The Effects of Male Womb-Mates


If a mother dog’s hormones can affect her fetuses in-utero, what about the hormones of their siblings? The effects of in-utero testosterone on females with all male siblings are well documented in rodents and cattle, and debatably exist in humans. These females undergo physical and behavioral masculinizing effects.

This effect is not believed to extend to dogs. The likely explanation?

There are three types of mammalian placentas:

-In primates (including humans) and rodents, blood exchange occurs directly from sibling to mom to the next sibling, allowing the indirect transfer of blood-borne testosterone between siblings.

-In ruminants (including cows), there is no direct blood exchange between mom and fetus, but blood vessels within an inner membrane can connect siblings to each other, allowing direct exchange of blood-borne testosterone.

-In carnivores (including dogs and cats), there is neither direct blood exchange between mom and fetus nor directly between two fetuses.

Therefore, there is no known mechanism for hormone-exchange between fetuses in dogs. However, anecdotal reports exist of masculinized female puppies in otherwise all-male litters. Is there some other mechanism we haven’t discovered? Or is this due to the post-natal environment of growing up with a bunch of brothers? We don’t know!