Goeldi's marmoset

Goeldi's marmoset, or Goeldi's monkey (Callimico goeldii), is a small New World monkey found on the South American continent, mainly in the upper Amazon basin of Bolivia, Brazil, Colombia, and Perú. It is the only species classified in the monotypic genus Callimico, thus these monkeys are sometimes referred to as "callimicos". The species takes its name from its discoverer, Swiss-Brazilian naturalist Emil August Goeldi.

Likely an evolutionary adaptation for camouflage and the evasion of predators, Goeldi's marmosets are a dark-furred species, usually a blackish-gray or darker brown in color. Superficially, the short hair on their head gives them a vague resemblance to the larger woolly monkey (Lagothrix); however, the back of their necks, their backside, and tails often display light, horizontal striping or highlights. Their bodies are about 8–9 in (200–230 mm) long, and their tails are about another 10–12 in (250–300 mm) long. Captive Goeldi's marmosets weigh around 480 g (17 oz), while observed individuals were approximately 500 g (18 oz) in the wild. Their digits have claw-like nails, except for the hallux, which serves for clinging, scansorial (arboreal) movement and escape, and the extraction of certain food sources from trees, such as fruits, honey, seed pods, etc.