Intrasexual aggression between paired and solitary gibbons is one factor that helps maintain monogamy (Mitani 1984). Although she is not involved in most group encounters, a paired female will approach an invading solitary female in order to exclude her from the territory, preventing her from stealing her food supply and her mate (Mitani 1984). Without the presence of solitary females, a gibbon male is limited to his monogamous bond. While females will only make an effort to evict solitary females, males will force out both solitary males and other gibbon pairs (Raemaekers and Raemaekers 1985).

Intrasexual aggression between females may also contribute to maintaining monogamy in that aggression between two or more females would decrease the male�s overall reproductive success (Raemaekers and Raemaekers 1985). The area defensible by a single male is also typically only enough to maintain one female and her offspring (Raemaekers and Raemaekers 1985).
Females tend to distribute themselves across wide areas in order to secure food supplies for themselves and their offspring (Reichard and Sommer 1997). Males tend to follow the females and thus distribute themselves in the same way (Reichard and Sommer 1997). This distribution of females is another factor maintaining monogamy in gibbons because males are left with few opportunities to mate with nearby females (Reichard and Sommer 1997). Thus, males are restricted to one female as a result of the females� �solitary nature, low density, and even dispersion,� as well as the need for paternal care of the offspring (Leighton 1986). While the female is tied up taking care of the slowly-developing offspring, the male is needed to defend the territory (and food resources), and sometimes to help care for the offspring as well (Leighton 1986).

The strong pair bond created through duetting helps maintain lifelong monogamous pairs in most gibbons. Since creating a synchronized duet requires so much time and energy put into practicing, males and females are not likely to want to start over forming a new pair bond. If pairs are constantly changed, much investment is required with each new pair bond formed, resulting in increased time and energy expenditure on the part of both individuals (Wicker 1980 in Geissman and Orgeldinger 2000). This energy could be used in other ways, like caring for young and foraging, which would help increase the reproductive success of the pair.

I am with you, H! I don't know how jealousy ever got such a bad rap... Dr Harley said this about it and I completely agree:

"Jealousy is a normal reaction to a threat to marriage. It's not a Love Buster."

creating a synchronized duet requires so much time and energy put into practicing, males and females are not likely to want to start over forming a new pair bond.

John Milton: Vanity, definitely my favorite sin.