# Eli5 do animals that wait for their prey to come to them, also get 10 percent of energy from the animal that they eat?

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In school I was taught that you when you eat something on you only get 10% of the energy because 90% percent is used, looking for the food or other things. If this is wrong an explanation would be awesome.

In: Biology

Any random number applied to that many forms of life is bound to be wildly inaccurate.
My guess the teacher was trying to make a point that energy from different types of food is relatively low.

The 10% vs 90% thing is really just a basic guideline, and honestly a little misleading. Everything that we take into our bodies becomes energy in one way or the other.

A better analogy would be a gas tank. You fill your gas tank with a big meal, but if you have to drive all over the city to find your next meal, then your tank might he close to empty again before you fill back up.

Ambush predators, on the other hand, use far less energy, and as such dont have to eat as often. Some large snakes, and lizards can eat a large meal, and then not have to eat again for weeks, because of how stationary their lives are.

That is generally pretty true when you look at the caloric intake at each level of a food chain.

Say we have a simple food chain of some grasshoppers, a carnivorous rodent, and an eagle. Say the eagle needs 1000 calories per week to survive. In order to get that, it eats those rodents’ meat. Let’s say each rodent only has 100 calories of meat to eat, so the eagle has to eat 10 of them per week to survive.

Now here’s where that 10% comes in. Each rodent has to have eaten enough grasshoppers to grow to the size it did. For each calorie of meat it has, about 10 were eaten to get there. So a 100 calorie rodent has probably eaten 1000 calories of grasshoppers. And each 10 calorie grasshopper has eaten 100 calories of grass.

So in this example, an eagle are 10 rodents to get 1000 calories. But that pack of rodents ate 10,000 calories of grasshoppers get to that size. And the swarm of grasshoppers had to eat 100,000 calories total to get to the size needed to feed 10 rodents for their lifetime.

It’s inefficient, right? That’s why there’s so much hubbub about cutting meat out of our diets; for every 100 calorie serving of beef, the cow that it came from ate about 1000 calories worth of feed. (For me, I reduce my meat intake in order to reduce the waste. But… They’re just too tasty to cut out entirely.)

I put it in a reply elsewhere, but just for people to see, what everyone is referring to is related to [trophic levels](https://en.m.wikipedia.org/wiki/Trophic_level), which is more or less the tier of things in the food web.

A primary producer/level 1 is plants. Herbivores are level 2/secondary, carnivores level 3/tertiary and so on.

Each jump up a level will see a large decrease in the amount of calories gained relative to what went into the entire line. That’s what everyone is talking about. Herbivores need to eat a lot of plants to get their energy. Carnivores that eat them miss a LOT of what energy went into that herbivore when they eat them. Carnivores that eat other carnivores, like humans, lose even more. We get a really small amount of the total energy that went to bring us our food.

The main advantage is that meat and stuff is really calorie dense. So even though we missed a huge chunk in the process, overall, we need to eat a lot less.

Lettuce is a good example. You need to eat a literal truck load to get any real nutritional value or calorie quantity, but a steak has like, 200 calories.