There is a published paper by Knott et al. (2009) from next month's Proceedings of The Royal Society B about female Bornean orangutans and their reproductive strategies. The authors propose that female orangutan's mating strategies is a product of coevolution from male coercion and also selective resistance.
There are two morphs in male orangutans; flanged or unflanged. Both are secondary sexual characteristics (traits that distinguish the two sexes of a species but are not part of the genitalia). Flanged males are usually adult dominant males. Unflanged males are either juveniles (who haven't developed their secondary sexual characteristics) or non-dominant adult males. These adult unflanged males might develop a flange when they become a "dominant" male or might never develop one in their lifetime. High levels of forced copulation in orangutan is common, especially by unflanged males. Orangutans are polygamous and males generally do not spend anytime with the female after copulation. Orangutans are usually solitary (with the exception of mother and infant pair), due to scarcity of food sources.
The authors found that females, when near ovulation, mated cooperatively only with prime flanged males. When the conception risk for these females was low, they willingly associate and mate with unflanged males. These observations supported the hypothesis that, together with concealed ovulation, facultative association is a mechanism of female choice in a species where females can rarely avoid coercive mating attempts. Female resistance reduces copulation time and may provide an additional mechanism for mate selection. However, female mating strategies is important to avoid aggressive interactions from flanged males and also as infanticide avoidance.
Knott, CD. Thompson, ME. Stumpf, RM. McIntyre, MH. 2009. Female reproductive strategies in orangutans, evidence for female choice and counterstrategies to infanticide in a species with frequent sexual coercion. Proceedings of The Royal Society B Retrieved October 9, 2009, from http://rspb.royalsocietypublishing.org/content/early/2009/10/06/rspb.2009.1552 [doi:10.1098/rspb.2009.1552]