Critically, however, freemartins are a form of microchimerism. They literally have cells with Y chromosomes in their bodies, received from the male twin through the shared chorion. You're correct that there is a shared mechanism - they receive testosterone and anti-Müllerian hormone through this link, which are primarily resonsible for masculinizing the freemartin.

In contrast, this mechanism is thought to be only testosterone exposure in utero.

A better comparison, then, might be intra-uterine position in mice [1]. This is similarly a pure testosterone-exposure mechanism (no microchimerism). Briefly, mouse pups are arranged in linear fashion within the uterine horn, and so a given (non-end) female pup has 0, 1, or 2 males adjacent to her. There are clear physiological and behavioral differences in these 0M, 1M and 2M mice, corresponding to the level of testosterone exposure.

[1]: https://en.wikipedia.org/wiki/Prenatal_testosterone_transfer...

Wow, from the article:

> In rural areas folklore often claimed this condition was not just peculiar to cattle, but extended also to human twins; this belief perpetuated for generations, as was mentioned in the writings of Bede. (de Albuquerque, Martim Notes and Queries Volume 2. 1857 by Oxford University Press, p. 149)

Actual observed effect that has now been confirmed by science? Or, a superstition that got lucky?