|If dolphins and whales are ocean mammals, how is
that whales are large and dolphins small? I am
asking this question because I read here that
whales being mammals can grow large as they
don't have the physical restrictions they would
have on land. So why does it not apply to
It’s great that you are questioning what you
read. That’s how we discover new things.
Mammals can be many sizes. A shrew only weighs
as much as a few paperclips. A blue whale is
the biggest animal that has ever existed on
Being in water ALLOWS blue whales to get that
big, but it doesn’t force them to be that big.
Bigger isn’t always better.
Blue whales filter tiny animals called krill
out of sea water. They basically swim up to the
krill, open wide, and swim right on through.
Then they squirt the water out of their mouths
and catch the krill on big comb-like things
called baleen. Krill can’t dodge and weave to
get away. Whales with baleen are called
Mysticetes and they tend to be very big.
Dolphins are hunters. They have to be fast,
smart, and agile to chase their prey, mostly
fish. If they were the size of blue whales,
they would be slower to move and turn. It would
be harder to catch the fast, maneuverable
fish. It would be like a tiger trying to catch
Sperm whales are the biggest whales with
teeth (Odontocetes). They specialize in eating
giant squid, so they may need to be big and
strong for this.
Odontocete whales tend to hang out in
groups. Any idea why they do that?
Thanks for asking.
That's a great question!
Although some whales can get large because the
buoyancy of water allows them to have less
restriction on mass than land-based animals, not
all mammals that live in the ocean get so large,
as you pointed out. Whales and dolphins are
both in the biological order of cetaceans,and
dolphins are actually in the suborder of toothed
whales (Odontoceti), Family Delphinidae
(Dolphin), which includes both common dolphins,
like the one you see off the coast of Santa
Barbara, and killer whales (Orcas). So dolphins
are a type of whale, just smaller than the
bigger whale species!
This difference reflects that different
species who have similar life history traits and
physiological traits (in this case marine
mammals) evolve different, specialized
characteristics (e.g. big, slow growing,
plankton-feeding baleen whales vs. smaller,
faster growing, predatory dolphins) to survive
in their environment and coexist with each
other. All organisms, both on land and in
aquatic environments do this.A familiar example
of this is the difference between domestic cats
and mountain lions.
Whales are larger than dolphins for the same
reason why elephants are larger than mice: the
physics of living on land versus in the ocean
impose a maximum size that an animal can attain,
and they also set a minimum, but between the
maximum and the minimum there is quite a range
of possible sizes. The actual size of an animal
is determined by its ecology, and dolphins have
different ecological preferences than whales
You asked an interesting question. There are
probably many reasons and events that led
dolphins and whales to evolve the way they have,
but let's consider an argument about obtaining
There are two broad categories in which we
can place whales: toothed whales vs. baleen
whales. Toothed whales typically hunt for their
prey, whereas baleen whales feed on prey low on
the food chain, and acquire their food in a more
passive manner. Let's think about what that
means in terms of anatomy: toothed whales that
hunt for their food end up being smaller and
more agile so they can catch their prey, while
baleen whales tend to be larger. That said,
let's think about how dolphins acquire their
food. Aren't most dolphins active predators,
like toothed whales? So wouldn't they also want
to be streamlined, and agile? In many cases,
dolphins may actually be of comparable size to
toothed whales. For the cases where they are
not, there are other factors that may also be
related to how whales obtain their food vs.
dolphins, which bodies of water you can find
them in, and so on.
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