Howler monkeys, known for their deep-throated roars, are dying from an unknown ailment in Nicaragua. The bodies of at least 75 have been found recently in the country’s tropical forests, with no obvious indication as to why. And similar die-offs have occurred recently in Ecuador and Panama. Now, a team of scientists is heading to Nicaragua to find out what is going on and whether these events are related. The researchers, from the University of Michigan, Ann Arbor, will be working with several other institutions to test their hypotheses. They’ll investigate whether the monkeys are suffering from lack of food or water (unlikely, because some monkeys already examined had full stomachs). They will see whether the howlers came into contact with pesticides or other contaminants or were poisoned after eating plants containing toxic compounds. They’ll also collect blood and tissue samples to test for pathogens, including the viruses that cause yellow fever or Zika. Howler monkeys (above) are highly susceptible to the yellow fever virus, but Nicaragua has not reported any cases of the disease for many years. Zika has sickened 24 people in the country so far, but scientists do not know whether the virus affects any species of New World monkey. Although howlers are not endangered, the International Union for the Conservation of Nature has classified them as “near threatened” because of habitat loss. The mass die-offs could lead to extinctions of some local populations, the scientists warn. And that could harm the howlers’ home—tropical dry forests that depend on these fruit-eating monkeys to spread plant seeds.