I’ve been working with golden lion tamarins (Leontopithecus rosalia), also known as golden marmosets, since 1983, when they were critically endangered and their global population had plummeted to about 200. Brazilian agencies had partnered with zoos inside and outside the country to reintroduce captive monkeys into their original habitat, the Atlantic Forest — South America’s largest coastal forest, at about 15 million hectares. I joined that partnership as a volunteer while still at school, and in 1992 the Golden Lion Tamarin Association took over the project.

I became a biologist in 2000, and started leading the association’s field team, based in Rio de Janeiro state. We collect data about the behaviour of all the groups of tamarins in our protected area, in the São João river basin. We examine births, deaths and transfers to other groups, take blood and fur samples and install collars with transmitters and identification marks.

In this picture, taken last November, I’m observing a group of tamarins that includes the alpha female and her juvenile brother. Golden lion tamarins have mane-like hair around their face and ears and bright orange fur, but are hard to spot; they can recognize individual humans and don’t like strangers. Many are afraid of me, because I’m the one who usually captures them for measurements. I use bananas as bait.

They’re very family-oriented: group members take care of each other and everyone helps with the infants. Bees sometimes invade their nests, and the stings can be fatal. Tamarins have about 20 types of call, which they use to warn each other about predators, keep in touch with other group members and communicate at ‘bedtime’, for example.

Our goal is to expand the population to at least 2,000 individual tamarins across 25,000 hectares of the Atlantic Forest by 2025. They’re not critically endangered any more, but there’s still a lot to be done.