[Harsh M (OP)]

Why is it that some animals (eg- elephants)are herbivores and do not eat meat?
how are they evolutionarily different from carnivorous animals?

[Don_1]

I suppose the simple answer to this is that in the early days of animal evolution, they had to eat something. Why look for other animals to eat when there are so few around and you could end up on the menu of the animal you are chasing? Its a bit pointless when as one of the new evolutionary life forms, you are surrounded by vegetation, which isn't going to run away or turn the tables on you.

If not for the herbivores, we would be innundated with vegetation and if not for the carnivores, the herbivores would get out of control and eat too much vegetation.

It boils down to a matter of ecological balance.

Unfortunately, man came along and eventually rather upset this balance.

[LeeE]

Herbivores take advantage of the fact that their dinner doesn't try to run away from them whereas having to chase and catch your dinner uses a lot of the energy you will gain from eating it, assuming that you do manage to catch it in the first place.  In short, it makes animals with relatively inefficient digestive systems viable.

[Harsh M (OP)]

Agree with the fact that plants are a ready source of food for which you dont have to run around, but what would stop a herbivore from eating meat, presuming, vegetation is scarce. Or worse, eating members of their own species or their own children in order to survive when there is no vegetation around ?

[Don_1]

The gut of any animal is adapted to the food it eats. If your gut is not adapted to certain food types, eating them could be of little or no use to you and in some cases can be positively dangerous.

Look at this tortoise fed on all the right things (dandelion, sowthistle, hawk bits, hedge mustard etc.):


Note the smooth rounded shell. A perfect example of a healthy tortoise.

Now look at this poor tortoise:


This poor creature was fed on root vegetables, fruit, egg and animal protein. The result of the wrong diet is evident.

Pictures from The Tortoise Trust.