These charming creatures are "saguis" (marmosets), in fact "Saguis de cara branca", White headed marmosets (Callithrix geoffroyi). One of the tamest wild animals, they are great favourites of tourists, eagerly taking fruit in their little hands. Without tourists, they have to fend for themselves, groups travelling up to 2 km through the forest. They eat fruit and any small creatures they can find, insects, snails or even frogs. They also eat a surprising amount of gum from trees and lianas, either gouging wounds in trees or searching for wounds made by insects. Gum apparently has a lot of carbohydrates, so it is sweet and gives lots of energy to little monkeys.
And saguis are little, which means they need lots of high energy food, but they can turn out to be food themselves. Hence the frequent nervous glances up to detect any eagles or falcons. Mothers carrying young seem to settle for fruit and gum, hunting for tasty insects takes too much attention, and they are too ungainly to bolt if needed. If predators appear there are basically two options. If they see a raptor, there is a brief cry and then they freeze still under cover. But if it is not immediately dangerous, a cat on the ground for instance, they will mob it - harassing it from above as a group and making loud "tsik" calls. Strangely, it has been shown that being part of a "mob" actually reduces rather than increases monkey stress - it is tempting to extrapolate that to humans!
One curiosity of saguis, and other new world monkeys, is that some individuals are trichomatic, and some dichromatic. What does this mean? Humans and higher apes have three different colour cells in their eyes, in combination they can distinguish up to 1 million colours. Most mammals have just two types, so can distinguish only about 10,000, kind of equivalent to colour blindness. So why are some monkeys one type, others another? Apparently trichromatic monkeys are better at finding fruit, especially when its red, but dichromatic monkeys see better in the low light of dawn and dusk, so it all balances out in the end.