Mice have litters of several pups at once. The pups are arranged in a line within the mother’s uterus, so many fetuses lie between two of their siblings. It has been shown that female fetuses located between two male fetuses (2M) experience higher testosterone levels than those adjacent to no male fetuses (0M), because the hormone is produced by the males and diffuses across the fetal membranes and through the amniotic fluid to adjacent females. The higher fetal testosterone levels are known to have several effects on the females later in life, including increased aggression levels, growth rates, and even which eye opens first. One study wanted to measure whether fetal testosterone levels affected how attractive these female mice were to male mice as adults (vom Saal and Bronson 1980).

Twenty-four male mice were given a choice between a female that was 0M and another unrelated female that was 2M. (Both the males and females were randomly chosen from their populations.) Each male was placed on a platform, and he could jump into the cage of whichever female he preferred. Nineteen of the 24 males chose the 0M female.

Is this evidence that the males prefer one type of female over the other?