Female house mice (Mus musculus) adjust the “fertilizability” of their ova depending on the social conditions in which they were reared.

Females are thus able to physiologically alter the external properties of their eggs to make them more or less fertilizable by sperm.

Let’s stop and think about that for a few minutes.

First, why would a female want to alter the fertilizability of her ova?

Ovary defensiveness is one mechanism by which females can inhibit polyspermy, or the incidence of more than one sperm molecule entering the egg.

Polyspermy is deadly to an embryo, so it is certainly something that a female would aim to avoid.

When female house mice have evolved in social situations that are polyandrous, their eggs become more resistant to penetration by sperm.

When females have evolved in monogamous situations, their eggs are less resistant.

This type of anticipatory phenotypic plasticity has long been demonstrated in males – who can alter the sperm content and components of their ejaculate based on the scenario at hand.

In this case, a polyandrous social situation will mean that males have aggressive ejaculates with high sperm counts. Monogamous males will have the opposite.

This is groundbreaking work: physiological changes to a female’s eggs based on the social environment has scarcely been documented before.

Having a more resistant egg when there is more likely to be a high level of sperm competition within her reproductive tract allows for successful fertilization by only the most competent sperm, and this evidence of cryptic female choice is clearly advantageous for the female.