Final answer:
A monogamous, small-bodied ape with low sexual dimorphism and known to duet with its partner is indicative of gibbons. These lesser apes are unique in their long-term monogamous relationships and vocal interactions with mates, standing out among primates.
Step-by-step explanation:
If we consider a monogamous ape that is small-bodied, exhibits low sexual dimorphism, and is known to duet with its partner, the description fits the gibbons quite accurately. Gibbons are lesser apes that are known for their monogamous pairings and reduced levels of sexual dimorphism compared to other primates. They are unique among non-human primates due to their long-term monogamous relationships and their singing interactions with their partners, often referred to as 'duetting'. Gibbons provide a stark contrast to their great ape relatives such as chimpanzees and bonobos, which exhibit more complex and less monogamous mating behaviors.
New World primates such as tamarins and marmosets also form monogamous pair bonds and show collective parental care, but they differ significantly in being considered monkeys rather than apes. Furthermore, it is important to acknowledge that there is no simple correlation between anatomy and behavior across primate species including aspects of monogamy and dimorphism. This variability among primates, including our closest relatives, indicates the biological flexibility and diverse social expressions of their gender and sexuality.