Near ovulation, females seek out and willingly mate with prime males - those with impressive flanges on the sides of their face - while avoiding lesser males. At times when the risk of conception is low, however, they consent to mate with any male they encounter. This means that even if prime males don't have more sex than other males, they will still have more offspring.
It is thought that this behaviour probably evolved as a result of male orangutans being about twice the size of females, meaning the latter find it hard to say no when it comes to sex. Mothers usually provide all the care for young orangutans and are keen to increase their chances of survival by ensuring they have the strongest genes.
Researchers also suggest that females use sex to protect their young from the risk of infanticide - thought more likely to occur at times when there is instability in a group, such as when a new alpha male takes over. Pregnant females actively solicit sex, especially from prime males, thereby causing confusion about who the baby's father is. It is thought that males are more likely to be protective of, or at least less likely to harm, a young orangutan they think might be theirs.
Results come from a study in Borneo which involved observing the natural behaviour of wild oraguntans, as well as collecting urine samples to test the hormone levels of different animals.