Will Helenbrook searches and listens for night monkeys at dusk at Manu Learning Centre in the Peruvian Amazon.

A juvenile night monkey peers down from the bamboo stand where its family nest during the day at Villa Carmen Biological Station, Peru.

Will Helenbrook and Megan Quirk use callback technology to study responses from nesting night monkey groups at Villa Carmen Biological Station, Peru.

Student researchers prepare to begin a night monkey survey at dusk at Manu Learning Centre, Peru.

A night monkey is silhouetted against the sky as it begins its nightly forage for food in the jungle with its family.

Researchers collect fecal samples from night monkeys to be used in parasitological research.

A parent and infant night monkey curiously look out from a bamboo stand at Villa Carmen Biological Station.

Isabelle Berman examines seeds in a night monkey fecal sample at the laboratory in Manu Learning Centre.

Night monkey researchers work on data analysis by candlelight at Manu Learning Centre.

Megan Quirk and Melissa Petrich search for the new sleeping site of a group of night monkeys. Groups sometimes change sleeping sites seasonly or in the case of an disturbance.

An infant night monkey, Salterino, perches on the shoulder of Will Helenbrook at Dos Loritos Animal Rescue in Pillcopata, Peru. This monkey like the other wildlife at the center, will be rehabilitated if possible and released back into the jungle.