I know pronghorns drop their horns by the end of November, yet I have seen pronghorns in February and March that seem to have a mature set of horns already. When do pronghorns start growing new horns, and when is horn growth complete? Thanks.

Pronghorn are quite a unique species.  They grow horns that shed annually unlike other species that grow horns that get larger each year like bighorn sheep and mt. goats.  Horns are different than antlers.  Horns are made from a keratinous sheath that grows from a core of live bone tissue.  In bighorn sheep, horn is added to each year.  In pronghorn, the horn is shed each year similar to antlers.  Antlers are in fact a porous fast growing bone tissue that are grown annually by the males of the species and are fed by a "velvet" tissue that is highly vascularized and feeds the rapid growth of the bone. A pronghorn's horn does not necessarily get larger each year and tends to top out between age 2-6.  Nutrition and moisture seem to have influence on antler and horn growth as does genetics.   The female also has a horn that grows similarly to the males, but is usually smaller.  The male's horns are shed from October through December after the rut and are shed when new horn begins to grow underneath.  The new horn is typically complete by late winter or early spring.  Females tend to shed their horns in mid-late summer, but sometimes they can be more variable and collect stacked horns (like paper cups) over 2-3 years.