The principal factors affecting the selection of sleeping sites of family groups include the following: 1) Protection and refuge from predators; 2) easy, routine, or circumstantial access; 3) shelter from weather conditions; and 4) sufficient space.
Protection and Refuge From Predators
The intrinsic security of a sleeping site is assured by its position, location, and its provision for escape when predator attack so requires. These in turn require 1) isolation of the lodging tree, 2) optimum visibility in all directions, and 3) easy access and exit routes permitting rapid displacement when necessary. Isolation is achieved by surrounding water bodies such a lakes, rivers, streams, and swamps, which serve as barriers. For example, at the locality of Bombonaje on the Amazonas River, in a Mauritia palm swamp or "aguajal," the sleeping site of a family group with six members including an infant was found in the dry trunk of an isolated Mauritia flexuosa surrounded by open swamp with a few dispersed trees. The only access route to the lodging tree was via a large leaf of Euterpe precatoria palm.
Easy, Routine, or Circumstantial Access
Lodging trees are generally mature, senescent, or lifeless and desintegrating trees. When sleeping sites are located on isolated trunks, the diameter usually does not exceed 30 cm, thereby facilitating vertical locomotion by short hops. When trunk diameter is greater than 30 cm, access is facilitated by climbers and vines hanging from the tree or by proximity of trees with thin trunks that lead directly or indirectly
Shelter From Rain, Wind, and Abrupt Drops of Temperature
Shelter is afforded by nearly total protection from direct rainfall and by water running down leaves and branches. Sleeping sites in holes of senescent or dry trunks offer optimal conditions by virtue of the subapical, subaxial, and lateral position of the openings (see Fig. 2). In sleeping sites positioned at the bifurcation of principal branches, among vines, climbers, and epiphytes, or in thickets, protection from running water is afforded by 1) accumulations of disintegrating organic remains, which fill the interstices and form an impermeable cover at contact points or nodes in the vegetation, and 2) the laminar superposition of foliage, which facilitates runoff of water laterally. This protection from rainwear has the simultaneous benefit of providing protection from wind and of adequate temperature maintenance.
Sufficient Space to Accomodate the Family Group
When sleeping sites are located in holes, the entrance is of variable dimensions between 14 and 35 cm, permitting the simultaneous entry of two or occasionally more individuals. The internal cavity can accomodate up to five individuals or, in exceptional circumstances, up to six. If the cavity is spacious and ample, simultaneous occupancy with other arboreal and nocturnal mammals can occur.
The 77 sleeping sites of Aotus nancymai, A. vociferans, and Aotus sp. [new karyotype, Ma et al, 1985] were grouped into four main types with all variant forms. The following three types coincide with those already described by other authors: a) tree holes [Thorington et al, 1976; Rathbun et al, 1980; Wright, 1981; Hershkovitz, 1983]; b) dense tangles of climbers and vines [Thorington et al, 1976; Wright, 1978, 1981; Rathbun et al, 1980; Hershkovitz, 1983]; and c) bamboos and dense thickets [Wright, 1981]. The fourth type described here occurs in concavities between the branches of trees, many are protected by dense entanglements of climbers, vines, root mats, and foliage of hemiepiphytes.
Aotus nancymai was found in all variants of all four types of sleeping sites, while A. vociferans was found only in tree holes of type A, coincident with the information of Hershkovitz [19831 that the "grey neck" Aotus of northern Colombia is found only in tree holes. This peculiarity of A. vociferans suggests the existence of a certain selectivity of habitat and occupation area and also might explain the absence or low population densities of A. vociferans in hillside forest (Aquino, unpublished data), where hardwood tree species predominate and trees with suitable nesting holes are rare.
Table II summarizes the observed variation in the position of sleeping sites with respect to height within the forest; Aotus nancymai appeared to prefer sites lower than 19.0 in, while Aotus vociferans often occupied also the middle and upper stories including emergents. Forest physignomy, degree of disturbance, topography, and drainage are factors that may influence the selection of sleeping sites by Aotus species. These observed differences in habitat preferences and behavior by species of Aotus in the natural environment should be studied in more detail.
