Wednesday, February 5, 2014

Herbivores and their microsymbionts

As the proud owner of pygmy goats, I cannot help marveling at how goats and other herbivores can digest grass and other plants, and “make a living” from it. I can't digest grass, and it seems to me that it would be a useful thing to be able to do. Why can't I or any other human digest grass? How do herbivores do it?

It turns out that vertebrates in general do not have the enzymes to digest cellulose, a key sugar in plant cell walls that is useless to animals, into glucose, a sugar supplying energy to the body. Thus, it would seem that vertebrates cannot gain much metabolic benefit from eating plants. This explains why humans can't cannot gain benefit from eating grass, but it does not explain why herbivores can break down cellulose in grass. After all, they do not naturally produce enzymes to break it down!

Yet, if you look closely into the stomach and intestines of herbivores, one will find that there are indeed the enzymes necessary to break down cellulose. Where do these come from?

Scientists have discovered that living in the guts and intestines of herbivores there are in fact a whole host of micro-organisms such as bacteria and protozoans, which make enzymes to break down cellulose. These organisms live in symbiotic relationships with their host organisms; the digestive tract provides a good living environment and food to these micro-organisms, and in turn the micro-organisms break down cellulose, some of it for their own consumption, and some of it for the host organism. Thus herbivores can't break down grass themselves, but enlist help from other organisms. How cool is that?



  1. I enjoyed this post very much. I too, sometimes, ask myself how herbivores are able to digest grass and how humans can't. But I do have a question for you, what makes grass so different that humans can't digest it, but we can digest other plants and vegetables?

    -Posted by Jacob Geier

    1. Most of the plants we eat are either fruits or roots. These are very different than the leafy portions of plants, because they contain lots of sugars and other nutrients to eventually support the growth of seeds into plants. We do eat some kinds of leafy greens, but in general we do not get many calories out of them.
      Posted by JE

  2. Wow! I had no idea this is how herbivores digest their food. It is interesting to see how both the bacteria and the herbivore benefit from their relationship, especially in your pygmy goats. Do you of any other instances that herbivores use bacteria in similar symbiotic relationships?

    Lindsey Janof

  3. Very cool! There is actually a lot of research happening in this field in order to try and more efficiently break down cellulose in biofuel plants. The difficulty in breaking down cellulose is actually one of the main factors that is slowing down the progress of biofuels.Chris Somerville has a couple of interesting videos on biofuels in general if you're interested.

    Posted by Kirk MacKinnon

  4. This was an interesting article, I remember learning in previous biology courses that we are unable to digest cellulose but never looked into how herbivores could. I think symbiotic relationships such as this are extremely interesting. Did you know termites actually do the same thing? they also have microorganisms that live in their gut to break down cellulose.
    Posted by Kristen Whitehead

    1. Termites have long been a poster child for symbiosis, but actually, that is now starting to change. Most termites do indeed have gut symbionts, but it has been discovered that termites can actually produce cellulose-breaking enzymes on their own (Tokuda et al. 2007). Thus, these symbionts are strong, important helpers, but do not seem to be quite as important as those in mammalian herbivores.
      Published by JE