Aye-ayes spend the night sleeping way up in the canopy in a nest of twigs, and the first task after emerging at dusk is to devote several minutes to giving the fur a thorough grooming, a task which is usually performed while the animal is hanging suspended upside- down by its feet. Aye-ayes sleep and forage alone for most of the year, although at certain seasons several animals may suddenly start sharing a nest in an abrupt outbreak of togetherness. It is quite common for several different aye-ayes to feed in the same trees on the same night, so perhaps home ranges show considerable overlap.

In several nocturnal prosimians sleeping huddles are common among the more sociable females, but the strictly solo life is the norm for the males. Such solitary habits are not found again among the diurnal lemurs and monkeys until we come to the virtual top of the scale with the orang-utan, one of the most advanced of primates yet with social habits not unlike many of the more 'primitive' examples of the primates.

Orang-utan populations fall into three distinct categories: adult males leading very solitary lives; adult females, mostly with one or two dependent offspring tagging along and sometimes associating with other similar groups for a day or two; and sexually immature animals of both sexes, also mostly leading a lonesome existence. In brief accidental encounters between adult males there is a general disinclination to seek further mutual contact and a strong tendency to shy away from one another; such fortuitous meetings are very rare, probably because the males' frequent renderings of their 'loud call' may help to prevent their coming too close to one another. However, if a female is present in consort with a male then the situation may take a very different and much nastier turn, sparking off a lengthy and deadly serious tussle disputing access to her sexual favours before one or other of the males gives up the struggle and makes off.

In fact most orang-utan social behaviour, if the term is really justified at all, takes place very much in a sexual context. A female in oestrus will probably quickly attract the attention of an adult or subadult male who will establish a consortship with her, perhaps leading to repeated bouts of copulation over several days. In common with most human youths eager for early and satisfying sexual adventures, subadult male orang- utans are not over-fussy about their partners and will enthusiastically copulate with any female who shows willing and sometimes with those who don't - hot-    blooded young male orangs tend to be somewhat forceful and reluctant to take 'no' for an answer, so an uncooperative partner may simply end up getting raped. This may be accompanied by anything from anguished squeals of protest to full-blooded battles of resistance.

Adult males, on the other hand, are more discerning in their appreciation of female values and less liberal with their attentions. So even some eager young seductress flagrantly exposing herself open-legged before his seasoned but disinterested gaze may fail to stir him. He is simply past the stage of temporary liaisons with all- comers which typified his youth, and only interested in investing the necessary time and effort into consorting with well-matured and experienced females capable of making a good job of rearing his offspring. When such an occasion does arise he will stir his great bulk to fierce defence of his investment. Thus the apparent lack of libido in these big old males and the seemingly early demise of their sexual powers may, it seems, merely be the mature development of a finely honed ability to judge feminine qualities.

Such carnal sagacity also extends to the actual act of mating, for whereas subadult males often 'try it on' with a female who is patently and often vociferously unwilling and end up taking her by force, adult males usually only consummate a relationship with fully adult females who openly solicit their attentions. This may be important, as in many mammals rape is thought to result in a far lower rate of pregnancies than is likely in copulations which have been actively encouraged by the female.

One step up from the 'solitary' lifestyle of frequent vocal or olfactory contact but little actual physical interaction is the 'family group' found, but only sparingly, in a wide variety of unrelated primates. A family group usually consists of a reproductively active male and female accompanied by one or more of their offspring. Among the small nocturnal species this lifestyle is found in the spectral tarsier from Sulawesi, a group of which will inhabit a remarkably small home range of only 1ha or so. Territorial considerations play a significant part in their lives. Scent marking by the males is noisily backed up by calling and at least fifteen different calls have been distinguished, many of which are used in shouting matches with opposing neighbours. It is in this species that the 'duet for male and female voices' forms part of the territory-delineating song repertoire.