The largest group of hoofed mammals is the Family Bovidae, which consists of 140 species. Bovids contain very important species that are vital to many of the world's ecosystems, such as wildebeest and impala on the African grasslands. Historically, bison or "buffalo" were critical to the natural succession of prairie plant communities in the western plains of the United States and Canada.

Today, many bovids -- like domestic cattle, goats, and sheep -- are important to humans as sources of meat, milk, and clothing.

Domestic cattle contain over 1,000 breeds, including the aesthetic Texas longhorn, which was introduced to the American West by Spaniards. Domestic cattle are the most abundant bovids, numbering about 1.3 billion worldwide.

The American bison is closely related to domestic cattle and can hybridize with cattle. The bison often is misnamed a "buffalo," but true buffalo are found in Asia and Africa and lack the distinct shoulder hump.

The American bison once numbered between 40-60 million in North American before European settlers, and a subspecies of American bison, the plain bison, extended their historic range eastward into central and western Pennsylvania, probably until the late 1700s.

By 1889, indiscriminate killing of the bison west of the Mississippi reduced the number to an estimated 835 animals -- an untold story is that this historic slaughter of bison was an attempt by white people to eliminate a food supply of the Plains Indians.

Today, these bison numbers over 65,000 (as in large herds in Yellowstone National Park), and the endangered subspecies of bison, the wood bison of western Canada numbers about 1,000.

By the way, if you encounter a female bison in Yellowstone, don't tell it (or a cow, for that matter) a joke because it will go in one ear and out the udder.

Horns are permanent structures present in males of most bovid species, and in females of about 50 percent of bovid species.

Horns are comprised of a bony core that is attached to the front of the skull -- a hard bony sheath covers the bony core. Horns are not exclusive to bovids, but can be found in giraffes, pronghorns, rhinos, and other mammals.

In giraffes, two to four horns occur, both sexes have horns, and young are born with horns (ouch, said the mother giraffe!)

Pronghorns are found only in North American and have horns with a deciduous outer sheath (shed yearly) and a permanent inner core.

All five species of rhinos have horns in both sexes and number 1-2, depending on the species. By the way, all rhinos are facing extinction, largely because horns of rhinos are believed to be valuable as an aphrodisiac.

Two interesting points should be made about horns. First, horns are much more likely to occur in bovid species that occupy open habitats and exceed 100 pounds in body weight.

Forest species of bovids typically are much smaller and rely on hiding to escape predators.

Second, horns of males tend to be curved or spiraled and thicker at the base compared to straighter, thinner horns of females. The thin, dagger-like horns of female bovids specialize as stabbing weapons to defend young against predators.

Thus, predator defense appears to be the major function of horns in female bovids. Predator defense, however, does not appear to be the primary reason for the evolution of horns in males of bovids. Instead, horns in males appear to have evolved for intrasexual combat.

The fact that horns of male bovids have some type of curvature or spiraling and are thick suggests that they serve to deflect blows from rivals and are capable of absorbing considerable force from opponents.

The familiar ritualized "head butting" of rival bighorn sheep attests to the importance of horns in mitigating injury of combatants during these fights to establish dominance hierarchies. In bovid species without ritualized male-male fights, e.g. mountain goats, horns in males are dagger-like rather than curved or spiraled.

In mountain goats, both males and females have unusually high levels of intrasexual aggression.