Some animal’s primary source of feed is plant-based, such animals are referred to as herbivores. Animals whose diet consists primarily of other animals are referred to as carnivores and animals whose diet consists of both plant and animal based sources are known as omnivores.
Animal feeds, much like human food, is composed mainly of protein, carbohydrates, lipids (fats), minerals, vitamins and water. All feeds are different in their composition. Some have more protein than others, for example legumes or animal muscle tissue, whereas others may contain relatively high levels of carbohydrate and lower levels of protein, for example, barley.
Naturally some feed types which are suitable for feeding one animal species, may be totally inappropriate for a different animal. Learning about feed composition and animal nutrition requirements is fundamental in any aspect of animal care.
Eating and Drinking Anatomical Adaptation
Different feeding behaviours have developed through evolution. For example, the earliest forms of land-based vertebrates were piscivores – large amphibians. These amphibians continued to feed on fish and insects, but some began to eat alternative feed types such as other vertebrates. In other words they became carnivorous. Later we know they also consumed plant sources and so also became herbivorous. As such we see the appearance of the omnivore. The way that an animal adapts towards a specific feed source is one of the main reasons underlying evolution in terms of their form and function.
It is widely understood and accepted that all animals must feed in order to replenish stores of sugars and fats which are converted into energy, even though some can hibernate for long periods of time their body utilises energy stores to survive during periods of hibernation. Naturally some animals can survive longer than others between meals, some animals are considered grazers and tend to eat relatively small quantities over long periods of time. Some animals gorge and binge – perhaps on a recent and, in some cases, infrequent kill – in the case of predator animals. Larger animals need to consume greater amounts of feed whereas smaller animals need to eat more frequently. Smaller body masses are simply not able to store as much energy.
Examples of evolutionary adaptations:
Beaks – the development of the beak in specific species such as hawks, woodpeckers, pelicans and humming birds has become specialised for specific feeding tasks like tearing flesh, tunneling for insects in (dead) trees, scooping up fish and probing flowers for nectar.
Mouth components and teeth – in animals these have also evolved differently to match different types of feeding behaviour, for example in vampire bats, whales, leeches, cats and fish.
Claws – these have developed in some animals as a way to catch and kill prey, such as the retractable claws of cats.
Camouflage – this allows some animals to change colour in order to surreptitiously snare prey, for example the blue ringed octopus or chameleon. Others use camouflage to avoid becoming a feed source, for example certain moth species.
Digestive system – some animals have developed specialised digestive systems to enable them to consume certain feeds. Ruminants, including sheep, goats, cattle, giraffes, llamas and yaks are an example of this. They are mammals which have a specialised digestive system. Feed is softened by bacterial action within their stomach’s first compartment. They then regurgitate the semi-digested mass (known as the cud) this is then chewed again, which is known as ‘ruminating’, and swallowed.
How the Digestive System Works
Herbivores and Omnivores
Animals eating lots of plant materials have evolved mechanisms for digesting large amounts of fibrous material. This includes rumen stomachs.
Ruminants have a much larger stomach than non-ruminants. An example of a ruminant is a cow and of a non -ruminant is a pig. The ruminant stomach is divided into four compartments and feed travels slowly through them so that a tough feed can be thoroughly digested. By contrast, the non-ruminant has a single (mono-gastric) or simple stomach. Examples of monogastric animals are dogs, pigs, humans, horses etc. Monogastric animals can include carnivores, herbivores and omnivores.
In very simple terms, the digestive system is a muscular tube extending from the mouth to the anus. Its function is to digest food reducing feeds to compounds which are simple (small) enough to be absorbed via the lining of the intestinal tract and utilised by the animal for energy and all cellular process. The digestive system eliminates waste products which cannot be digested.
The mouth is a cavity that has several functions. Some of the functions of the mouth are to:
- Gather food.
- Grind food into small pieces.
- Mix food with saliva and mucous to form a slippery ball (called a bolus) that can be easily swallowed by the animal.
- Can be the start of chemical break down of food, as some enzymes are added to the food here.
Tongue: The tongue helps in the grinding of food, the formation of the bolus, and in the swallowing of the bolus. The surface of the tongue contains glands and taste buds which play an important part in the selection of food. In grazing animals, the tongue is also covered with a layer of small, stalk-like structures called papillae, which help the animal to grip the blades of grass.
Teeth: An animal’s teeth play an important part in the biting, tearing, and grinding of food. There are three types of teeth:
- Incisors – the sharp cutting teeth at the front of the mouth
- Canines – the conical, pointed teeth used for ripping
- Molars and Premolars – the blunt, irregularly shaped teeth used for grinding food into small pieces
Farmers and veterinary surgeons look at an animal’s teeth to estimate its age.
The oesophagus is a thick, muscular tube connecting the mouth to the stomach. Once the bolus has been forced into the oesophagus it is automatically squeezed along the tube by an action known as peristalsis. The animal has no control over this process.
The Simple Stomach
Non-ruminants are said to have “simple” stomachs, the walls of which are made of involuntary muscle. The arrangement/shape of organs in the digestive system differs between species.
Food enters the stomach via the oesophagus. Once food has been processed it passes out of the stomach into the small intestine. Both the entrance to the stomach (from the oesophagus), and the exit (into the small intestine), are controlled by narrow rings of muscle called sphincters. When the sphincter is contracted, it prevents the flow of food. When the sphincter is relaxed, food can pass through.
The Ruminant Stomach
The ruminant stomach is found in cattle, sheep and goats. Ruminants share the mouth, tongue, teeth, and oesophagus of simple stomached animals but their stomachs are very different. The ruminant stomach is designed to deal with large amounts of roughage such as grass and crop residues. It is much larger than the simple stomach and is divided into four compartments – the rumen, reticulum, omasum, abomasum.
Simply, the functions of the reticulum, rumen, and omasum are to break down the food material to the point where it can be dealt with by the true stomach (the abomasum). This is necessary because the ruminant animal lives on raw, uncooked vegetable matter, much of which is poor in feeding value. Vast quantities have to be eaten and digested to provide enough nutrients for the animal. Most simple stomached animals (except the horse) eat much more concentrated foods and even horses need some form of concentrated food to keep them in good condition.
The Small Intestine
The small intestine is a long, muscular tube from the stomach to the large intestine divided into three parts. The stomach leads into the duodenum, which leads to the jejunum and the last part is called the ileum.
The lining of the small intestine contains many glands which produce mucous. The glands also produce enzymes that are required for the further digestion of food that has passed from the stomach. In addition, the lining contains many small stalk-like projections called villi (singular = villus) (see diagrams). Digested food passes through the villi on its way from the small intestine into the bloodstream. Digested carbohydrates and proteins pass into the bloodstream while digested fats pass through the villi into the lymphatic system.
The large Intestine (including the caecum)
The ileum part of the small intestine leads into the caecum which is a large pouch designed to hold food for further digestion and absorption. The horse has the largest caecum because, being a non-ruminant but at the same time a grass-eating animal, much of the breakdown of roughage takes place there. In other non-ruminants the caecum is small.
Food passes from the caecum into the colon where minimal further absorption takes place. Water is also removed here and re-absorbed by the animal’s body. If this did not happen, the animal would have to drink far more than it does normally.
Once water has been re-absorbed by the animal, all that is left is waste material. This passes into the rectum and out of the body through the sphincter at the anus.