Why are so many South American animals smaller than their counterparts on other continents?

 Smaller animals have an easier time navigating dense rainforests, says Melissa Hawkins, curator of mammals at the National Museum of Natural History. Deer in the Amazon, for instance, evolved to have narrower antlers than North American deer. Human activity can also influence animal size. The African elephant evolved to roam wide-open savannas, but trophy hunters selectively killed off the largest members of the species. You’d be hard-pressed nowadays to find one as large as Henry, the 13-foot-tall elephant on display at the Natural History Museum.