not do well in mixed-species groups, and formation of such groups for social enrichment is not recommended.

Old World monkeys exhibit true menstrual cycles with the sloughing of the uterine wall lining at the end of the luteal phase of the cycle if fertilization has not taken place (Catchpole and Van Wagenen 1975). Some species (e.g., rhesus, bonnet, and Japanese macaques) are seasonally polyestrous with a hiatus in ovulatory cycles during late spring and summer months. Others (e.g., crab-eating macaques) show seasonality only in some circumstances. Other Old World monkeys might show a tendency to give birth more frequently in some months (e.g., sooty mangabeys), whereas others seem to give birth with equal frequency in all months. Indoor housing can be expected to alter these observations from wild populations. Menstrual cycles are generally 4–5 weeks long, and copulations tend to be concentrated at midcycle, although some species (e.g., Macaca arctoides) seem less hormonally controlled and others respond more to social conditions than to absolute hormone concentrations (e.g., rhesus monkeys).

Sexual swellings (edematous engorgement of the skin, usually in the perineal area) are distributed in an erratic pattern among Old World monkey taxa. In species in which they do occur, females will cycle between full swellings at or near the time of ovulation to no swelling during the late luteal early follicular phase. Some taxa also maintain partial or even large swellings during pregnancy. Sexual swellings and other visual, chemical, auditory, and behavioral cues might synchronize the reproductive activity of the two sexes. The size of sexual swelling varies among species and among individuals within a species. In some taxa, it is common only among adolescent females and disappears in fully mature females.

In some species, clear evidence of male–male competition is seen (e.g., baboons), but female choice also plays an important role (e.g., in rhesus monkeys). In some, consortships are formed that endure beyond the time required for copulation; but in most multimale groups, females will mate with more than one male, and the degree of correlation between dominance, observed matings, and paternity is highly variable.

Male–male competition is highly variable in expression, but in most groups male aggression is ritualized and produces few serious injuries. Under natural conditions, male transfer between groups during the breeding season might account for more wounding than males fighting over females (Ruehlmann and others 1988). Infanticide has been reported in several locations for hanuman langur when a new male takes over a group (Blaffer Hrdy 1977). Although suggested in a wide variety of other species, its prevalence is debatable. In most cases, even when it does occur, it is most likely when a new breeding male replaces the former resident male.