Animate creatures become miserable in the next world from their own actions and as the result of those miseries they are (again) born as lower animals.
जन्तुस्तुकर्मभिस्तैस्तैः स्वकृतैः प्रेत्य दुःखितः। तदुःखप्रतिघातार्थमपुण्यां योनिमाप्नुते॥