males might compete for a receptive female, most competition consists of noisy display.

Gibbons and siamangs will breed when maintained as pairs. Males are reported to show care of older offspring (they might sleep with a juvenile and spend much time with it) after new infants are born. These animals do well in family groups.

Infant apes can be nursed for 3 years or longer, and interbirth intervals of 4 years or more are not unusual (Fritz and others 1991). Development to full adult size can take 9–10 years in gorillas and even longer in orangutans. One curious observation in orangutans is that the presence of a fully adult male inhibits the development of cheek flanges and other secondary sex characteristics, but not fertility, in maturing males (Kingsley 1980; Rodman 1988).

As with New World and Old World monkeys, sexual maturity can precede full physical size, and pregnancies have been reported in exceptional females as young as 6 or 7 years old. A period of adolescent sterility might follow puberty and first menstrual cycling. Females also reach sexual maturity and full body size several years before their male age peers.

Genital rubbing between bonobos of every age-sex combination has been observed in natural groups. This should be regarded as part of their social repertoire, rather than sexual behavior, and it neither enhances nor decreases reproductive activity (Thompson-Handler and others 1984).

Cognition

Although gibbons and siamangs have not been noted for advanced cognitive capacities, great apes exhibit remarkable complex learning and tool-using skills. They readily develop concepts in formal training, and orangutans and chimpanzees can recognize themselves in mirrors and in televised images (Gallup 1977, 1982; Lambeth and Bloomsmith 1992; Menzel and others 1985). Enrichment devices to stimulate activity will occupy much of the time of captive great apes (Bloomstrand and others 1986; Paquette and Prescott 1988; Rumbaugh and others 1989). They interact with objects creatively and will spend substantial amounts of time with simple tools and sticks. They like fabrics—such as squares of carpet, various types of cloth, or blankets—for examination, destruction, and use as nesting materials (Bernstein 1962, 1967). Chimpanzees also become adroit in performing a wide variety of complex video tasks, using a joystick to control events on a monitor.

If they are reared from birth in an environment where humans speak to them communicatively and extensively, there is evidence that at least bonobos and chimpanzees can understand many requests. That can be helpful, for example, when an attendant asks a chimpanzee to trade a cage lock or piece of equipment for food (Laule and others 1992).

If chimpanzees are allowed to see and hear real-world events on video moni-



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