The cow as a herbivore and a mutualist
Cows are ruminants i.e. their digestive system contains a structure known as a rumen
Remember that in humans cellulose is indigestible
The rumen is one of several structures in the ruminant digestive tract which allow ruminants to derive energy from cellulose
The structures additional to those found in humans are:
- rumen
- reticulum
- omasum
These three structures are found before the abomasum (the structure equivalent to the human's stomach)
These four structures are sometimes referred to as a four-chamber stomach
Picture taken from www.mun.ca/biology/scarr/ Ruminant_Digestion.htm
Picture taken from www.gov.on.ca/OMAFRA/english/livestock/ beef/facts/91-066.ht
Note the correct spellings of structures shown in these diagrams:
caecum NOT cecum
oesophagus NOT espohagus
Cows "chew the cud" i.e. they chew food for long periods, swallow it, regurgitate it and then chew it again.
The chewing motion of ruminants is in a circular motion providing a grinding action to break the food into small pieces.
Cows' teeth are well adapted to a herbivorous lifestyle. There are:
- 6 incisors at the front of the lower jaw for clipping plants
- 2 canines
- 12 premolars
- 12 molars
- The molars and premolars are the flat teeth found to the rear of the jaw that grind food. In herbivores these are particularly large.
- By contrast the human dentition has 8 incisors, 4 canines, 8 premolars and 12 molars
Skull pictures taken from http://www.d91.k12.id.us/www/skyline/teachers/robertsd/skulls.htm
The large space between the incisors and the rest of the teeth is called a diastema
This gap enables the cow's tongue to manipulate the bolus of food (the cud) moving it repeatedly over the grinding surfaces.
When the food is swallowed it becomes mixed with anaerobic bacteria that are present in the rumen. The chewing (mechanical digestion) increases the surface area available to the bacteria which carry out anaerobic fermentation on the cellulose.
A typical reaction series occurring in the rumen is:
cellulose -> glucose -> ethanoic acid + carbon dioxide + methane
The ethanoic acid is absorbed across the wall of the rumen (and of the omasum) and used by the cow as fuel molecules
The gases are lost (by eruction i.e. belching).
The energy released by the fermentation reactions is used by the bacteria themselves
Note that the rumen bacteria also digest proteins and make some of the amino acids available to the cow.
The bacteria also utilize other nitrogen sources (e.g. that in urea) to make amino acids which can also be made available to the cow
Once the food has been fermented it passes into the reticulum.
The reticulum passes the food back into the oesophagus and it is regurgitated.
After more chewing the food is re-swallowed but, this time, bypasses the rumen and enters the reticulum directly.
From the reticulum it enters the remainder of the digestive tract.
The food now enters the omasum where water and organic acids (such as ethanoic acid) are absorbed
The food then enters the abomasum (which is equivalent to the human stomach).
From this point on the process is equivalent to that in humans
Remember that humans have bacteria in their colon which digest starch
The diagram below illustrates the similarities and differences between humans and cows with regards to intestinal bacteria
Picture taken from wwwscience.murdoch.edu.au/centres/ rumen/biosafety-page.html
An adaptation common to herbivores is the position of the eyes.
Since herbivores are not predators they have eye positions characteristic of prey animals - the animals are at the side of the head giving good all-round vision
Note, this is not an adaptation FOR a herbivore lifestyle; rather a consequence of it, so avoid using it in exams unless it is appropriate