See the lightning talk for the AI challenge.
Players take turns. A turn consists of an optional exchange, followed by rolling dice. The first player to get (at least) a rabbit, a sheep, a pig, a cow, and a horse, wins.
The player rolls the two (dodecahedral) dice, which look like this:
Fox die: 6 rabbits, 2 sheep, 2 pigs, 1 horse, 1 fox Wolf die: 6 rabbits, 3 sheep, 1 pig, 1 cow, 1 wolf
Depending on the outcome of the roll, the animals may "breed" and the player may end up with more animals as a result. The process behind this is best explained by example:
Player has no animals, but rolls two rabbits => 1 pair of rabbits. That gives the player 1 rabbit.
Player already has three rabbits, but rolls at least one rabbit => 2 pairs. That gives the player 2 rabbits.
Player has three rabbits and five sheep, and rolls a rabbit and a sheep => 2 pairs of rabbits and 3 pairs of sheep. Player gets 2 rabbits and 3 sheep.
Player has 10 cows, and rolls a rabbit and a sheep => no new animals. Only the animals on the dice get to breed.
So, the animals breeding is determined by the dice, and the total number of animal pairs (and thus the total number of new animals) is counted from the total number of pairs in the player's inventory combined with the animals on the dice.
All the animals are taken from a place called the stock, which works like the bank in many other games. If the stock cannot deliver a certain amount of animals, it just delivers the maximal amount instead.
At the start of the game, the stock contains
60 rabbits 24 sheep 20 pigs 12 cows 6 horses 4 small dogs 2 big dogs
A roll turning up a fox will result in all the player's rabbits being "eaten" and returned to the stock, unless the player has (at least) one small dog, in which case the small dog will be returned to the stock.
A roll turning up a wolf will result in all the player's rabbits, sheep, pigs and cows being eaten and returned to the stock, unless the player has (at least) one big dog, in which case the big dog will be returned to the stock. Wolves don't eat small dogs or horses.
Before each roll, a player may make exactly one trade, either with the stock or with another player. The other player may accept or deny; the stock always accepts. There is no haggling, and the conversion rates between animals is fixed:
6 rabbits <=> 1 sheep 2 sheep <=> 1 pig 3 pigs <=> 1 cow 2 cows <=> 1 horse 1 sheep <=> 1 small dog 1 cow <=> 1 big dog
An exchange may be struck as long as the total worth of the animals exchanged is the same. For example, 2 pigs, 1 sheep, and 6 rabbits may be exchanged for 1 cow (and vice versa). The following deals are allowed: one animal for one animal (for example 1 sheep against 1 small dog), one animal for many animals (for example 1 cow against 3 pigs), many animals for one (2 sheep against 1 pig). However, many animals for many (for example 2 sheep and 5 pigs against 2 cows) isn't allowed even when (as in this case) the total worth of the animals exchanged is equal.
If the exchange is with the stock, and the stock has fewer animals of some kind (for example 3 cows) than the amount desired (for example 4 cows), the lower amount is traded. This is the only case where the total worth of the animals is allowed not to add up.