DISCUSSION

Our population density estimates for lowland and highland forests agree with those of other authors (Table IV). These include estimates obtained by Soini and Moya [1976] for A. nancymai [their A. trivirgatus] on the Isla Iquitos; by Wright [1978, 1985] and A. nigriceps [her A. trivirgatus] of Puerto Bermudez, Pasco, and Cocha Cashu, Madre de Dios; and by Aquino and Encarnación [1986a] for A. nancymai of the Rio Tahuayo area. Estimates differ slightly from those recorded by Heltne [1977] for A. lemurinus of northern Colombia (Table IV).

The relatively high densities recorded for Aotus in lowland forests, as compared with densities of highland forests, suggest that night monkeys are better adapted to the lowland forests. Differences in density after other ecological factors have been considered appear to be related to the availability of suitable sleeping sites. In highland forests the trees, predominantly hardwood, provide few hollows for the nesting required by A. vociferans. Lowland forests, on the other hand, with predominance of softwood trees, provide hollows used by both A. vociferans and A. nancymai, as well as abundant tangled vines and branch platforms, which are used by A. nancymai but not by A. vociferans [Aquino & Encarnación, 1986b].

The census also revealed that the area between the lower and middle Ríos Tigre and Pastaza northward, possibly to 03'S, is inhabited by A. nancymai exclusively, in what otherwise appears to be A. vociferans territory. That Aotus nancymai individuals were trapped live in San Miguel de Pucate, on the left bank of the Rio Huallaga, and in San Felipe, on the north bank of the Rio Marañon, indicates that the range of the species extends farther west than previously known [Hershkovitz, 1983, map p. 214], perhaps to the base of the eastern slopes of the Andes.

CONCLUSIONS

1. Greater densities of both A. nancymai and A. vociferans were recorded in lowland forests than in highland forests.

TABLE IV. Population Densities of Aotus in South America (Names of taxa follow Hershkovitz [19831)

2. Among other possible factors, the lower densities recorded for highland forests appeared related to a scarcity of suitable nesting sites, particularly hollow tree trunks used exclusively by A. vociferans.

3. The enclave of A. nancymai populations discovered by Hershkovitz [1983] in the Ríos TigreTigrillo area extends westward to the Rio Pastaza.

4. The two karyotipic forms of A. vociferans occupy identical habitats throughout the Peruvian range of the species.