|Why did large animal evolve in nature? Aren't
they kind of useless like too big need lots of
space and food?
|Question Date: 2003-09-26|
Size has distinct advantages and disadvantages.
Large animals, as you say, need lots of food and
space, and there are design complications with
supporting the animal. However, larger animals are
also unlikely to become prey for smaller animals,
and larger animals (if warm-blooded) also lose
heat slower than smaller animals (when you take
geometry, if you haven't already, you'll see why).
So, there are advantages to being big. Whales
are much bigger than we are so that other
creatures in the sea cannot prey upon them and so
that they retain their heat inside their bodies.
We, likewise, are bigger than ants, and for the
same reasons: an anteater is harmless (to us), but
it's a deadly terror if you're an ant!
Being a big animal has both advantages and
disadvantages. Big animals lose far less body heat
than small animals, and therefore don't need to
rely as much on consuming high calorie food. Big
animals conserve body heat very efficiently, and
often don't even need hair (an elephant barely has
any hair). They also can use their size for
defense, and can live in places that have large
fluctuations in heat and cold because they monitor
their own temperature so well.
For many, many
millions of years, dinosaurs dominated the planet,
and small mammals existed in relatively small
numbers. The benefit/cost of being a large animal
depends entirely on the current environment. Some
paleontologists think that the reason that mammals
thrived after the meteor that hit Earth and caused
the extinction of the dinosaurs is because they
were small enough to be able to hide from the
aftermath of the meteor strike. In that case,
being big would have been a very bad disadvantage
after many millions of years of large size being a
That's a great question. You're right that big
animals need a lot of food, so they need a lot of
space for that food to grow. On the other hand,
there are some benefits. For one, you may be so
big that no predators can kill you. Elephants do
not have any non-human predators. Even their
babies are protected by the herd. Sometimes
predators get together to attack large animals,
but they can usually only kill the weakest.
As any object gets bigger, the ratio of
its surface area (its outside) to its volume (its
inside) gets smaller. In other words, its inside
grows faster than its outside. You can see this
by calculating the surface area and volume of a
cube with 2-cm sides and one with 3-cm sides.
What this means to the animal is that it loses
heat much more slowly so they are much more
efficient with their food. Mammals and birds have
to spend huge amounts of energy to make heat.
Small animals like shrews and hummingbirds have to
make so much heat that they can starve in only a
couple of hours. Their hearts have to beat almost
as fast as a heart can beat. Huge animals have
very slow heart rates. So they need LESS food PER
KILOGRAM even though they need more total
So if you were a dinosaur, would you
rather be the biggest in the herd or the smallest?
Does your food source matter?
Does the environment matter?
Do your predators matter?
By the way, there were once dwarf
elephants on islands in what is now Greece. Why
do you think island elephants might be so much
smaller than their mainland relatives?
Organisms don't stick around too long if they're
"useless." In some cases being large probably has
advantages. Being big, for example, decreases the
amount of surface are an animal has (per unit
volume), which means they lose less heat (per unit
volume) than do smaller animals, which in turn
means that they can survive on poorer quality food
than can smaller animals.
Being huge also protects some animals from
Large animals are not useless - This is
attested to by the fact that big animals have
repeatedly evolved over time from smaller
ancestors in a really wide range of animal groups.
Some reasons include that
- when you are big, you have a
lower metabolic rate. Thus, while you need more
food overall, you need less food per pound of you
than if you were tiny
- when you are big, you
have a bigger stomach. Particularly among
herbivores, this means you can eat very low
quality food and then digest it over a long time
frame to gain nutrition. A small stomach cannot
- When you are big you do not
particularly have to worry about predators
When you are big you have a reduced surface area
relative to your volume
- so you lose less heat
from your surface. This is good in cool
places(polar environments; the ocean).
That's a good question, and I have both a short
and a long answer for you.
The short answer
is that animal species only ever evolve into forms
that work best in their environment. That's the
only reason any species ever evolves into any
other species. "Working best in their environment"
really means reproducing the most. Living longer,
finding more food, being stronger or faster, or
being better at hiding from predators all only
matter because these things allow an animal more
time to reproduce. Producing more offspring is,
ultimately, the only evolutionary advantage that
Sometimes a species will evolve into a
new species not because the new species is better
able to reproduce, but just because the new
species is not WORSE at reproducing.
longer answer I have is a list of possible reasons
why being big like a dinosaur or a blue whale
might allow some animals to reproduce more. In the
ocean, there are fewer disadvantages to being
huge, since you can float in the water and let it
support your weight instead of having to have your
legs support your whole weight. The food that blue
whales eat is extremely abundant--there are
literally tons of it everywhere. Being unable to
find enough food is often a thing that stops
animals from getting bigger, but large whales
don't have that problem. Also, the huge size of a
Blue Whale protects it from most of the ocean's
predators, since few of them are big enough to
kill and eat them. There might be other advantages
to being that big, or it might be simply that
there aren't many disadvantages.
probably had similar reasons for evolving into
such big species,but scientists don't really know.
The ones that lived on land didn't have the ocean
to help support their weight, but the warmer
climate probably made it easier for them to
maintain their body temperatures (whether they
were cold-blooded or warm-blooded). The body
temperatures of small animals change much more
quickly than those of large animals. Generally,
though, I think the fact that there are such huge
marine animals but so few huge land animals means
that being huge on land doesn't work as well as
being huge in the ocean.
So, remember the
short answer: all species only ever evolve into
new species if doing so makes them better at
reproducing (or, at the very least, if it doesn't
make them worse at reproducing). Also remember
that individual animals don't evolve, animal
SPECIES evolve--and that evolution happens over
many generations. Keep those things in mind, and
you'll be on the right track in thinking about
evolution! And keep asking great questions!
"Useless" is a little harsh, don't you
Things evolve to be big because that's one of the
only sure-fire ways to avoid getting eaten. The
blue whales are larger than the largest predatory
animals in the sea:
killer whales and white
sharks. Sure, if enough killer whales and white
sharks ganged up on a humpback whale it might not
be able to escape predation, and humans have
certainly evolved ways of killing blue whale with
guns and harpoons, but even then, it still
involves a huge struggle.
has its advantages. In fact, you could have asked
why aren't most animals as big as blue whales?
Most animals are limited in their size by
something. Insects, for example, are pretty much
limited in their size by the fact that they don't
have lungs. Most insects rely on oxygen diffusing
into their bodies through tiny holes in their
sides,near the legs. They can't just take deeper
breaths to get more oxygen. In prehistoric times
when there were lots more plants, there was
actually more oxygen in the atmosphere than there
is now, and insects were bigger (imagine fending
off four inch cockroaches and giant predatory
centipedes five feet long and a foot wide!). Blue
whales have an advantage in the ocean because
water bears most of their weight, and they don't
need to worry about having massive skeletons to
support all the blubber, but the dinosaurs showed
us that it was definitely possible to have
skeletons strong enough to support lots of weight.
It does seem like many organisms in the past were
larger: there are fossil penguins from Antarctica
that stood almost 6 feet tall. On the other hand,
humans have been getting taller since we first
evolved, probably as our diet improved because of
agriculture and tools for hunting. We may be 20
feet tall someday.
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