Adaptations to the environment

A cactus and a seal

A cactus in a desert and a seal in a polar sea look nothing alike. Yet each is well fitted to its home. We call such a fit an adaptation.

An adaptation is any feature of an organism that helps it to survive and to reproduce in its habitat. It can be in the shape of the body (morphological), in how the body works (physiological) or in how the animal acts (behavioural). Most adaptations took a long time to evolve, and they are carried in the genes.

Adaptations to a dry place

A desert has little water and a hot day. Here are the standard examples.

• Opuntia, a desert plant, has no leaves in the usual sense. The leaves are reduced to spines. The flat stem does the photosynthesis. The stem has a thick cuticle, and its stomata are sunk in deep pits. All these reduce the loss of water.
• The kangaroo rat of the North American deserts lives without drinking. It gets all its water from the oxidation of fat in its body. Water is a by-product of that reaction. It also makes very concentrated urine, so little water is lost in removing wastes.
• Some desert lizards bask in the sun when it is cool, and move into the shade when it gets hot. Some animals burrow into the soil to escape the heat above the ground.

Adaptations to cold

Heat is lost through the surface of the body. A body of the same shape with a smaller surface loses less heat.

• Allen's rule: mammals from colder climates usually have shorter ears and limbs. Shorter parts have less surface, so less heat is lost.
• A seal and other aquatic mammals of the polar seas have a thick layer of fat, called blubber, under the skin. It acts as an insulator and cuts the loss of body heat.

Small bodies are at a disadvantage. A ball of radius r has the surface area $4\pi r^2$ and the volume $\tfrac{4}{3}\pi r^3$. The surface for each unit of volume is

So the smaller the body, the more surface it has for each unit of volume.

Adaptations to a mountain

You may have felt unwell on a high mountain. Many people feel nausea, tiredness and a fast heartbeat at places such as the Rohtang Pass near Manali or at Mansarovar in Tibet. This is altitude sickness. The air has the same share of oxygen as at sea level, about 21 per cent. But the pressure of the air is lower, so each breath brings less oxygen.

After some days the body compensates in three ways.

1. It makes more red blood cells.
2. The binding capacity of haemoglobin decreases, so it gives its oxygen to the tissues more easily.
3. The breathing rate increases.

A change of this kind in one person, over days, is called acclimatisation. It is not the same as a genetic adaptation. Many mountain peoples have both.

This graph uses the standard atmosphere. NCERT gives no pressure values.

You should find that the area for each cm³ falls as the radius grows, and it is 3 divided by the radius. A body 8 times wider has 8 times less surface for each cm³.

Try it in Python

The program makes a table of the surface area, the volume and the area for each cm³ of balls of five sizes.

Practise with new numbers

Common mistakes

• Saying that an adaptation is a change that happens in one day. It is a feature that has evolved over a long time. A short change in one person is acclimatisation.
• Saying that the air has less oxygen at high altitude. The share is the same. The pressure is lower.
• Mixing up the kinds of adaptation. Basking in the sun is behavioural. Blubber is morphological. Concentrated urine is physiological.
• Applying Allen's rule to the wrong climate. Shorter ears and limbs go with the colder climate.

Remember these

• Adaptation: a feature that helps the organism to survive and to reproduce. It can be morphological, physiological or behavioural.
• Opuntia: spines, a flat stem for photosynthesis, a thick cuticle and stomata in pits. Kangaroo rat: water from fat, concentrated urine.
• Allen's rule: shorter ears and limbs in cold climates. Seals have blubber.
• At altitude: more red cells, less haemoglobin binding, faster breathing.
• A ball has $3/r$ of surface for each unit of volume.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Give one morphological and one physiological adaptation of a desert organism.

2. State Allen's rule.

3. What does the blubber of a seal do?

4. Name the three changes that the body makes at a high altitude.

5. A ball has a radius of 4 cm. What is its surface area for each cm³ of its volume?

References

• NCERT (2023). Biology, Textbook for Class XII, chapter 11, Organisms and Populations, section on adaptations.
• Campbell Biology, 12th edition (2021), Pearson, chapter 52, An Introduction to Ecology and the Biosphere.
• OpenStax Biology 2e (2018), chapter 44, Ecology and the Biosphere.