At least 75 howler monkeys living in the tropical forests of southwestern Nicaragua were recently found dead. Concerned about the unusual mortality event, two researchers from the University of Michigan are investigating various scenarios that might explain the sudden deaths. 

"It's really, really, really unusual to see this many monkeys sick all at once and to see this many monkeys dead all at once," ecologist Kimberly Williams-Guillén, who has studied wild howler monkeys in Nicaragua since 1999, said in a news release

Of the 75 confirmed deaths, Nicaraguan landowners and forest rangers have reported 70 since mid-January. The Nicaragua deaths follow two smaller howler monkey mortality events in Ecuador and Panama. 

While researchers are only in the early stages of their investigation, they have for main cause-of-death hypotheses: drought or other environmental stress resulting in lack of food or water; poisoning by ingestion of plants containing high levels of toxic compounds; pesticide use or other form of environmental contamination; or a mosquito-transmitted pathogen such as yellow fever or Zika virus

"We don't know why this is happening, and we need to find out," co-author Liliana Cortés-Ortiz added in the university's release. "So we are putting together a team of experts to test all possible scenarios in the three countries where howler deaths have been reported."

So far, examination of some of the dead monkeys revealed mild dehydration; however, the animals had full stomachs, which rules out lack of food or water as the primary cause of death -- for now.  

The next step, which requires certain federal permits, will be to collect tissue and blood samples from several of the dead monkeys to test for the presence of various pathogens, including yellow fever and the Zika virus, and to see if there is any relation between the deaths reported in the three countries. 

While howler monkeys are known to be highly susceptible to yellow fever, the disease has not been reported in Nicaragua for many years, researchers say. Furthermore, more than two dozen human cases of Zika virus have been reported in Nicaragua, but the affect this emerging infectious disease has on monkeys remains unknown. 

Currently, howler monkeys in Nicaragua, Panama and Ecuador are not listed as threatened or endangered. However, the International Union for the Conservation of Nature recently categorized the animals as "near threatened," due to habitat loss and increased forest fragmentation. 

Researchers warn that major die-offs, such as those in Nicaragua, could result in local population extinctions. The loss of these primates would greatly impact tropical forests, as howler monkeys are fruit eaters and disperse plant seeds when they defecate, which ultimately helps maintain forest health.

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