Subject: why are whales so big?

Mike Williamson (m.dalebout@auckland.ac.nz)
Fri, 19 Sep 1997 4:23:39 GMT+1200

Dear Louise, 

You asked why it was that whales were able to grow so big compared to 
land animals.

The secret to it all is their aquatic environment. The average blue whale 
for example is 85 feet long and weighs 106 tons, which is as much as 4
Brontosauri or 30 elephants. Other large baleen whales such as fin and
sei whales are much smaller than blue whales (tipping the scales on average 
at 58 tons and 13 tons respectively),but they are still considerably larger 
than the largest living terrestrial animal, the elephant. The reason whales
are able to support such weight is the buoyancy of the water in which they
live. 

The heavier an animal is the greater its relative surface area. The power 
of the legs and muscles which help support an animal is a function of the
surface of their cross-sections.  As the total weight of an animal increases
a point is reached where the legs would just collapse.  So once a certain
maximum weight is reached, life on land becomes impossible.  The situation
is quite different in water however, where the buoyancy counteracts the 
gravitational pull on the body. But note that even aquatic animals have
some limits on the size they can attain, for the surface area of the lungs,
intestines, red blood cells and kidneys becomes relatively smaller with
increase in total weight. This means that above a certain point the organs 
would not be able to handle the metabolic requirements of such a gargantuan
body, but the problems of food supply would probably set a limit on further
growth long before that point is reached.

You also asked how it was that whales differ from fish.

Whales and fish, although both supremely adapted to life in the sea, are 
different in many fundamental ways. Whales do look so much like fish,
 especially sharks, at first glance as they both have streamlined
body shapes, dorsal fins, flippers and large tails.In fact for a long 
time whales were actually thought to be 'spouting fish'. But whales are
actually mammals just like us,  and are descended from a cow-like 
ancestor which began to make the transition to aquatic life sometimes 
during the early Tertiary about 50 million years ago.  Being mammals,
whales are warm-blooded,breathe air, and give birth to live young which 
they nurse on milk.

Fish on the other hand never left the water, are cold-blooded, and use 
their gills to extract oxygen straight from the water.  The best way to 
tell a fish and a whale apart is just to look at the tail. A whale's tail 
is horizontal and moves up and down while a fish's tail is vertical
and moves from side to side.

Hope that answers your questions, Louise.

cheers

merel
***********************************
Ms. Merel Dalebout
Ecology & Evolution Research Group
Thomas Building, Level 1
School of Biological Sciences
University of Auckland
Private Bag 92-019
Wellesley Street
Auckland, N.Z.
Ph:09-373-7599 ext. 4588
Fax: 09-373-7417
e-mail: m.dalebout@auckland.ac.nz