At times, the DWA troop of Red howler monkeys, unique in North America, are heard long before they are seen. The mechanical-sounding, rumbling growl is produced by an enormous larynx (voice box) and can be heard three miles away. The only animal sound that is louder is made by Blue whales. The howlers are often next to the ceiling of their high enclosure. While this South American primate was notorious for being difficult to keep in zoos, it has done well at the DWA, where a number have been born and raised.