I agree with moonstar2001 & Nithin Sridhar! They may not have self consciousness but again it purely depends on spiritual stage of the soul. Punishment is the same even if we know or dont know reason behind such punishment and by that that karma will be cleared off.
May it be human or any other animal, birth itself may be considered as punishment. We need to experience the result of good or bad deeds done by us. We may be not acknowledge but it can be related to anything like if a human/animal gets a comfort or discomfort of any sort it is result of a previous act. We may need to refer sastras for understand the same.
As discussed here sometimes animals may be more lucky than humans in reaching higher spiritual states, so we need not under estimate animals' birth.
Further suggested discussions are Does karma of previous life affect new life? and How exactly does karma work?