No primate species is purely polyandrous. Some mating relationships in tamarins are polyandrous but most are monogamous. Mates provide extensive paternal care in polyandrous relationships. A male has several female mates. Male reproductive variance is greater than females'.
Reciprocity is facilitated by individual recognition and long-term memory.
Natural selection favors traits improving reproductive success. Resource-defense polygyny is expected in areas where critical resources are unevenly distributed and clumped. Baboon males have larger canines and body size than females. Human females have larger breasts and hips than males. Species are reproductively isolated from each other.