The lion was responsible for hunting and providing food for the lion and the two cubs. So, everyday, he would go out in the morning and would come back within evening with food, and provide it to the lioness, who fed the cubs and herself.
One day, it was nearing sunset and but the lion had not found any prey after wandering throughout the jungle.
He thought of returning empty-handed. On his way, he found a baby jackal. He would finally be able to provide food for his family. But he took pity on the jackal, because it was just a baby. So, instead of killing him, the lion carried the baby jackal home and presented him to the lioness.
The lioness said, "My dear, He is only a baby. How can I kill and eat the jackal that you have not killed yourself. Let us then raise the jackal with our own cubs. They will soon like each other and play together as brothers."
And so, the lion and the lioness started caring and raising him as their own, without making any differentiation or partiality towards any of them.