Duetting is symptomatic of monogomy?

At least in tropical birds and some primates, says Sarah Hrdy in "The Woman that Never Evolved" (page 46, last paragraph):
"Before the report by Telson and Tenaza (published in 1976), no one suspected that Old World Monkey-much less a langur- would be monogomous. Until then, known cases of monogomy in higher primates came from either New World monkeys or gibbons. My own first respnse was increduality. It was only when Ron Wilson played me a recording of a male and female Mentawei langur singing duets that I believed my ears. I had to. As has long been known for tropical birds, duetting is symptomatic of monogomy"