That would give us a realistic feeding and herbivore movement but of course every animal dies and every animal reproduces by definition. So there would have to be another timer, dependant on the time it spends hungry (time where it can't find grass objects), size and a bit of randomness. Once that timer runs out (or reaches a certain time) the creature dies.
With death comes birth, this would depend on the animal. Bigger animals (that live longer) breed less and smaller (shorter life) animals will reproduce a lot more. So the amount of 'eggs' laid will be proportionate to its size. Of course for birth you need male and female, so you have male and female and in most animals the percentage of males is a lot fewer than the females. So giving birth I think about 3/4 will be female, therefore the amount of 'reproduction' is also dependant on the sex of the animal.
Since I started thinking about this when i read about the ever developing forest I figured the every birth the animals ought to get slightly better, they travel slightly faster, require less food etc. They would also differ from their parents in size and in colour, so every birth the colours from the two parents are taken (HEX) and a new colour with a possible difference of +5 or -5 in R, G or B is created. I originally stated that the 'babies' should start as eggs, this would add a bit of competitivity between animals as of course eggs feed better than grass

Quite a lengthy Idea but I think It would be quite fun to leave some animals around and see what happens after a month or two.
As well as C & C I ask that people post what stats they think animals should have (and how they effect them). Refer to the slightly randomness at birth as the birth difference.
So far I think-
Size - base animals' stats and birth differences.
Hunger - covered in post, related to size and birth differences, animals will eat more until they reach 1/2 age (hungrier animals will run faster than towards food).
Speed - how fast it can run... (faster animals require more food) (based on base animal and birth difference). Age effected.
Smell sensitivity - how far it has to be to from 'an edible' to 'lock on' and eat it. Age effected.
Lifespan - size, base animals' stats and birth differences.
Health - How much it can be hit by other animals before it dies, base animals' stats, age, hunger and size.
Damage - Size age and base animals' stats.
Pack - (name pending) The loyalty animals have to their own kind, these animals will travel in packs or herds and will fight for their species eggs (will run away at different levels of health depending on their loyalty). Base animals' stats and birth difference.