Systematic study of nocturnal primates in general is difficult due to observation conditions at night and to lack of ability to detect signals that animals may use to communicate with one another. Nocturnal primates, including aye-ayes, often communicate with olfactory signals that are temporally deferred in their delivery, and with vocalizations that researchers cannot always trace to their source (Sterling and Richard, 1995).
Nocturnal primates were long believed to have simpler and more homogeneous social systems than diurnal primates. The vast majority of solitary primate species are nocturnal, and without the ability to observe interactions between individuals, researchers tended to associate a lack of gregariousness with a lack of social complexity. Nocturnal primates tend to be predominantly nongregarious and spend much of their time alone, whereas most diurnal primates live in social groups consisting of individuals that know one another, interact regularly, and spend most of their time nearer to one another than to nongroup members (Sterling, 1993b). Nevertheless, nocturnal primates sometimes form social networks between animals that recognize one another and interact regularly, but that may not spend a significant amount of time in proximity to one another. Richard (1985) suggests that many solitary foragers live in "neighborhoods," in which individuals do not live in distinct social units but are most familiar with those individuals whose home ranges overlap the most with their own home ranges. Nocturnal primates appear to communicate with each other in a variety of ways and to develop complex relationships, but our measurement techniques and sensory capacities remain unable to grasp the majority of these interactions. Nevertheless, research is showing aye-aye social systems to be much more complex than was originally thought (Sterling and Richard, 1995).
Vocal and olfactory signals are particularly important in the social organization of aye-ayes, as their nocturnal and generally solitary habits preclude the use of visual signals in many situations. Aye-ayes on Nosy Mangabe communicated primarily by means of calls and scent-marks (Sterling and Richard, 1995). No research to date has explored vocal communication between aye-ayes in the wild, but one study of captive animals (Stanger and Macedonia, 1994) has provided information on the number and structural complexity of vocalizations as well as some of the contexts in which they are used. Captive aye-ayes were found to emit six different vocalizations in a variety of situations, and three additional vocalizations have been heard from free-ranging animals. The aye-aye's primary contact call has enormous acoustic variation and is used in many different contexts, whereas most other lemurs use several contact calls, often for spacing purposes. At first glance, this repertoire of nine vocalizations appears to be rather small for a primate, but given that aye-ayes are relatively solitary animals and that other nocturnal lemur species also have small vocal repertoires in captivity, this situation may not represent an aberration among primate vocal patterns (Stanger and Macedonia, 1994).
Olfactory signals, which can provide important information about an individual's age, sex, reproductive status, and territory, can be an effective mode of communication between individuals that may not come into direct contact very often (Kappeler, 1998). Wild aye-ayes on Nosy Mangabe exhibited three kinds of scent-marking: ano-genital rubbing, head and chest rubbing, and overmarking, where individuals urinated or dragged their genital region over an area previously marked by another animal (Sterling and Richard, 1995). Price and Feistner (1994) found evidence that captive aye-ayes can discriminate between the scents of conspecifics from different age-sex classes. The structure of the aye-aye's nasal cavity is unusual when compared to other lemurs in that the maxilloturbinal is somewhat stirrup-shaped, with a single inferior scroll (Tattersall, 1982), and as in many nocturnal species, the olfactory bulb forms a greater percentage of brain volume than in diurnal species. However, many of the details regarding the role that olfactory signals play in social and sexual communication between aye-ayes have yet to be explored.
In addition to communicating across space and time by means of calls and scent-marking, aye-ayes do sometimes interact directly with other members of their species. Sterling (1993a) reported that aye-ayes on Nosy Mangabe generally spent less than 20% of their time within 20 m of another aye-aye of either sex. Males and females differed in their reactions to conspecifics of the same and the opposite sex. Females rarely came into proximity with one another, and when they did, their interactions were usually aggressive and involved fighting or chasing. Interactions between males and females occurred more frequently than those between two females, and the nature of these interactions was highly variable. Tandem foraging, where individuals foraged in the same or adjacent trees and called to one another prior to moving in tandem from resource to resource, made up the largest percentage of time that males and females spent in close proximity. Similarly, affiliative vocalizations between the sexes were heard more frequently than agonistic ones. Males interacted with each other more often than they interacted with females, and certainly much more often than females interacted with each other. Relationships between pairs of males ranged from tandem foraging to avoidance and aggression.
Tandem foraging deserves special attention in a discussion of aye-aye sociality, as it may demonstrate that the species is not entirely solitary, as has long been assumed. Aye-ayes do often forage alone, but Sterling (1993b) documented groups of up to three individuals foraging and traveling together on Nosy Mangabe. Foraging associations were observed between adult males, adult and young males, and adult males and females. Aggregations of several aye-ayes have also occasionally been seen foraging together in Madagascar's mainland forests (Ancrenaz, 1991; Sterling, personal observation). The repeated occurrence of tandem foraging associations suggests that aye-ayes may be more social than was originally thought, and that established relationships may exist between pairs or groups of individuals.
The occurrence and composition of sleeping groups is another major axis organizing the social diversity of solitary primates (Kappeler, 1997a; Müller and Thalmann, 2000). Although aye-ayes show a tendency toward solitary resting, they do sometimes sleep in nests together or near one another. Aye-ayes sleep during the day in oval-shaped nests located 7-20 m from the ground in the fork of a tree or in a tangle of lianas, constructed of branches and lianas from contiguous vegetation. A single individual usually occupies a nest for a few days at a time, frequently refreshing it with new vegetation. Multiple aye-ayes may use the same nest at different times, as previous occupants vacate the nest and move on to new areas. Sterling's field study on Nosy Mangabe (1993b) revealed that females never slept near other females, whereas males slept near other individuals (male or female) only during the mating season. Several aye-ayes on Madagascar's mainland have been seen to build and use nests in a single tree during the same day (Ancrenaz et al., 1994), and males and females have been seen to sleep in nests located only 5 m apart (Andriamasimanana, 1994). Adult males have been observed to share a nest on mainland Madagascar (Sterling, unpublished data). Existing data are not sufficient to draw any conclusions regarding the relationships between individuals or groups of aye-ayes based on choice of sleeping site, but the possibility of social relationships between individuals that sleep near one another should not be ruled out.
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