Seemingly extinct animals who are back from the dead

This monkey is rated "endangered" on the Red List of the International Union for Conservation of Nature (IUCN). The twist: Until 2015, when some of the red monkeys were seen in the Congo, the species was believed to be extinct. This is called the Romeo error ― when a species is declared extinct while it is still alive, named after the tragically mistaken lover in Shakespeare's "Romeo and Juliet."

Animals that return from (falsely assumed) extinction like this are so-called Lazarus species, named after the man who was risen from the dead by Jesus in the Bible. Among them: The Hula painted frog. It's endemic to the Lake Hula marshes in Israel and was thought to have gone extinct after they'd been drained. It hasn't, but it is critically endangered according to the IUCN Red List.

This species of Galapagos tortoise is also critically endangered according to the IUCN. It was considered extinct until a single female was discovered on Fernandina Island in 2019. She's estimated to be over 100 years old. Researchers discovered the species in 1906 when they found a single male animal ― which promptly died, leading to the belief the species was extinct.

This critically endangered bird is only found on the South Island of New Zealand. Its North Island relative is extinct, and for a long time, it was believed the South Island one was, too. Its population had dwindled after rats, cats and dogs had been introduced to New Zealand by colonists. But in 1948, 50 years after it was declared extinct, the bird was rediscovered in an isolated valley.

The endangered Chacoan peccary lives in the Gran Chaco region of Paraguay, Bolivia, and Argentina. When researchers discovered a fossil of the animal in 1930, they believed the species to be extinct. But in the 1970s, they were proven wrong. Indigenous people of the region had known the peccary was still alive, it had just taken Western researchers decades to catch up.

This glamorously named bird lives in high altitudes in Colombia's western Andes. For a long time, researchers only knew about its existence from a specimen in a museum and believed it to be extinct. It was rediscovered in 2004. Today its population is increasing, though it is still endangered, according to the IUCN.

This frog species is critically endangered. In the wild, it mainly lives in Costa Rica (the image seen here is from a frog farm in Colombia), where it was rediscovered in 2003. As amazing as it is to find a Lazarus species ― researchers stress that many rediscovered species are likely to go extinct without aggressive conservation efforts. (Editor: Fabian Schmidt)