Why do mammals lack the ability to breakdown cellulose?

[kaminarigaston]

Hey, you, scientist guys & girls! Vegan mammals, what's up with them, huh?

 

Although a lot of mammals (probably most of them) are mac user, new age vegans, none of them has developed a digestive enzyme capable of breaking down cellulose, so they all rely on bacteria or fungi to do it. In ruminants almost all the glucose produced by the breaking down of cellulose is used by the symbiotic bacteria. Ruminants get their energy from the volatile fatty acids produced by these bacteria: acetic acid, propionic acid and butyric acid, it´s like they live off of bacterial farts! And what about elephants, most of the food they eat ends up undigested, in huge mounds of sh*t.

 

Was there no advantage at all in an enzyme capable of breaking down cellulose? Wouldn't herbiborous animals need to ingest less food if they had one? Wasn't there enough time for at least some species to develope one?

 

Further more this was writen by user Boerseun in another thread, and it seems to imply that a more efficient processing of cellulose wouldn't hurt:

 

"From what I've read up to date, it seems that the dinos grew to such proportions simply because of their diet. The herbivorous dinos dined on leaves and twigs, which are very low in nutrients. In order to garner as much as possible from their diet, they had to eat an enourmous amount, which needed a big vessel in which to ferment to release the usable nutrients. Hence, diplodocus and his kin.

 

Problem, of course, for the carnivoures, is that the bigger the prey became, the bigger they had to get themselves, in order to ensure dinner. And from there you ended up with T-Rex and his buddies.

 

Mammals could never grow to the same size, because they are warm-blooded. They need even more energy, if only to sustain their body temperature. It's been calculated that a cold-blooded reptile need only 10% of the energy a mammal needs (for the same size, of course) because it don't need to keep itself warm. So, if a diplodocus needs a ton of food a day, for the same body size a mammal would need ten tons. And ten tons of twigs could mean a lot to a reptile, but not to a mammal."

 

So please explain me!

[hallenrm]

I am a bit confused on reading your post, what are you trying to say?

 

Do you mean that mammals never had a digestive mechanism to proccess cellulose?

 

That is really not true, because mammals that survive on cellulose, like cattle, do have a mechanism in their body to digest cellulose. The bacteria that does this job in their stomach, is a part of their body and so far as I know, nobody has ever produced evidence that these organisms ever existed without this symbiotic relationship.

 

I think, your confusion arises, because you are considering the body of a mammal as a sum of its organs and not as a whole as it exists and has existed. :shrug:

[kaminarigaston]

My question is this ¿Wouldn't mammals get more from the cellulose if they could break it down and take the glucose monomers, instead of taking just the less energetic fatty acides, a great amount of wich get lost by farting and burping? ¿Wouldn't mammals get more from the cellulose if they contributed to break the cellulose, since most of it pass through to the other end undigested?

 

¿Yes/no/maybe?

 

You ask if I mean that mammals never had a digestive mechanism to proccess cellulose. Didn't I explicitly stated that mechanism? By stating that some (most?) animals feed on it, didn't I implicitly stated that they have a mechanism to do it?

 

I'm not saying that they don't have means to do it, I'm just saying that bacterial digestion of cellulose is not a thorough job, but if it's the only method for mammals, there's surely a reason for it. But what reason is it?

[hallenrm]

I'm not saying that they don't have means to do it, I'm just saying that bacterial digestion of cellulose is not a thorough job, but if it's the only method for mammals, there's surely a reason for it. But what reason is it?

 

The reason that I can think of is the process of evolution through which their body acheived the structure it has. Every design, whether natural or engineering has certain constraints. Not every design incorporates every desirable trait :)