossil fuels (gas, coal, oil,
etc.), we are burning the remains of living things
that got their energy from the sun, either
directly (plants) or indirectly (animals). These
things died millions of years ago and became coal,
oil, and natural gas. Even wind energy is caused
by the sun heating some places more than others.
Can you think of any forms of energy that do
not come from the sun?
Thanks for asking,
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