Introduction
Animals require food in order to obtain the energy and nutrients needed for survival. Unlike green plants, animals cannot manufacture their own food because they lack chlorophyll and therefore cannot carry out photosynthesis. For this reason, animals are described as heterotrophs, meaning they depend on other organisms for their food.
Animals obtain their nutrients in several ways depending on their feeding habits and the type of food they consume.
Animals obtain food by:
- Eating plants directly (herbivores)
- Eating other animals (carnivores)
- Eating both plants and animals (omnivores)
Food is essential to animals because it provides:
- Energy for movement and body activities such as breathing, circulation and growth
- Materials for growth and development of body tissues
- Substances for repair of damaged or worn-out tissues
- Nutrients for maintaining body functions, including regulation of body processes
The overall process through which animals obtain food and utilize it within the body is called nutrition.
Meaning of Nutrition
Nutrition refers to the process by which living organisms acquire food and use it to provide energy, growth and maintenance of the body.
In animals, nutrition involves several physiological processes that occur mainly in the digestive system. These processes ensure that food is broken down into forms that the body can absorb and use.
The major processes involved in nutrition include:
- Ingestion – taking food into the body
- Digestion – breaking down complex food into simpler substances
- Absorption – movement of digested nutrients into the bloodstream
- Assimilation – utilization of absorbed nutrients by body cells
- Egestion – removal of undigested food materials from the body
These processes ensure that complex food substances such as carbohydrates, proteins and fats are converted into simpler soluble molecules that can be absorbed and used by body tissues.
Modes of Nutrition in Animals
Animals obtain nutrients in different ways depending on their structure, environment and feeding behaviour. These different ways of obtaining food are known as modes of nutrition.
The four major modes of nutrition in animals are:
- i. Holozoic nutrition
- ii. Parasitic nutrition
- iii. Saprophytic nutrition
- iv. Symbiotic nutrition
Figure: Major modes of nutrition in animals.
Each mode of nutrition represents a different strategy through which organisms obtain nutrients necessary for survival.
Many animals have evolved specialized feeding strategies that allow them to survive in very different environments, from deep oceans to dry deserts.
Parasitic Mode of Nutrition
Figure: Parasite–host interaction showing how a parasite obtains nutrients from a host organism.
Parasitic nutrition is a type of nutrition where one organism obtains food from another living organism without providing any benefit in return.
In this relationship:
The organism that obtains food is called the parasite.
The organism that provides food and shelter is called the host.
The parasite benefits from this relationship while the host is usually harmed, weakened or infected with disease.
Characteristics of Parasites
Parasites generally show several distinctive features:
- They depend partially or completely on the host for nutrients.
- They may live inside the host’s body or on the surface of the host.
- Many parasites possess special adaptations for attaching to the host.
- Parasites often cause diseases, weakness or irritation in the host.
- Some parasites absorb nutrients directly from the host’s tissues or bloodstream.
Types of Parasites
Parasites can be classified into two major groups based on where they live in relation to the host.
Figure: Comparison between ectoparasites and endoparasites.
1. Ectoparasites (External Parasites)
Ectoparasites live on the external surface of the host’s body.
Examples include: Ticks, Lice, Fleas and Jiggers
Effects on the host:
- Cause irritation and itching
- Suck blood from the host
- May transmit diseases
- Weaken the host over time
2. Endoparasites (Internal Parasites)
Endoparasites live inside the body of the host, often in organs such as the intestines, liver or blood.
Examples include: Roundworms, Tapeworms, Hookworms and Liver flukes
Effects on the host:
- Compete with the host for nutrients
- Cause diseases and infections
- Damage body tissues and organs
- May interfere with digestion and absorption of food
Some parasites such as tapeworms have no digestive system. They absorb nutrients directly through their body surface from the host’s intestine.
Saprophytic Mode of Nutrition
Saprophytic nutrition is a type of nutrition in which organisms obtain nutrients from dead and decaying organic matter.
Organisms that feed in this way are known as saprophytes.
These organisms play a very important role in ecosystems because they help in the decomposition of organic materials.
How Saprophytes Obtain Food
- They release digestive enzymes onto dead organic matter.
- These enzymes break down complex organic substances into simple soluble substances.
