| Why are elephants so big?|
|Question Date: 2017-10-02|
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
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
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
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
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
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
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
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
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
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
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
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!
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