Households near the reserve burn plastic waste, releasing pollutants that travel by air and settle on the leaves the monkeys eat.By Andrés MuedanoFor thousands of years, Mexican howler monkeys have inhabited the lush rainforest canopy of what is now Los Tuxtlas—a 155,000-acre UNESCO biosphere reserve in the eastern state of Veracruz, 325 miles southeast of Mexico City. Every morning at around 4:30 a.m., these dark-haired primates wake up and begin their day the same way. First they howl. Then they poop.