Nose-horned megafauna hunted for keratin worth fortunes
Rhinoceros means nose-horned in Greek. These megafauna all exceed half a tonne as adults, carrying one or two horns and hide 1.5 to 5 centimeters thick. Poaching for black-market horn has pushed most living species toward crisis.
Five species survive of a once-diverse family of odd-toed hoofed mammals. Linnaeus named the Indian and black rhinos in 1758, though he wrongly grouped them with rodents because the Indian species has incisors, and John Edward Gray named the family in 1821. Living species average from 775 kilograms in the Sumatran rhino to 2,300 in the white; fossil relatives spanned the pig-sized Menoceras of Miocene North America to the 4,500-kilogram Elasmotherium of Ice Age Eurasia. Brains are small for such bulk, about 531 grams in an adult black rhino, and the black rhino carries 84 chromosomes where the others have 82.
Their armour is a folded hide of densely crosslinked collagen, stiffer than other mammals' skin and mostly hairless except in the shaggy Sumatran rhino. Horns grow like fingernails from packed keratin cells anchored to roughened patches on the skull; many extinct relatives had none. The African species lack front teeth and pluck food with their lips, and hindgut fermentation lets rhinos fall back on tough fibrous plants. The white rhino, a grazer with a broad flat mouth, averages 1,600 kilograms for females and 2,400 for males, with a front horn around 90 centimetres long.
Adult males are usually loners, as are Asian females, though African females sometimes group. Both sexes mate with several partners; males sniff out a female's condition by tasting her urine and curling the upper lip, and rivals may fight seriously. Pregnancy lasts well over a year, about 460 days in black rhinos and 504 in whites. Calves stand within half an hour, stay vulnerable to predators until about three, and reach maturity between five and eight, though male whites take about twelve years.
Poaching drives most species toward extinction, fed by demand in China and Vietnam for traditional medicine despite no evidence of benefit, and earlier by Yemen's market for dagger handles. The southern white rhino, with 20,405 wild animals in 2013, is the most abundant; the northern white is down to two captive females.
Source: Rhinoceros
