2.5 … and getting larger

Size offers a number of biological advantages, including some protection from predation, but it can also help in reducing heat loss. A large mammal has a large body mass (which generates heat) relative to its surface area (which loses heat). But there will be constraints on any evolutionary trend of increasing body mass in some aquatic mammals. Seals, sea lions and toothed whales have to remain agile enough to chase their prey, and the pinnipeds as a whole have to be able to move about on land, but they are all still fairly large animals.

Manatees live in relatively warm waters. Their metabolic rate is comparatively low and they normally expend relatively little energy on keeping their body temperature constant. Their large size (up to 590 kilos and 4 m in length) helps conserve body heat but they are very vulnerable to cold water, below about 15 °C.

Figure 10 Manatee

Show description|Hide description

A manatee feeding under water.

Figure 10 Manatee

The baleen whales, however, feed by filtering fish or invertebrates as they cruise the colder oceans of the world. As their prey is not chased, and their bodies are well supported by the density of the water, over the many millions of years of their evolution they have grown to immense sizes. The blue whale is the largest animal that has ever lived, a point that the following images will demonstrate.

Figure 11 The african elephant is the largest terrestrial mammal, at around 6 tonnes

Show description|Hide description

The African elephant is the largest terrestrial mammal.

Figure 11 The african elephant is the largest terrestrial mammal, at around 6 tonnes

Figure 12 The blue whale, at up to 150 tonnes, is the largest animal that has ever existed

Show description|Hide description

Thought to be the largest animal that has ever existed, the blue whale can weigh up to 150 tonnes.

Figure 12 The blue whale, at up to 150 tonnes, is the largest animal that has ever existed

Maximise

Figure 13 This comparative image then demonstrates the immense size of the blue whale, when shown to scale against the elephant, and a variety of other mammals

Show description|Hide description

This diagram, created on a computer, shows the scale of a blue whale in comparison to a variety of other mammals assembled in a row, including: gorilla, horse, brown bear, elephant, wolf, rhinoceros, polar bear, lion, tiger, bison, ostrich, hippopotamus, giraffe, human, dog. The blue whale looks roughly as large as all of them combined.

Figure 13 This comparative image then demonstrates the immense size of the blue whale, when shown to scale against the elephant, and a ...