The monkey shares the fig. Not because he should. Not because he is being watched. He shares because there is more fig than monkey, and a monkey beside him with no fig.
He shares the warm spot. He shares the troop's heat in the night. He shares the fruit-bearing tree he found, and then everyone has fruit, including him, who found it.
We were told scarcity was the law, that you keep what you find or lose it. The capuchin did the math the other way. The capuchin found that giving fig to monkey makes monkey give fig back, eventually, mostly, and that this is what a troop is.