Every cell of the body needs nourishment in large animals. Most of their cells cannot leave their position in the body. They cannot move toward food sources. So food must be delivered to these cells. The digestive system provides water, electrolytes, and other nutrients to the body. So the digestive system is specialized for the ingestion of food, propulsion of food through the digestive tract, digestion of food, and absorption of water, electrolytes, and other nutrients from the lumen of the digestive tract. Undigested food is removed from the digestive tract.
The animals show different types of modes of nutrition. Animal nutrition may be classified into different groups on the basis of mods of nutrition.
Different Methods of Animal Nutrition
- Filter Feeders
- Fluid Feeders
- Microphagous Feeders
- Parasitic Nutrition
The animals that feed on detritus are called detritivores. The detritus is organic debris. It is formed by the decomposition of plants and animals. The earthworm is a common example of a detritivore. It moves on the soil surface or makes burrows in the soil and ingests fragments of decaying organic matter, especially vegetation.
The animals that feed on plants are called herbivores. The typical herbivores are insects, reptiles, birds, and mammals. Two important groups of herbivore mammals are rodents and ungulates. The ungulate mammals are hoofed grazing animals like horses cattle and sheep.
Dentition in Herbivores: The premolar and molar teeth of herbivores have large grinding surfaces. There is a large gap between incisors and premolars. Canine is absent in them. The upper incisors are absent in grazing and browsing herbivores like deer and sheep.
The animals that feed on other animals are called carnivores. A predator is an animal that captures and readily kills animals for its food. The animal which is eaten is called prey.
All the predators are carnivores. The carnivores have large canine teeth for catching and tearing the prey. The incisors, premolars, and molars are all adapted for cutting flesh and cracking bones. They break the chunks (large pieces) into a size suitable for swallowing. Cats, dogs, lions, and tigers are common examples of carnivores.
The predator-prey interaction helps in maintaining the ecosystem stable. If a specie does not have a natural predator, it may cause disastrous results. The rabbit was introduced in Australia without its natural predator. It multiplied and produced a large number of rabbits. These rabbits become a menace to the farmers.
These are the animals that eat both animal and plant foods. Examples of omnivores are crows, rats, red foxes, bears, pigs, and men. Their teeth are structurally and functionally intermediate between herbivores and carnivores.
5. Filter Feeders
Many aquatic animals filter the water and digest the particles they extract from it.
A common mussel contains two large gills. These gills are covered with cilia. The cilia help in the movement of water current into their body. Water current enters the animal through the inhalant siphon.
This water current contains food like microscopic algae and protozoa. Some secretary cells (glands) are present among cilia. These cells produce sticky mucous. Food particles are entangled in this mucous. The ciliary movement pushes these food particles into the mouth. Certain whales are also filter feeders.
6. Fluid Feeders
The animals which ingest food in the form of liquid are called fluid filters. Aphids and mosquitoes are examples of fluid feeders:
- Aphids insert their delicate stylets into the phloem of the green stems and suck its juices.
- The female mosquito is also a fluid feeder. It pierces the skin capillaries and sucks blood with its tubular mouthparts.
7. Macrophagous Feeders
The animals which take food in the form of large pieces are called Macrophagous feeders. Tentacular (capturing food with tentacles), scraping, and seizing (capturing) prey are common methods of Macrophagous feeding.
- Hydra is an example of tentacular feeding.
- Scraping type of feeding occurs in the garden snail (Helix). It feeds by using a rasping organ called a radula. This radula is present in the mouth. The lips of the snail hold the leaves. The radula moves back and forth on the leaves and its teeth scrape the food. In this way, tiny fragments (pieces) of leaves are obtained. These fragments of food are gradually pushed back into the pharynx.
- Seizing and swallowing type of Macrophagous feeding is found in spotted dogfish.
A parasite is an organism that lives on or within another organism called the host for obtaining its food. There are different types of parasites:
A parasite that lives upon its host is called an ectoparasite.
- Fleas and lice are ectoparasites. They live in the fur or feathers of mammals and birds. They suck blood from the skin of their host.
- Ticks and mites are common ectoparasites in non-human mammals.
- Leech is also an example of ectoparasite. It attacks both aquatic and terrestrial animals.
- Aphid is an ectoparasite of plants. It sucks food from leaves or stems.
A parasite that lives within the host is called an endoparasite. Endoparasites also occur in both aquatic and terrestrial animals. These parasites are most commonly found in the intestines of a vertebrate host like a man. They absorb the digested food of the host in their intestine.
Entamoeba histolytica, tapeworms, and roundworms are common examples of endoparasites. The presence of some parasites or their excretory products upset the metabolism of their host. So these parasites weaken their hosts.
3. Obligate Parasites
The organisms which live parasitically at all times in their host are called obligate parasites. These parasites cannot survive without their specific hosts.
4. Facultative Parasites
The parasites that can live independently (without their host) for some time are called Facultative Parasites.