breed is a special primitive that can only be placed in the top of a netlogo code tab. Using breed, you can define custom kinds or breeds of turtles. It is most useful when we have different groups of turtles that each need to have their own behavior and we want to have an easy way to reference each group within your code.

To create a new type of turtle, we use the breed primitive followed by an opening square bracket [, the plural version of the breed, a space, the singular version of the breed, and a closing square bracket ]. For example, breed [dogs dog], breed [buildings building], and breed [cars car].

NetLogo requires both the plural and singular versions of a breed because:

• Different spoken languages may have different conventions for plural and singular words
• Some things may have non-conventional forms for plural and singular such as breed [cacti cactus], breed [mice mouse], or breed [leaves leaf].

breed is also special in that it creates other related commands that can be used later on in your code. For example, if we create a dogs breed, we can use primitives such as create-dogs, dogs-here, hatch-dogs, sprout-dogs and so on.

We can create as many breeds as you wish in your model. We can also use the link-breed primitive to create different groups of links.

In the example model example below, we use breed to create two types of turtles with very different behavior: trees and lumberjacks. Trees just sit there, while lumberjacks wander around and cut down the trees if they happen upon a patch with a tree. While it wouldn't be too difficult to create a version of this model without breed using custom turtle properties defined with the turtles-own primitive, breed allows us to easily differentiate between different types of turtles. Using breeds also makes it easier to read the code.