A monkey imitates what others do. Nala, a monkey in Sugriva's army, once resorted to penance, simply because it saw some sages doing so. When Lord Shiva appeared before the sages and asked them what boon they craved, the sages said they sought nothing, except the ability to worship the Lord always. Shiva granted them their wish.
Lord Shiva then asked the monkey what it wanted. Nala wanted to ask for something original. It so happened that at that time someone flung a stone into a river and the stone sank. So Nala prayed that any stone touched by him should not sink. The Lord granted Nala his wish.
Later, when a bridge was being built to enable the monkey army cross over to Sri Lanka, Jambavan recalled this incident, said Goda Venkateswara Sastrigal.
Jambavan thought that enlisting the service of Nala would speed things up. So he told Nala to sit near the sea. All Nala had to do was to touch every stone brought by the other monkeys. Once the stones were touched by Nala, they would not sink, and so the bridge would be ready in no time at all!
So Nala, who had started out doing penance just because his nature as a monkey was to imitate, ended up being a very important person, when it came to helping Lord Rama in His quest to rescue Sita from Ravana's clutches.
Thus every monkey was a cog in the wheel, doing its bit towards rescuing Sita. Watching the monkeys helping Rama, a squirrel thought it too should help. The squirrel didn't have the strength to lift stones as the monkeys did, and Nala was anyway making sure that the stones did not sink. But the squirrel could see that there were gaps between the stones and it decided that it should seal the gaps. It therefore wet its body and then rolled on the sand and then came and shook the sand from its body in the gaps between the boulders, hoping that the sand would seal the gaps.
Moved by the squirrel's desire to help, Lord Rama ran his fingers affectionately down its back.
It is said that the three stripes on the back of all squirrels came from the finger prints that Lord Rama left on the back of that helpful squirrel.
