|The 31 Planes of Existence|
The karma to be reborn a human being -- a rare and fortunate opportunity -- is due to having kept the Five Precepts, the basic morality of abstaining from killing, stealing, sexual misconduct, deceiving, and intoxication.
Violating these basics gives rise to the possibility of falling into the animal world or any of the four lower planes.
Animal womb (tiracchana yoni loka)
|I'm a lucky house cat, but I want to run free!|
As we wander through samsara, rebirth on this plane (or any of the lower realms among the 31 Planes of Existence) is brought about by the Ten Unwholesome Courses of Action (MN 41). They are called "courses" because they lead to rebirth in unwelcome, unwished for, unwanted destinations.
Lack of virtue, holding wrong views, particularly pernicious ones like not believing that actions bear results, and actions based on ignorance (delusion) will lead to this plane of existence and worse. If one has been generous to monastics, however, one may be reborn as an "ornamented" animal (i.e., a bird with bright plumage, horse with attractive markings, etc. AN 10.177).
Behaving like an animal (MN 57), such as eating flesh or hunting, can also result in being reborn that kind of animal. It is far better to be a vegetarian/vegan. Chart