The black lion tamarin (Leontopithecus chrysopygus), also known as the golden-rumped lion tamarin, is a lion tamarin endemic to the Brazilian state of São Paulo, almost exclusively at the Morro do Diabo State Park. Its limited geographical range makes it the rarest of the New World monkeys, with little known about it. It was thought to be extinct for 65 years until its rediscovery in 1970. In 2016 an adult couple was found to the east, in the Caetetus Ecological Station, after six years with no sightings. A 2020 assessment by the International Union for Conservation of Nature (IUCN) estimated that there was 1,600 individuals living in the wild, 1,200 of which are found in Morro do Diablo State Park. They are usually found in groups of 4 to 9, living in the secondary and primary forests along the circumference of its home range. On average, the black lion tamarin weighs . Taxonomy The classification of the black lion tamarin was debated, as one group of taxonomists classified the lion tamarins by their geography, while other taxonomists placed them all into one species and then divided them into subspecies. More recently, taxonomists have agreed to base classification predominantly on their geography, though sometimes characteristics such as long calls are used to classify different species, similar to the use of bird songs in taxonomy. For differentiating within Leontopithecus, the black lion tamarin is categorized as starting its call at the lowest note and going through the greatest range of pitch. Behaviour and ecology Diet The diet of the black lion tamarin is seasonal and varies with the habitats it moves through. When the tamarin is in the dryland forest, it usually eats a variety of fruits, whereas in a swampy environment it predominantly feeds on the gum of various trees. In addition to seasonal variation, the black lion tamarin exhibits daily and monthly cycles of food preferences. Independent of the environment it occupies, a tamarin spends long periods each day searching for different types of insects and spiders to feed on. On average, 80% of its time foraging is spent searching for insects, such as by foraging the forest floor. The tamarin's foraging locations are very intentional: it spends extended periods of time looking under dry palm leaves, in loose bark, and in tree cavities, with hands that have specialized fingers for prying. The tamarin also positions itself in trees and