His interest piqued, the scientist and his team returned a few times to the spot in the Amazon rain forest and observed more of the primates, which also have a light-gray forehead stripe and ochre sideburns.
Further research revealed the team had discovered a new species of titi monkey—small, tree-dwelling primates that number more than 30 species across South America. (Related: "Pictures: Bushy-Bearded Titi Monkey Discovered.")
The newfound species, Callicebus miltoni—also known as Milton's titi monkey or the fire-tailed titi monkey—was named in honor of Milton Thiago de Mello, a Brazilian primatologist, according to a recent study in the journal Papéis Avulsos de Zoologia.
Dalponte says he was surprised to discover a new monkey in the Brazilian Amazon, especially since that region was explored as part of the Roosevelt-Rondon Scientific Expedition of 1913-14, which was jointly led by Theodore Roosevelt.
"A century has passed before we learned of the existence of Milton's titi monkey in this region," said Dalponte, of Brazil's Institute for the Conservation of Neotropical Carnivores.
Like their relatives, Milton's titis live in small family groups consisting of a monogamous adult pair and two to three of their offspring.
The fruit-eating, 3.3-pound (1.5-kilogram) monkeys spend a lot of time grooming each other, and sometimes sit next to one another on branches with their tails endearingly entwined.
"These behaviors may support the ties between family members and the male-female pair bond," says Dalponte.
When startled, Milton's titi monkeys quickly disappear from sight and hide in dense parts of the forest canopy. (Also see "Monkey Day Pictures: Our Favorite Primates Around the World.")
However, Dalponte and his team were sometimes able to find the monkeys by listening for and playing back recordings of their vocalizations, which are among the most complex in the animal kingdom.
Stuck in Place
Milton's titi monkeys live in a small area of lowland rain forest between the Roosevelt and Aripuanã Rivers.
Because these monkeys cannot swim well or cross mountainous terrain, they are stuck there.
"The rivers fragment the forest and isolate the population of the new species, leaving them vulnerable to various types of human disturbance, such as forest fires," says Dalponte.
About 57 percent of the monkeys' territory is protected as either a conservation area or indigenous land. However, deforestation and development is common in the remainder of the monkeys' habitat.
Another potential threat, Dalponte says, is the Brazilian government's plans to construct new hydroelectric dams and extend the road system within the Amazon.
It's too early to say whether Milton's titi monkey is endangered, he added, but it's certainly not out of the woods.