Oh now that is unique! Haven't seen anything like - makes sence on aging them then...thanks, quite eye-opening - I always thought they were growing horns constantly during their life time - like Chamois - that reminds me of.

Also pictures here speak volume:

http://www.onthewingphotography.com/wings/2012/11/06/the-horns-of-pronghorns/

Their body size is quite misleading out in the open (to me that is) - they look quite bigger than they actually are...

Well I am glad I have learned something new today - thanks!