Very interesting question, Emily. The size of animals depends very much on what animals eat, believe it or not. Animals that eat low quality (meaning, low calorie =low energy-containing) food tend to be large, whereas small animals tend to each food rich in calories. Cows eat grass, after all, and rodents prefer seeds and nuts.

Why is this? A cow (or elephant) is happy to eat a nut, if it happens to find one. But there aren't enough high-calorie foods in the environment to sustain a very large animal. (An elephant needs to find more calories per day to survive than does a mouse.) Small animals need to consume fewer calories per day overall than does a very large animal, so small animals can "get by" looking around for small amounts of hard-to-find, calorie-rich food.

Why don't small animals just eat lots of low-calorie, easy-to-find food (for example, grass) instead? Through an accident of geometry, it turns out that small animals burn more calories per unit volume than big animals do. That's because as things get smaller, they acquire more surface area per unit volume--the area of the surface of their bodies is large compared to how space their body actually occupies. This means than an elephant has less skin given how much it weighs, than does a mouse. Since a mouse has lots of surface area (skin) for its size, it loses heat (energy = calories) to the environment, meaning it needs calorie rich food. Mice can't eat enough grass (low quality food) to produce the calories needed to survive.

On the other hand, an advantage of being big, is that you can "get away with" eating large volumes of lower quality (lower calorie) food. There's plenty of that lower quality food around, because small animals can't eat it.

A good analogy would be the following: if you could eat nothing but carrots for a month, your body couldn't process enough to extract the calories you need per day to survive. As a consequence, you'd lose weight (so don't try this experiment!).

If you had a truck-load of carrots, however, an elephant would be perfectly happy. It would eat vast quantities of carrots, but because it has less surface area per unit volume (skin area compared to mass) than a person does, they lose less heat to their environment, and therefore are fine with eating large amounts of low calorie food.

Per unit volume, large animals are more efficient than small animals (it takes fewer calories to sustain 1 pound of elephant per day, than one pound of mice).

So...large body size seems like an evolutionary strategy for surviving on lousy food. Elephants eat stuff (bark etc.) that nobody else can survive on. The large size of some dinosaurs is probably result of the same phenomenon.

We are what we eat, perhaps more than you ever thought possible.

There are really two lines of thought involved with your question.

First, animal size is often tied to its trophic level, or what position the animal occupies on the food chain.

Plants are the primary producers, turning sunlight into energy.
Herbivores are the primary consumers, eating plants and turning plants into energy.
Carnivores then eat the herbivores, turning the herbivores into energy.

At each higher trophic level, there is a loss of energy.

You might notice that most carnivores are relatively small compared to herbivores, for example elephants and moose are the prey of lions and wolves, respectively. If predators were extremely large, they would have to eat a lot more prey animals, which means those predators would run out of food more quickly than smaller predators. Large herbivores also need to eat a lot of food. Adult African elephants eat between 200-600 pounds of food each day
elephant .

In fact, the ten largest land animals today are all herbivorous mammals. And the largest land animals to have ever existed were sauropod dinosaurs, also herbivores. For this reason, there are also not many large herbivores that an environment can sustain. And likely animals will not reach the large sizes that were once common during the Mesozoic (i.e. when dinosaurs dominated the land). Ocean dwelling organisms are not considered here because different environmental factors have dictated their evolution.

The second aspect of your question is how elephants became so large evolutionarily. A study on the maximum rate of mammal evolution, through documenting changes in the fossil record over the past 70 million years (Evans et al., 2011, PNAS v109 n11), predict that it would take 5 million generations of change for an animal the size of a rabbit to evolve to the size of an elephant. And microevolution still only lowers that evolutionary change to less than 1 million generations.

In summary, it takes a really long time to become so big. Predictions for why elephants evolved to become bigger often fall into the success for survival of the animals that were larger. The idea being that the larger elephants were better able to survive attacks and fend off smaller predators. Though this is not the only method for survival against predators.

Gazelles have the same predators as elephants, yet they are relatively small in size. Instead of size and strength resulting in survival for gazelles, agility and speed resulted in survival. Thus gazelle are smaller and fast.

Another line of thought to compliment the first, is that animals that live in colder climates also tend to be larger due to metabolic requirements for survival. This is more speculative, but wooly mammoths were in fact larger than modern day elephants. And since DNA analyses have shown that Asian elephants are more closely related to mammoths than to African elephants, and that African forest elephants and African Savannah elephants are two separate species also evolutionarily distant.

Good question! The short answer is that we don't know. Part of it, probably, is that elephants are big in order to avoid predation by lions, but lions can still kill elephants, and there may be other reasons as well. Elephants are hardly the largest land animals in Earth's history - some of the giant sauropod dinosaurs weighed as much as a herd of elephants, and traveled in herds themselves. This is an area of ongoing research.

Elephants have evolved to be larger so they can escape being preyed on by other animals, such as leopards, hyenas, wild dogs, and individual lions. Being larger allows prey to ‘escape’ from predation because they are too large to fit easily in a predator’s mouth and too large to be easily killed.

Why are mice so small? Why are we middle-sized? Plants and animals are the sizes they are, because the ones that grew to those sizes survived better and/or had more offspring than the ones that are other sizes.

Thanks for the great question!

Evolution by natural selection provides an answer to questions like the one you ask. If you see a trait in all members of a species, like the big size of elephants, it is possible that the trait is an adaptation favored by natural selection. This means that the trait has improved the chances of the animal’s genes of reproducing.

Scientists think that ancestors of the modern elephant that happened to be bigger than other elephants were better able to survive and reproduce. Therefore, the genes for being a big elephant were selected for, and on average, the big elephants became more common than smaller elephants. Being big was an advantage as it helps protect against predators that live in Africa and Asia where elephants live, like lions and tigers. In fact, elephants are so big that even a pack of lions cannot attack a single elephant. It is clear then, that being big, provided an advantage for these animals. Being big has its downsides, however, for example elephants need to find and eat a lot of food every day to support their massive size, up to 300 pounds of food a day!

The beauty of the theory of evolution by natural selection is that it uses a few principles that can answer many questions like the one you asked.

Thanks for the great question,

That's a good question. I think the short answer is that it comes down to evolutionary pressure. Elephants are herbivores, which means they don't eat other animals. Being an animal that doesn't eat other animals often means that other animals probably want to eat you.

However, it's much more difficult for say, a lion or a hyena to take down a large elephant than it would be to get a baby elephant. That's not to say that elephants are never caught by lions or hyenas -- just that because of their size, they are less likely to be eaten. I think it's important to clarify something about how evolution works though: evolution isn't about making the "perfect organism." Rather, evolution comes about based on organisms trying to survive in the given environment in which they live, given whatever resources and predators or prey they have to work with. The organisms which survive best and live to reproduce will likely pass on their characteristics to their offspring, which may benefit from them. In this way, traits are selected for based on circumstances, but "getting the job done" can often happen in ad-hoc and less-than-perfect ways. Hope this helps!