Thomas Jefferson used to boast to European statesmen of the greater dimensions attained by American animals than their Old World counterparts. Though it’s not strictly accurate, the claim has an element or two of truth: Several mammals also found in Eurasia reach their maximum size in North America. The mammoths, mastodons and giant short-faced bears of the Pleistocene may be gone, but the United States still has a few colossi among its land mammals: specifically, some jumbo-sized ungulates...
