Animal Pens are storage buildings for animals. The first, the Wildlife Habitat, was released on August 17th, 2011. A constructed Animal Pen will allow you to store up to 20 animals, species dependent upon building. Each expansion will allow you to store 10 more animals of whichever type the building holds. The number of buildings is not limited, however you can only have one un-constructed building of each type, per farm, at a time. All mystery babies grow up to be the same types of animal as their parent. The type of mystery baby depends on building and parent type.