2.1 Land versus water
Mammals share a number of biological characteristics that mark them out as members of the class Mammalia. Many of these are adaptations to a life on land. For example:
Mammals give birth to young at a relatively advanced stage of development and feed their young on milk.
Most mammals have hair, or fur, covering part or all of the body.
Mammals have a high metabolic rate and maintain a relatively high body temperature, irrespective of the temperature of their environment.
Most land mammals walk or run with a leg at each corner – a limb pattern they share with most other land vertebrates, including reptiles (except snakes and legless lizards) and most amphibians.
Mammals breathe air and have a four-chambered heart, allowing them to split their circulation into a system for passing blood through the lungs and a system for transporting oxygen-rich blood around the body.
Almost all mammals can swim to some extent – even bats – and many make their way into the water from time to time for one reason or another.
Activity 3 Mammals in water
Figure 4 is a series of photos that show an African elephant crossing a river and snorkelling using its trunk. Study the photos and make notes about what might tempt any land-dwelling mammal into the water in the first place, and the main challenges it would face in the new environment.
Answer
The obvious attraction for many would be food, but other mammals might take to the water to escape from predators, pests or extremes of temperature. The elephant is a land mammal, but it enters the water to drink or bathe and as you have seen, it can swim across open water using its trunk as a snorkel.
The main challenges for a mammal moving into water would be: holding its breath long enough to find and catch its food; moving about in the strong current; and staying warm in the water. A good example would be the sea otter, which holds its breath while it uses webbed feet and a rudder-like tail to search for food in shallow coastal waters, its thick fur keeping it warm enough to live in Alaskan seas.
The evolution of elephants
There are features of elephant anatomy that suggest that they might have adopted a semi-aquatic way of life at some point in their evolutionary history. The lungs of modern elephants are surrounded by loose connective tissue, not the bag-like ‘pleural cavity’ found in other land mammals, and this allows them to resist some of the damaging effects of water pressure. In addition, the kidneys of elephant foetuses possess features that are common in aquatic mammals but not land mammals. If the theory is true, the ancestors of today’s elephants must have moved into the water and then returned to the land, while their sirenian cousins went on to become fully aquatic.
You might have thought of other examples of aquatic mammals, but they all commonly face the same challenges. They are the result of matching their mammalian characteristics against a number of differences in the physical properties of air and water, the most important of which are listed below.
Water contains less oxygen than air does.
Water is denser and more viscous than air.
Water has a higher specific heat and a higher thermal conductivity than air (you will meet these terms later in Section 2.4).
Light is much more readily absorbed by water than by air, and visibility in water is often poor because suspended particles make it turbid.
Sound travels faster and further in water than it does in air.