Each animal is made up of 3 traits, a head trait, a middle and a tail (plants have flower, stem, root). Each trait has a part of a name, a description of the traits, and (in small print) a list of which climates the trait is good (rated 3+ on a 0-5 scale). With the expansions, there are something like 432,000 possible animals. Gnools and Ebbedings are the favorites we've come up with....
There are three types of niches, Plant, Herbivore and Carnivore. Each has an upper and lower niche. The upper niche is the dominant creature of that kind, the lower niche isn't 2nd, and only has 2 traits. (I suppose the lower niche is a class of animals). During a players turn, he can either change an animal (mutate, replace a trait with a trait from his/her hand or from the deck at random) or challenge the animal above it for dominance.
During the challenge, each animal may mutate (to improve, if the owner wishes). Then both players decide if they want to fight it out. If either animal backs down, then that settles it. If both want to fight, then they look at each others animals and try to compare which is better. If one is obviously superior, then the other owner may back down again. If they fight it out, then a card-in-screen-system is used to determine the value of each trait in that climate. (The idea is not to konw the exact numbers). Animals are also modified by the food chain. If they animals are herbivores, for example, they gain a bonus if they eat