Two critically endangered monkeys being held in captivity at a Louisiana zoo tragically froze to death last week after their caretaker failed to notice them, according to reports.

It still remains to be seen how exactly the victims - a pair of tiny cotton-top tamarins - were overlooked at the Alexandria Zoological Park, but city officials have launched an investigation to get to the bottom of these senseless deaths.

These monkeys are known for their extremely small size (weighing less than a pound) and black and white face with a fan of hair atop their head. They also require temperatures between 76 and 85 degrees like that of their native tropical forest habitat in northern Colombia, according to The Associated Press (AP). So when temperatures in Alexandra last Wednesday were in the 50s, dropping to the teens at night, needless to say the tamarins were cold (though a third monkey managed to survive).

"The incident is under investigation," Public Works Director David Gill told The Town Talk. "The employee, who was initially on administrative leave and had been removed from any animal care, has resigned. This appears to have happened as a result of human error and not a system problem."

Cotton-top tamarins (Saguinus Oedipus) are a critically endangered species, with about 1,800 in captivity and only about 6,000 in the wild, according to the International Union for Conservation of Nature (IUCN). In the 1960s and '70s, tens of thousands of them were brought to the United States for biomedical research, and today the illegal pet trade and habitat loss continues to decimate their numbers.

So as can be expected, this latest incident outraged People for the Ethical Treatment of Animals (PETA), which opposes animals being kept in zoos.

"These tamarins lived a sad life of deprivation in captivity, and their deaths were totally preventable," lamented PETA Foundation Deputy General Counsel Delcianna Winders. "This fatal neglect is all too common in zoos and other places where animals are displayed for human amusement, and that's why PETA's motto reads, in part, that 'animals are not ours to use for entertainment.'"

The zoo appears to have a clean inspection record, and is fully accredited by the Association of Zoos and Aquariums, the AP notes.

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