John Hale, of Dunedin, asks :-

What determines the maximum size of an animal? Is it the bodily materials or the construction, as it might be with engineering?

Reg Dunlop, a retired robotics engineer at the University of Canterbury, responded.

There are several factors that limit the size of the largest animals.

1) The environment. Land animals experience different forces on their bodies than those that live in water or fly through air. A large land animal must be strong enough to manage gravitational forces whereas a marine mammal such as a whale has to be strong enough to resist the pressures experienced during diving. The weight of their bones is not so important as the bone strength. Birds must have bones that are very strong for their weight.

The load carrying capacity of bones depends on the area through the bone. Thus if the creature is made twice as big, the length of the bone is also doubled (x2), but the area becomes four times larger and the bone can carry four times (2x2) as much weight. The body weight depends on the volume of the creature, so if the size is doubled, the weight increases by eight times (2x2x2), and the bones would need to be eight times area i.e. almost three times thicker. The largest dinosaurs were probably getting near the size limits for land based animals moving about under the gravity of the earth. This isn't such a problem for wales which are very large. The gravity load is cancelled by the water buoyancy but the bones of a wale need to be strong enough to transmit the muscle loads and to withstand the crushing pressure of the ocean depths.

2) Muscle systems The heavier the load that the muscles need to move, the more muscle (and bone strength) that is needed. The limit depends on how fast the muscles have to move the load. Good examples of the limits can be seen in the dinosaurs that were very large: The meat-eating dinosaurs had to hunt and to move fast to catch other animals for food so cannot be as big as a the huge vegetarian Brontosaurus that moved quite slowly. It ate plants that couldn't run away.

3) Blood supply A heart is used to pump blood around the body, and to lift the blood high enough to reach the brain. This might be a problem for a Brontosaurus with a very long neck as considerable pressure would be needed to supply blood to the head (for the brain and jaw muscles) that was high above the ground eating tree tops. The arteries would need to be strong enough to withstand the blood pressure. It isn't such a problem for a whale as the head is on a similar level to it's heart so that not much pressure is needed to move the blood. Interestingly, some creatures have multiple hearts to share the load, but humans have definite limits which is why fighter pilots wear special suits to prevent them from blacking out during high "g" (gravity) turns.

(4) Oxygen Content Another aspect is the oxygen content of the air and the oxygen carrying capacity of the blood. For mammals, a log-log graph of the blood carrying capability of an animal's blood graphed against the animal's body weight (mass) is pretty much a straight line. Two obvious exceptions on the graph are chihuahua (not good blood for carrying oxygen) that are not good at running far, and horses (very good oxygen carrying blood) that are very good at running long distances.

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