Different Methods of Food Taken by Animals

Advances in technology have expanded the scope of Biology Topics we can investigate and understand.

How do animals take food ?

We have studied in the previous chapter that plants can make their own food by the process of photosynthesis. Animals, however, cannot make their own food by photosynthesis. Since animals cannot make their own food, they need readymade food. This readymade food comes from plants’ or from ‘other animals’.

Thus, animals obtain their food from plants or from other animals (which eat plants). Some animals, however, eat both plant food as well as animal food. We (human beings) are also animals. We obtain foods like wheat, rice, pulses (dal), fruits and vegetables from plants. And the foods like milk, curd, cheese and eggs are obtained from animals. Some people also eat meat, chicken and fish as food. These foods are also obtained from animals (which eat plants as food).

All animals (including human beings) require food for obtaining energy, growth and repair of damaged body parts. The process of taking in food by an animal and its utilisation in the body is called animal nutrition. Nutrition in animals takes place in five steps : Ingestion, Digestion, Absorption, Assimilation and Egestion.

1. The process of taking food into the body is called ingestion.
2. The process in which the food containing large, insoluble substances is broken down into small, water soluble substances (which can be absorbed by the body) is called digestion.
3. The process in which the digested food passes through the intestinal wall into blood stream is called absorption.
4. The process in which the absorbed food is taken in by body cells and used for energy, growth and repair, is called assimilation.
5. The process in which the undigested food is removed from the body is called egestion.

Animals Take in Food by Different Methods

The first step in the process of nutrition in animals is ‘ingestion which means ‘taking food into the body’ (or eating of food). Different animals use different modes (or methods) of taking food into their body. In fact, every animal has some special structures (or organs) for taking food inside its body. The methods used by some of the animals to take in food (or eat food) are described below.

Frog is an animal which eats flying insects as food. The frog uses its long and cleft tongue (forked tongue) to catch its prey like insects. A wall lizard and a chameleon (girgit) also catch their prey (insects, etc.) with the help of their fairly long tongue. The butterfly belongs to the category of insects. The food of butterfly is nectar (Nectar is a sugary liquid present inside the flowers). The butterfly uses its long feeding tube to suck nectar from flowers (just as we sip Pepsi or Coca-Cola) (see Figure). Bees and hummingbirds also suck the nectar from flowers in plants. Both male and female mosquitoes feed by sucking nectar from flowers. The female mosquitoes also suck blood from other animals (including human beings).

Infants (small babies) of humans and many other animals (such as cows, dogs, etc.) feed on mother’s milk by sucking. Lice are wingless insects which live on hair. Lice feed by sucking blood from the skin of scalp. Houseflies feed on filth and refuse. They take in only liquid food by sucking. Houseflies spit out saliva on solid food to convert it into a liquid and then suck this liquid. Ants feed on plant material and other animals by biting and chewing. Snails feed on algae by scraping it from rocks. Snakes are flesh eaters. They eat animals like rats, etc. Snakes (including pythons) swallow the animal ‘whole’ which they prey upon. Eagle is a large bird which feeds on the flesh of its prey by tearing its flesh with powerful hooked beak. Amoeba is a tiny aquatic animal which ingests its food with the help of its pseudopodia (or false feet).

Hydra is an animal which uses its tentacles with sting cells to kill the prey and put it into the mouth (body cavity). A spider weaves a web {jaal) to catch its prey. A crow uses its long beak to put the food into its mouth. And we (human beings) use our hands to put food into our mouth. Starfish is a sea animal which has an amazing way of taking in food (or ingestion).

Starfish feeds on sea animals (such as shellfish and oysters, etc.) which are covered by hard shells of calcium carbonate) After opening the shell of its prey, the starfish pops out its own stomach through its own mouth. This stomach surrounds the soft body of the prey (which is inside the shell). The starfish then brings back its stomach containing the prey (or food) into the body. This food is digested slowly by the Starfish. From the above discussion it is clear that there are many different methods of eating food (or ingesting food) in the animal world.

Digestion

Our food consists mainly of substances such as carbohydrates (like starch), fats and proteins (with small amounts of minerals and vitamins). Now, starch carbohydrate, fats and proteins are all large, insoluble substances which cannot pass through the walls of our intestine and get absorbed as such. So, before the food can be used by us for various functions like getting energy or for growth, it must be broken down into small, water soluble substances which can be absorbed by our body. The process in which the food containing large, insoluble substances is broken down into small, water soluble substances which can be absorbed by our body, is called digestion.

In most simple terms, digestion means dissolving of solid food. Digestion makes the food soluble so that it can be absorbed and utilised by the body.

We use both physical and chemical methods for digesting (breaking up) the large substances present in food. Physical methods include chewing and grinding the food in mouth and chemical methods include the addition of digestive juices to food by the body itself. During the process of digestion, the complex starch carbohydrate present in our food is broken down into a simple sugar called glucose.

Fats are broken into simpler substances called fatty acids and glycerol. And proteins are broken down into simple substances called amino acids. Now, glucose, fatty acids, glycerol and amino acids are all water soluble, simple substances which can pass through the wall of our small intestine into the blood and hence get absorbed in the body. The process of digestion takes place inside our body. Before we describe the human digestive system in detail, we should know the meaning of alimentary canal. This is described below.

Alimentary Canal

A long tube running from mouth to anus of a human being (or other animals) in which digestion and absorption of food takes place is called alimentary canal. Alimentary canal is also known as gut or digestive tract. It is about 8 to 9 metres long in humans. The alimentary canal is a continuous canal which has many parts such as mouth (buccal cavity), oesophagus (food pipe), stomach, small intestine, large intestine, rectum and anus. Three glands are also associated with alimentary canal. These are salivary glands, liver and pancreas.

The food enters the alimentary canal at the mouth (or buccal cavity). As the food travels through the various parts of alimentary canal, it gradually gets digested. During the passage of food through alimentary canal, the various glands (salivary glands, liver, pancreas) and inner walls of stomach and small intestine, secrete digestive juices. These digestive juices convert the complex substances of food into simpler substances which can be absorbed by the body. The undigested part of food is defecated (thrown out) through the last part of alimentary canal called anus.

An important question now arises : How does food move forward in the alimentary canal ? The food moves forward in the alimentary canal by the process of peristalsis. Peristalsis is the wave-like movement caused by the alternate contraction and relaxation of the muscles of alimentary canal which pushes the food forward in the alimentary canal. We will also be using a term called ‘buccal cavity’. The mouth cavity by which food is taken into the alimentary canal and chewed is called buccal cavity. We will now describe what happens to the food when it passes through the different parts of the alimentary canal (or digestive tract).