Digestive Systems in Different Animals
Different species of animals have
different digestive systems which are adapted to their unique
requirements. The type of food, method of food gathering and energy
needs are some factors that influence the type of digestive system
an animal needs in order to survive.
Herbivores have a more specialised digestive system
than that of a carnivore because it is more difficult to digest
vegetation than meat. The teeth are flat so that grass and plant
material can be ground down, rather than the sharp teeth of
carnivores designed to tear flesh.
Animals which eat both plants and meat, such as
humans, have both types of teeth so that they can perform both
functions. In simple animals the digestive system is not complex,
usually containing a single tube. As the animal becomes more
complex organs with specialised functions develop. Higher order
animals require a storage organ, such as the stomach, which allows
them to take in large amounts of food in one feeding and then use
its energy over a long period of times. This makes it possible for
them to devote time to activities other than feeding.
Birds need a high body temperature
which requires a large amount of energy to maintain. This means
that birds need to eat larger amounts of food to gain the energy
they need every day to survive.
In order to do this they have a specialised digestive
system where there is an efficient absorption of energy. Food
passes through very quickly and is all absorbed, leaving little
waste. Birds have no teeth so digestion does not begin in the
mouth, all of the food breakdown must occur within the digestive
system. Food enters through the mouth where it passes down the
esophagus into the crop. This organ is where the food is stored and
begins to soften. From here it moves into the stomach, which is
called the proventriculus. This acts as a true stomach where
digestive juices continue to chemically break down food. The
partially digested food moves into the muscular gizzard, which has
a rough lining to break down the food further. It sometimes
contains sand or pebbles which have been swallowed by the bird,
which add to the grinding process.
The food moves into the intestine, first into the
small intestine and then onto the large intestine. At the point
where the small and large intestine meet are two pouches or caeca,
which absorb the water from the food. In herbivores this is the
site of cellulose deposition. The food becomes harder and enters
into a chamber called the cloaca. It then passes out of the body
through the cloacal lining.
An adult horse, over five years old,
has 40-42 teeth which include incisors, canines and cheek teeth.
The incisors work together with the lips to grasp and move food
around the mouth. The molars are used to grind down food, making it
easier to digest. The digestive system has developed to effectively
break down and digest fodder, with the stomach being smaller than
that of other cud chewing animals.
The average adult horse is able to hold 7.5-9.5
litres (2-2.5 gallons) of chyme in their stomach. The small
intestine is 18-21 metres (60-70 feet) long, while the large
intestine is enormous, adapted to digesting grass and hey. The
caecum is a pocket between the large and small intestine and is
able to hold 15-65 litres (4-17 gallons) while the large colon can
hold 60-150 litres (16-34 gallons). In the large intestine food is
broken down through fermentation by both bacteria and protozoa.
Food may remain in the intestine for up to 55 hours.
The digestive system in insects is
basically a tube that begins at the mouth and ends at the rectum.
It can be divided into a pharynx, esophagus, stomach, intestine,
colon and rectum. The stomach or midgut has glands called the
gastric ceca which secrete the digestive juices. The malpighian
tubes removes the nitrogen rich waste from the blood. Each tube
empties at the connection between the stomach and large intestine,
producing an end product of uric acid, which is passed with
Insects such as the praying mantis are carnivorous
and begin to digest their food by chewing. Other insects such as
wasps paralyse their pray and lay their eggs in the bodies. This
provides a living food supply for the young, providing their
immediate source of food. Termites gain their energy from wood and
begin to digest the cellulose before it enters the digestive
system. Protozoans are released onto the wood and begin its
breakdown, continuing to act on the wood once it is passed onto the
stomach. Flatworms have a simple digestive system, with a single
tube serving as both mouth and anus.
The digestive system of a crustacean varies
depending on the species. In simple species it is a single tube,
while others have a specialised system with chambers and organs,
each with a specialised function. This often reflects their feeding
habits, some animals are scavengers and their food is often
beginning to decay. Others require more complex systems as they
fully digest the food.
Snakes eat all parts of their pray and
need a specialised digestive system to gain the most nutrients from
their food. Their teeth are very thin and usually curve backwards.
Their function is not to grind down food as it is in most animals,
rather it is to capture prey. The food is swallowed whole, thus the
teeth perform a specialised function. They have powerful digestive
enzymes to break down the hair, feathers, bones, organs and other
parts of their food.
The salivary glands also produce strong enzymes which
are also used to kill the organism. If saliva enters the wounds of
the animal it will begin the digestive process and cause severe
tissue damage, which can often lead to the death of the animal.
These toxic substances are found in the saliva of many non
poisonous snakes. In poisonous snakes it is the salivary glands
which have developed into venom sacks, with the venom being a
highly toxic form of saliva.
The mollusk digestive system has
millions of microscopic hair like fibres along the main digestive
tract and has several divisions for the different organs.
The first section contains the mouth and esophagus
and is the site of the initial breakdown of food. There is a
specialised filelike radual found in the mouth, which acts like
teeth or a tongue in the food breakdown.
Oysters, clams and muscles do not feature the radula,
as they are filter feeders, the food is already filtered when it
enters the mouth and continues down the digestive tract. It reaches
the liver and stomach, which continues the digestion. In many
mollusks the stomach has a flexible rod, which is made up of mucus
and proteins in a crystalline structure. This secretes the
digestive juices and enzymes and acts as a kind of stirring stick,
mixing up the stomach contents to aid digestion. The final section
of the digestive tract contains the intestine and anus, from which
the waste is removed.
Britannica, 15th Edition. (Chicago: University of Chicago,
The Book of Popular Science
Encyclopaedia. (New York: Grolier, 1961)
The Software Toolworks Multimedia
Encyclopaedia, Release 6. (New York: Grolier, 1996)