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He peered through his binoculars towards the treetops in awe. Perched in the trees were three small marmosets with white tails. Rodrigo Costa Araújo of the National Institute of Amazonian Research, Brazil, was gobsmacked: most other Amazonian marmosets’ tails are black.

That was the moment that Araújo realised he might have a new species on his hands. “I thought, ‘Oh my God, they are different, they are really, really different’,” he says.

Araújo and his colleagues from institutes including the Federal University of Amazonas, Brazil, first spotted the marmosets in a…