This close-up of a greater one-horned rhinoceros appears to be a male not because of the horn – both genders have them – but because of the thick neck folds. This fifth largest land mammal in the world averages 4,500 pounds but the largest was nearly twice that heavy. Also called the Indian rhinoceros, only about 3,000 live in the wild. But you can see this one, plus a white rhino and a calf born in 2012, at the San Diego Zoo in California.