Comparative Biology
Comparative Biology is when we look at other animals. We can look at other animals to learn about them. The phrase 'comparative' means that we are looking at their anatomy and physiology, in comparison to each other and to us. To learn about how anatomy and physiology ties in with the evolution and specifications of different species. So we can learn principles of biology.
Here we will compare Cow, Rabbit, Cat and Humans. We will start with cows because they are quite different
Cows eat around 100KG of grass per day. Click on the big picture of the cow above to use an interactive exploring the digestive system of the Cow.
Grass. Being made of cellulose, mammals don't produce the enzyme cellulase that breaks down the cellulose back into sugar. So grass is of little nutritional value if you can't break down the cellulose
However bacteria do.
So cows eat grass, swallow it and it goes to their Rumen
This is different from many other Herbivores
Cows, Sheep, Goats and Deer are called Ruminants
So if its something that you farm, then it is likely to be a Ruminant.
Horses are different, their digestive system is more like ours, but with a bigger Large Intestine. Horses are not Ruminants
Cows will often regurgitate their food and rechew it
This regurgitated grass is called Cud
Ruminating is to chew something over
A Ruminant has a Rumen
A Ruminant has a Rumen
The rumen is full of bacteria
The bacteria do extra-cellular digestion. Releasing cellulase and other bacterial enzymes that break down the cellulose back into sugars
The chud is then moved to the Reticulum
Food small enough to move into the Omasum does.
Anything that it too big, is moved back into the mouth where it is rechewed and then swallowed back down into the rumen
From the Rumen to the Reticulum where size of the digested material is checked
From the Reticulum to either the mouth for rechewing (Cud) or to the omasum
The Omasum absorbs water
From the Omasum to the Abomasum
The Abomasum is like our stomach. It contains acid that denatures the proteins of bacteria and the proteins in the grass
In the Abomasum, proteins from the grass and the bacteria are digested by proteases such as pepsin and turned into amino acids.
The Abomasum of young calves secretes a cocktail of digestive enzymes called Rennet. Rennet is a group of digestive enzymes (including pepsin and a lipase) that are really good at digesting milk.
To have dairy cows, the cows need to get pregnant and have calves. This stimulates milk production. But, the calves are not wanted so they are butchered into meat (veal). the abomasum of the calf is processed separately to get the enzymes out of it. These enzymes are then mixed with milk to make cheese (back in the old days)
The chyme now moves to the small intestine
Here other digestive enzymes from the cows pancreas mix in, including amylase and lipase. Also bile mixes in from the cows gall bladder. In the small intestine chemical digestion of carbohydrates, lipids and proteins occurs. Absorption of these nutrients occurs here, along with more absorption of water.
The chyme then moves to the large intestine. Here more water is absorbed and more digestion by bacteria occurs, making some short chain fatty acids.
Mouth
Molars and bottom Incisors only
Esophagus
Rumen
Full of Bacteria
Bacteria extracellular digestion of cellulose
Reticulum
Filters food
Too big, sent back to Mouth for re-chewing and then back to Rumen. This is called Cud.
Small enough bits are sent to the Omasum
Omasum
Absorbs water from the slushy mixture of water, grass and bacteria
Abomasum
Acid containing
Contains Pepsin
In a calf the Abomasum contains Rennet - this is a cocktail of digestive enzymes that effectively digests milk
Rennet used to be harvested from slaughtered calves for cheese making
Acid kills bacteria
Acid denatures proteins from the grass and from the bacteria - these proteins are cleaved by proteases such as pepsin and broken into amino acids
Duodenum
Cells in the duodenum secrete hydrogen carbonate to neutralize the stomach acid
Pancreatic enzymes enter
Lipase
Amylase
Trypsin
Bile from the gall bladder enters
Jejunum then Ileum
Villi absorb Amino Acids, sugars, lipids
Amino acids and sugars go into blood stream
Lipids go into the lacteals attached to the lymphatic system
Caecum
Store of bacteria, but due to the large bacterial digestion that has already happened, the further breakdown here doesn't contribute much calorically. Note, this is opposite in horses
Will still get some vitamins from the bacteria here
Colon
Absorb water, vitamins, minerals
Rectum
Stores waste till fill then egestion occurs
The clip to the side shows how large the Rumen is.
