Final answer:
The least common mating system among primates is polyandry, where one female has multiple male partners. This rarity is seen in the contrast with more common systems such as monogamy and polygyny within varied primate social structures.
Step-by-step explanation:
The least common mating system among primates is polyandry, where one female mates with multiple males. This system is comparably rare in primate societies. While most primate species exhibit monogamous or polygynous mating systems, very few demonstrate polyandrous patterns. Polyandry may occur in conditions where females are scarce and this ensures that no reproductive opportunities are missed.
Primate societies are highly complex and exhibit a variety of mating strategies. These strategies often involve intricate social dynamics, such as female choice, male-male competition, and cooperation. Primate behavior, including mating systems, shows that apes, along with humans, display remarkable biological flexibility in their social and sexual behaviors.
New World species like tamarins and marmosets exhibit monogamous relationships whereas Old World monkeys generally do not. Within the primate species closest to humans, both bonobos and chimpanzees manifest very different social and sexual behaviors despite their genetic similarity to us.