In this study, population censuses were taken periodically from 1982 to 1985 along the north bank of the Peruvian Rio Amazonas and along its principal tributaries, the Nanay, Tigre, and Marañon, in lowland and highland forests.
The transect method used has been described by Southwick and Cadigan [1972] and modified by Freese et al. [1982]. Tails cleared by hunters were used for censusing by foot. During inundations, surveys of river banks were made moving upstream by motorized canoe. Both methods involved two observers equipped with portable recorders and/or field notebooks and hand counters.
The surveys were made during clear still nights from 0300 to 0600 hr and 1800 to 2000 hr local time. These periods correspond to the times of greatest movement and foraging (personal observation). The distance covered during each time period ranged from 1 to 6 km, for an average of 1.5 km per hour on foot, approximately 2 km per hour by canoe. The distance by canoe, powered with a 6hp motor, was estimated on the basis of the number of minutes per km moving upstream.
Night monkeys were detected by 1) the sound of falling fruit and debris caused by leaping, and 2) alarm calls. The animals were then illuminated by our lanterns. Maximum detection distance was about 20 m. For calculating population densities, 40 m was taken as the full field of detection. Densities per km 2 were calculated by multiplying the number of family groups by the average group size, which was 4.1 for A. nancymai and 3.3 for A. vociferans. These values were extracted from results obtained by trapping 75 A. nancymai and 62 A. vociferans complete family groups (Aquino, personal notes).
Following Hershkovitz; [1983], identification of the Peruvian species of Aotus was made from animals collected and preserved as museum specimens or from animals captured live in the study area and in various other localities along the principal affluents of the Ríos Napo, Curaray, Tigre, Marañon, and Tapiche. Both preserved and live animals are in deposit at the Centro de Reproducción y Conservación de Primates NoHumanos (CRCP) of the Instituto Veterinario de Investigationes Tropicales y de Altura (IVITA, in Iquitos, Peru).
A nancymai occurs.in Peru south of the MarañonAmazonas from the Rio Yavarí on the east to the east of the eastern slopes of the Andes on the west, and on both banks of the Rio Ucayali south to approximately 07'00'S. A. nancymai also is found north of the Marañon in an enclave between the lower and middle Ríos Tigre and Pastaza (Fig. 2).
A. vociferans occurs north of the Rio Marañon from the Rio Amazonas and west to the Eastern slopes of the Andes, excluding the enclave between the Ríos Tigre and Pastaza inhabited by A. nancymai (Fig. 2). The two karyotipic forms of A. vociferans [Ma et al., 1985] are syntopic (Aquino, personal observations), whereas A. vociferans and A. nancymai are strictly allopatric.