This surgery is performed on a street cow in India
The cow has eaten plastic bags.
These need to be removed before they cause a blockage in the digestive track and the cow dies
The surgery is done under local anesthetic
The cut is into the very large Rumen.
You can see the vets arm disappear into the Rumen
This shows that the rumen is very large.
Remember that the rumen is the fist chamber of the cows 'stomach'. It is full of bacteria that digest the grass for the cow.
Describe and explain the function of each of the four chambers of a Ruminant Herbivore
1. Rumen
2. Reticulum
3. Omasum
4. Abomasum
Cows 4 stomachs are RROA
1. Rumen - Cows like to Rumen about things all day long
2. Reticulum - Round and Round. After ruminating they decide to go, or to ruminate some more
3. Omasum - O for amazing - sucking up water
4. Abomasum - This is absolutely o-mazing - in the Abdomen
Hindgut Horse
Horses digestive systems are different from cows.
Theirs looks a bit more like ours.
Food goes from the mouth to a normal stomach
From there it goes into the small intestines then into the large intestines
Without the large rumen that cows have the horses are pickier eaters
They choose newer shoots to eat, along with grains and grass seeds.
They are also more sensitive eaters than cows and need to be looked after differently
To get sugar from the cellulose of plant material, the horses still use bacteria
However, instead of having the bacteria before the acidic stomach, like in the foregut of ruminants, horses have the bacteria afterwards in their hindgut, like us. But there hindgut is huge!!
Note, foregut means before pyloric sphincter, hindgut means after pyloric sphincter
Also note, Bovine = cow. Equine = horse.
Horses have huge Large Cecum and a very Large Large intestine
The other difference is that bacteria digestion in a rumen destroys the plant cell allowing the proteins and oils out of the cell for absorption in the small intestines. However this cant happen in horses
Because Horses are hind-gut fermenters, they don't get to have bacteria helping them before the food gets to the small intestine. Thus the horses themselves need to break up the cell wall to get to the proteins contained inside
For this, the horses will chew their food for longer than what cows will.
Cows will chew their food twice, horses only once so this once is very thorough.
Horses need to chew their food for longer to break up the plant cell walls so that enzymes can get to the proteins and lipids inside the cells for digestion
In fact the large intestine in cows is so large that it has two parts
Large Colon (large large intestine)
Small Colon (small large intestine)
With the horse we can see the importance of the Cecum and the Large intestine for herbivores
We can look at the percentage of the digestive tract that is large intestine in several animals we can see the importance of the large intestine and cecum and its bacteria for horses.
Percentage of digestive system that is Cecum + Large Intestine
Dog - 14.4%
Cow - Ruminant herbivore 10.7%
Horse - Herbivore - 62%
Human - 35%
We can also see by looking at these numbers how effective the Rumen is for getting nutrients out of food
We can also see how important the Cecum is for hindgut fermentation
However, for similar sized animals, the Cow has a larger digestive system overall. I wonder how this contributes to other differences between ruminant herbivores and non ruminant herbivores. Such as the ability of horses to run for long distances.
Also looking at this, we can see how nutrient rich meat is, with dogs having smaller digestive systems in relation to the rest of their body.
When we look at the Cecum, in humans it is only small (about 6cm in diameter).
In organisms where foregut fermentation, aka the breakdown of cellulose by bacteria, is really important for their ability to get enough calories. Then the cecum will be bigger. Like in Horses and Koalas and to a less extent, cows
Eat your poop, Rabbit
Like Cows and Horses, rabbits also eat a lot of grass
Rabbits have strong lips that hold grass or their food (prehensile lips), then their very big incisors cut the grass and then chew it with their big molars.