- The simpler nutrients are then absorbed directly into the organism’s body.
- This process is known as external digestion because digestion occurs outside the body of the organism.
Importance of Saprophytes
Saprophytes are extremely important in maintaining ecological balance.
- Break down dead plants and animals
- Recycle nutrients back into the soil
- Increase soil fertility
- Help in the formation of humus
- Prevent accumulation of dead organic matter
Common examples include: Mushrooms, Bread mould, Bacteria and Fungi
Symbiotic Mode of Nutrition
Symbiosis refers to a close association between two different organisms living together, where at least one organism benefits from the relationship.
The organisms involved in symbiosis depend on each other in different ways.
Types of Symbiosis
1. Mutualism
Mutualism is a relationship in which both organisms benefit from living together.
Example:
Bees and flowering plants
Bees obtain nectar and pollen as food.
Flowers benefit because bees transfer pollen between flowers during pollination.
Other examples include:
- Lichens (algae and fungi living together)
- Bacteria in the intestines of animals.
2. Commensalism
Commensalism is a relationship in which one organism benefits while the other organism is neither helped nor harmed.
Example:
Birds sitting on cattle feeding on insects around them.
The bird obtains food while the cattle is unaffected.
Lichens are actually two organisms living together — a fungus and an alga. The fungus provides protection and moisture while the alga produces food through photosynthesis.
Holozoic Mode of Nutrition
Holozoic nutrition is the mode of nutrition in which animals ingest complex solid food which is then broken down inside the body into simpler soluble substances.
This is the most common mode of nutrition in animals, especially in higher animals such as mammals.
Holozoic nutrition involves a series of processes that occur in the digestive system.
Steps in Holozoic Nutrition
- i. Ingestion – intake of food into the body
- ii. Digestion – breakdown of food into simple substances
- iii. Absorption – movement of nutrients into the bloodstream
- iv. Assimilation – use of nutrients by body cells
- v. Egestion – removal of undigested food materials
Classification of Animals Based on Food They Eat
Figure: Classification of animals based on dietary habits (herbivores, carnivores and omnivores).
1. Herbivores
Herbivores are animals that feed mainly or entirely on plants and plant materials.
Examples include: Cows, Goats, Sheep, Rabbits and Antelopes
Characteristics of herbivores:
- Strong grinding teeth
- Well developed premolars and molars
- Digestive systems adapted to digest plant cellulose
2. Carnivores
Carnivores are animals that feed mainly on the flesh of other animals.
Examples include: Lions, Leopards, Dogs, Cheetahs and Hyenas
Characteristics of carnivores:
- Sharp incisors for cutting meat
- Long pointed canines for tearing flesh
- Strong jaws and powerful muscles
3. Omnivores
Omnivores are animals that feed on both plant materials and animal flesh.
Examples include: Human beings, Pigs, Bears, Chickens
Characteristics of omnivores:
- Teeth adapted for cutting, tearing and grinding food
- Flexible feeding habits
Video: Explanation of the different modes of nutrition in animals.
Teeth are specialized structures found in the mouth that help animals capture, cut, tear and grind food before it enters the digestive system.
Mammals possess four main types of teeth.
Figure 1: Types of teeth in the human mouth.
Adult humans normally have 32 permanent teeth, while children usually have 20 temporary (milk) teeth.
1. Incisors
Characteristics:
- Chisel-shaped edges
- Sharp cutting surfaces
- Usually have one root
Functions:
- Cutting food
- Biting food into smaller pieces
2. Canines
Characteristics:
- Sharp and pointed
- Conical in shape
- Long strong roots
Functions:
- Tearing flesh
- Holding or seizing prey
Carnivorous animals such as lions and wolves have extremely long and sharp canines that help them capture and tear the flesh of their prey.
3. Premolars
Characteristics:
- Broad surfaces
- Possess ridges called cusps
- Usually have two roots
Functions:
- Crushing food
- Grinding food
4. Molars
Characteristics:
- Largest teeth
- Broad flat surfaces with cusps
- Usually have three roots
Functions:
- Grinding food thoroughly
- Chewing food before swallowing
Molars are the strongest teeth in the mouth and are specially adapted to crush and grind food during chewing.