Polyandry: An evolutionary puzzle

The fact that males want to reproduce with as many partners as possible can easily be explained in terms of evolutionary biology, since they thereby increase their chance of having large numbers of offspring. Things look different for females; their number of offspring does not increase with the number of their sexual partners. Nevertheless, polyandry is widespread in the animal kingdom.

Scientists from the German Primate Center in Göttingen investigated in a study with mouse lemurs (small primates from Madagascar) whether females mate with different males in order to avoid permanent sexual harassment. They found that the lemur females actively look for different partners rather than just giving in to the males’ pushing. (Proceedings of the Royal Society B).

The classical socioecological model in biology assumes that males want to mate with as many females as possible in order to pass on their genes to the next generation through a large number of offspring. Females, on the other hand, are more discriminating; they look for the best father for their offspring. Sex with changing partners mainly involves disadvantages for them, because they may catch sexually transmitted diseases or could be injured during the sexual act.

Why does polyandry nevertheless occur in the animal kingdom? One explanation could be that the females give in to the courting by males in order to avoid permanent sexual harassment. Elise Huchard and her team at the German Primate Center (DPZ) in Göttingen tested this hypothesis experimentally.

As objects of study, they chose gray mouse lemurs, small Malagasy primates that weigh only about 60 grams, because males and females of this species are almost the same size. The scientists were able to manipulate the size of females by an increase or reduction of the food supply in such a way that in one experimental approach the females were larger than males and smaller in the other group.

The scientists expected that the larger females would mate with fewer males, because they would be better able to defend themselves against undesired overtures. But things turned out very differently: the large females had considerably more sexual partners than the little ones. “The lemur females actively look for changing sexual partners, so this must be advantageous for them“ said Elise Huchard from the German Primate Center.

The scientists also found that the large number of sexual acts causes energetic costs to both males and females. “We therefore assume that polyandry is a flexible strategy which leads to moderate advantages for the female mouse lemurs”, Huchard states. For weaker females it therefore seems to be better to save their strength and to be satisfied with few partners, whereas strong females have an evolutionary advantage if they mate with several males.

Original Publication

Huchard, E., Canale, C., Le Gros, C., Perret, M., Henry, P.-Y., Kappeler, P. (2011). Convenience polyandry or convenience polygyny? Costly sex under female control in a promiscuous primate. Proceedings of the Royal Society B