Four bad destinations
(4) Host of asuras
(3) Domain of ghosts
(2) Animal realm
(1) Hell realms
The early Buddhist texts envisage a universe with three principal tiers subdivided into numerous planes. The lowest tier is the sense-sphere realm (kāmadhātu), so called because the driving force within this realm is sensual desire. The sense-sphere realm (in the oldest cosmology) contains ten planes: the hells (niraya), planes of extreme torment; the animal realm (tiracchānayoni); the domain of petas or ghosts (pettivisaya), shade-like spirits subject to various kinds of misery ...
[url]http://www.wisdompubs.org/book/connected-discourses-buddha/selections/connected-discourses-part-i-introduction
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It is not difficult to see the reason why Buddhism regards animal birth as a state of life included in the plane of misery.
First of all, there is no morality in the animal kingdom; animals live according to the wild law of
nature. Among animals, the strongest is the survivor; the bigger eats the smaller. Danger to life is the
crucial problem for them; it may come at any moment from animals themselves as well as from
human beings. They have to struggle hard for their survival, especially for food which is not always
certain. Sometimes they have enough to eat, but sometimes they have to starve for many days.
Moreover, they have to bear nakedly the hot and cold, the wet and dry seasons of the year without
proper protection. With all these difficulties, animal life, although it is not as much suffering as that of
the beings in hell, is rationally included as one of the miserable existences.
Another reason why Buddhism includes animal birth in the apāya-bhūmi is that whosoever, because
of his evil kamma; attains the birth of animal is naturally obstructed from realising the ultimate truth,
from following the path of virtue which leads to emancipation. This is because animal existence is the
negation of such realisation and practice. Lacking the capacity for realising the ultimate truth is
considered critical since such realisation is the prime aim of life that we struggle for.
http://www.bps.lk/olib/wh/wh462.pdf