From the mouth the grass and plant material goes to the acid filled stomach
From the stomach to the small intestine and then to the Caecum
Caecum to large intestine and then out
Being smaller, the bacteria in their caecum and large intestine do not have as much time with the cellulose as they do in horses
So, the feces leaves the body with a lot of cellulose left
So the rabbit has a solution
It eats it again!
Rabbits are called Hindgut digestors
This means that most of the digestion occurs in the Caecum and Large Intestine by Bacteria
The first set of feces that it creates are called caecotrophs, or cecotropes depending if you are going for the English or the American spelling. You can also call it a Caecal pellet.
The Caecal pellet is made in the Caecum and the Large intestine
These caecal pellets are fill of bacteria, about half of the weight of the feces is bacteria.
When the rabbit eats the caecal pellet a second time it eats its own bacteria
The bacteria is destroyed in the acid of the Stomach. This bacteria is then digested.
The second pass through the digestive system allows:
More proteins that have been released from digested plant cells to be absorbed in the small intestine.
More sugars that have been released by the digestion of cellulose to now be absorbed in the small intestine
Further use of the sugars by bacteria in the caecum
Further Short Chain Fatty Acid absorption in the large intestine
After this second pass, a new pellet is created. This new pellet is hard and the rabbits do not eat this
Carnivorous Pets
Carnivores have short digestive systems. This is because meat is easily digested in the stomach and duodenum. Whilst proteins and fats are effectively absorbed by the Villi in the small intestine.
Thus the Large intestine is only needed for the absorption of water.
Carnivores may still have a Cecum, but this as a store of bacteria for the creation of vitamins rather than as a digestive role.
Carnivores do not eat Starch or Sugar. As such, they do not have amylase in their saliva. However, they will have small amounts in their pancreatic juices (dogs have lots due to them eating our scraps, wolves only have tiny amounts)
Carnivores do not eat Starch or Sugar, however their brains still run on sugar. So their livers convert some of the fats from the meat that they have eaten into sugar and glycogen
This is another advantage that we have as omnivores. Our liver also does this, but doesn't do it to the same level because of all the starch that we eat. In fact it is hypothesized that the reason our brain is so huge compared with other carnivores is our mixed diet, with hunting and communication mixed in with regular fire cooked starch for long continuous food
Mans best friend is a carnivore.
Mans second best friend is also a carnivore.
Carnivores are generally quite intelligent.
Think about their lifestyle and compare this to herbivores
If we think about a horse or a cow, they spend most of their day eating. Where as lions and wolves eat and then sleep and do other things.
Herbivores spend most of their day eating and avoiding being hunted
Carnivores spend most of their day hunting
Hunting is often more effective in groups
Hunting involving groups is most effective if the group members can communicate. The better the communication the more likely a successful hunt will result.
The young train for a hunt by playing. Play interactions involves nonverbal communication. The more intelligent the animal, the better the communication, the better the hunt, the more meat they get.
Meat is far higher in protein and energy (fat) than plants
So when an animal eats meat, they don't have to spend all day eating. Whereas a plant eating animal needs to eat more and eat for longer.
Because they don't eat all day, they have time to play and communicate.
The higher then intelligence, the better the communication, the better the hunt, the more meat
This is why humans are the most intelligent animal on the planet.
We hunt, we communicate, we play, we communicate, we hunt.
The transition to eating meat was vital in our evolution as a species.
By eating this energy rich food source, we had time to play and communicate. We also had time to focus on how to hunt and how to catch prey as it became harder to hunt and find. We also had to think about what other things we could eat, and then communicate this through our tribe.
Our ability to eat everything is vital to our success as a species. Cats and dogs are Carnivores. We are not.
Cats and dogs have shorter digestive systems for their body size than we do or than herbivores do
Short small intestines
Very short Large intestines
Due to the high energy of meat and the easy digestion of meat
The End
