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Why are whales so big?
Question Date: 2009-11-03
Answer 1:

This is a really interesting question!

Some background on whales first:
Whales are mammals (like us), and are descended from a cow-like ancestor which began to make the transition to aquatic life about 50 million years ago. Because they are mammals, whales are warm-blooded, breathe air, and give birth to live young, which they nurse on milk. There are two broad types of whales:
(1) Baleen whales, which have a sieve like structure in their upper jaw made of keratin (the same stuff that makes your nails) that they use to filter plankton from the water. These whales are the largest, even though they feed on tiny planktonic creatures.
(2) Toothed whales, which use their sharp teeth to eat fish, squid and smaller marine mammals. This group is also able to sense their environment through echolocation.
Like people (and sea turtles) they tend to be long-lived, with life spans ranging from 50 years to over a century. Like sea turtles, many species of whales are threatened or endangered. Whales have been hunted for their meat and as oil sources for centuries and by the mid 1900s, industrial whaling had left many species of whales on the brink of extinction. Most countries have now banned commercial whaling and other threats to whales survival.

Whales are large animals. The biggest organism by volume and weight is the blue whale, an endangered species whose record length is 33.58 meters (110.2 ft) and 210 tons (for a pregnant female). Living in water makes such an enormous size possible. When you get in a pool, you float, because the buoyancy of water supports your weight. Whales are much, much bigger than you, but the same principle applies. For example, an average blue whale is 85 feet long and weighs 106 tons, which is about 4 times a Brontosaurus (dinosaur) or 30 elephants. Even the smaller baleen whales (such as fin and sea whales, at about 58 tons and 13 tons, respectively) are still much larger than the largest living terrestrial animal, the elephant.

Why is it so difficult to get so large on land? The bigger an animal gets, the smaller its relative surface area to its volume (its size). In other words, a larger proportion of their body mass is internal, putting increasing strain on external structures that hold everything together and support their weight. The power of leg muscles that support an animal depends upon the muscles surface cross-section (e.g. how wide are the leg muscles?) Once a certain maximum weight is reached relative to the animals volume, life on land becomes impossible because the legs would just collapse. Waters buoyancy works against the gravitational pull on the body, so animals that live in water have the potential to get bigger than land animals.

What keeps whales from evolving to be even bigger? Even aquatic animals are limited on how big they can get. This is because the surface area of their lungs, intestines, red blood cells and kidneys becomes relatively smaller with increasing total body size. This means that above a certain point, big animals cannot grow organs that are able to handle the metabolic requirements of their giant bodies. Finding enough food to support extremely large sizes also limits how big animals can grow.

Please write Science line or contact me if you have any more questions!

Answer 2:

It is not really known why large body size is evolutionarily advantageous. Of course there are a few theories. Large body size gives them advantages over predators and possibly increased intelligence (bigger body, bigger brain). A huge body might also be helpful during times where there's not a lot of food to eat (increased ability to live off body fat.) Whales also live in an environment that supports their large size. There is a lot of room in the ocean and also whales don't have to fight gravity like large land animals (think of an elephant the size of a whale!!)

Another interesting fact is that in mammals there is a correlation (or a relationship) between body size and life expectancy. Larger mammals tend (though not always) to have longer lives. Longer lives mean they may have more chances to reproduce or an extended ability to raise their young. Whales and elephants live long lives whereas mice and small fish have shorter lives. Smaller animals have a greater surface area and require a higher metabolism to keep themselves warm. Higher metabolism can lead to the build up of chemicals that cause cells to age. However, it is important to note that these are generalizations and is not applicable to every single animal.

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