The Beni titi monkey, Callicebus modestus
Lönnberg desribed in 1939 a new species of titi monkey, Callicebus modestus, based on two specimens collected at El Consuelo, Rio Beni, Bolivia. He describes the species as “”in appearance resembling generally to a certain degree that of some members of the cupreus group as it presents a similar grizzled brown colour. This is affected by the outer half of the hairs being annulated with alternating buffish and blackish brown rings. The lower side of the body is rufous, not annulated, which increases the likeness mentioned, but the limbs are not rufous as in cupreus but annulated as the back. The upper parts of the head are looking somewhat more brightly rufous in consequence of the tips of the hairs being more vividly coloured. The extreme frontal edge is somewhat blackish produced by the basal parts of the hairs. The face is beset with short, adpressed whitish hairs. The whiskers are not well developed, tipped with rufous. The hairs of the ears are white. The hairs of the arms are annulated with rufous and black all the way to the black hands. The hairs of the upper or outer side of the fingers are also black, but on the sides between them are also seen some whitish hairs. The hind legs are also annulated as the back, although somewhat brighter rufous, but towards the feet the black becomes dominating, and the upper side of the toes is pure black. The tail is speckled blackish and greyish white. In the older specimen the black is dominating in the middle for about two thirds. The fur of the back is long, generally from 4,5 to somewhat more than 5 cm, and the under wool is also very dense, but the long hairs are not so silky soft as in C. olallae.
This is one of the least know species of titi monkeys, and its relation to Callicebus olallae remains unclear. It is endemic to Bolivia.