Both male and female oryxes have horns, with the females' being more slender. The horns are long, thin, and symmetrical; curve backward (a distinctive feature of this species); and can reach in both sexes. The hollow walls of the horns are so thin that they can easily break. The female has four nipples. The large, spreading hooves are well adapted to allow these antelopes to walk on the sand of their dry habitats. A scimitar oryx can live as long as 20 years. At Smithsonian National Zoo, a female oryx died at 21, an exceptional age since females generally have a lifespan of about 15 years.