Adult pygmy marmosets weigh in the region of one hundred grams, with a head-and-body length near fifteen centimetres and a tail somewhat longer. That places them below every other monkey, though not below every primate: several mouse lemurs of Madagascar are smaller still, and Madame Berthe's mouse lemur weighs about thirty grams. The correct claim is smallest monkey, not smallest primate.
Being that small in the Amazon means a narrow way of life. A family group depends on a handful of gum trees, gouged with the chisel-shaped lower incisors shared with other marmosets, and defends a territory that may be under half a hectare. The animals move in short darting runs along trunks and freeze against bark when threatened, since almost every forest raptor and small carnivore is a potential predator.
Recent genetic work has split the pygmy marmoset into two species, divided by Amazonian rivers, which changes how the conservation status of each population should be read.