Battle of the blastocysts!

When a female pronghorn gives birth, she typically gives birth to 2 fawns at a time; these fawns have already won a life-or-death battle before arriving in this world. Research suggests that 3-7 eggs are fertilized when a pronghorn is bred and they develop into blastocysts, which is the stage immediately following fertilization but before the egg is implanted into the uterine wall.

Typically only two of those initially fertilized eggs are ultimately implanted in the uterine wall to later become fawns, because to get to the wall, they first must undergo a battle to the death! Each fertilized egg wields a weapon—a spear-like protrusion known as the necrotic tip. As the unfertilized eggs race to reach the safety of the uterine wall, they can become entangled, impaling one another with their necrotic tips. Only the most intrepid will survive. As far as we know, this form of battle in the uterus is unique to pronghorn. We always knew that pronghorn are a strange critter, and this unique behavior is further proof!


Reference: O’Gara, B. W. 1969. Unique aspects of reproduction in the female pronghorn (Antilocapra americana Ord). American Journal of Anatomy 125:217-232.


Description by Rhiannon Jakopak. Illustration by Rachel Smiley.
Pronghorn blastocysts duking it out!
Pronghorn blastocysts bump around the uterus and use sharp necrotic tips to destroy competing blastocysts.