Light-fingered monkeys threaten critically endangered Príncipe’s lily of the valley

  • Camera traps have confirmed suspicions that mona monkeys eat the eggs of the critically endangered Príncipe thrush.
  • Monkeys and several other invasive species were brought to the then uninhabited islands of Príncipe and São Tomé by Portuguese sailors from the 15th century.
  • Conservation authorities are considering allowing monkey hunting in the Príncipe Natural Park to reduce their numbers, but more research is needed to understand their place in the ecology.

Environmentalists using camera traps have obtained evidence showing that mona monkeys from Príncipe Island off the coast of central Africa pinch the eggs of Príncipe’s thrush, a critically endangered species with a total population of estimated at less than 250.

Although this was considered probable, until now there was no strong evidence to prove that the apes (Mona cercopithecus) were stealing the nests of Príncipe’s thrush (Turdus xanthorhynchus), a starling-sized bird with an olive-brown back, white-speckled breast and bright yellow beak.

Mona monkeys arrived in Príncipe and its sister island São Tomé centuries ago, possibly as pets of Portuguese sailors. The islands, now the Democratic Republic of São Tomé and Príncipe, lie in the Gulf of Guinea.

A mona monkey caught in a camera trap. Image courtesy of Fundaçao Príncipe.
Mona monkeys arrived in Príncipe and its sister island São Tomé centuries ago, possibly as pets of Portuguese sailors.

Originally uninhabited and covered with evergreen forest, São Tomé and Principe were colonized by Portugal in the 1470s. In addition to monkeys, sailors also brought with them dogs, cats, pigs, African rats, mice and civets (Civettictis civetta) through the centuries.

“This is indeed a very global problem – invasive species – and there is so much to discover and do that I think we are only beginning to see the tip of the iceberg,” Estrela said. Matilde, Executive Director of Fundaçao Príncipe, a local conservation NGO that studies the impact of these introduced mammals on Príncipe’s thrush and other native species.

The camera traps were set to monitor 55 artificial nests of thrushes baited with quail eggs during a survey conducted from September 2020 to August 2021 in the Príncipe Natural Park, the last stronghold of the bird in the south of the ‘Isle.

Of the 55 nests, 42% were stolen by animals within six days, wrote a team led by Fundação Príncipe in a brief report published in the journal Oryx. The Mona monkeys were responsible for 18% of these events. The other predators could not be identified. Matilde told Mongabay it was because the footage was overexposed or the cameras were malfunctioning.

One of the active thrush nests monitored by the team was visited five times by mona monkeys until parent birds abandoned it on July 6.

These results offer “convincing evidence that the reproductive activity of this critically endangered bird is disrupted by mona monkeys,” say the authors.

A mona monkey raiding caught red-handed. Image courtesy of Fundaçao Príncipe.

Prior to this study, only one Príncipe thrush nest had been found. The Fundação Príncipe team found four nests of active thrushes, which are woven from moss and plant roots and nestled in rocky cavities and trees. They didn’t see any eggs or chicks, although the team spotted two young thrushes in late 2020.

The monkeys were probably not the only ones responsible for the theft of the nests. A 2019 survey found other potential predators at thrush breeding sites.

“We have pictures of the highest peaks, and there are cats and dogs there and there are rats everywhere,” said Matilde, noting that the rats are often stealthy enough to avoid setting off camera traps. and can be responsible for a large number of nest thefts. .

Mona monkeys are a popular game species with local hunters, but this pressure has pushed them into the Príncipe Natural Park where hunting is prohibited, and their numbers have increased inside the park.

Matilde says plans are being made to allow controlled hunting in the park to reduce their numbers in an attempt to protect the thrush. But each intervention must be carefully considered.

“We don’t yet understand the dynamic between invasive and native and there may be a certain balance that can be upset if we start hunting monkeys,” she said.

Support from the island’s hunters could help researchers in another way, by shedding light on exactly what the animals are eating. Forty-three samples were taken from stomachs of monkeys slaughtered by hunters in the buffer zone and forests around the park and sent to a laboratory in Portugal for analysis.

The stomach contents of three civets were also sent. The results are not yet available.

Martim Melo, Center for Biodiversity and Genetic Resources Research at the University of Porto, Portugal, said that while controlled hunting of monkeys in the park can reduce the pressure on lily of the valley and benefit several other species, complete eradication of invasive mammals is usually the only practical solution.

However Melo, who was not involved in the Oryx study, said that this is rarely possible. In Principe, where monkey meat provides both food and income for islanders, it is also reportedly unpopular.

Melo and his colleagues conducted a study in 2007 that listed Príncipe’s lily of the valley as Critically Endangered on the IUCN Red List. They also used analyzes of the song, DNA and physical traits of the thrush to confirm it as a distinct species from the São Tomé thrush (Turdus olivaceofuscus).

The two had been combined into a single species since 1924, initially masking threats to Príncipe’s Thrush.

Its struggle for survival, compared to that of the thriving São Tomé thrush just 150 kilometers (93 miles) away, has sparked much speculation among scientists.

The Príncipe’s Thrush is now listed as Critically Endangered on the IUCN Red List. Image courtesy of Fundaçao Príncipe.

“We have always wondered whether the introduction of mona monkeys resulted in the extinction of several species before their discovery for science, and whether the native bird community that we see now represents the species able to withstand their pressure – the Príncipe’s thrush being a case of a species that struggles to do so, ”said Melo.

In addition to having different songs and different plumage, a key trait that distinguishes the Príncipe’s Thrush from its São Tomé relative is its docility.

This is something Matilde of the Fundação Principe witnessed nine years ago after leaving Portugal to settle on the island.

She and a group of friends were camping at Pico Mesa, a large mountain in Príncipe Natural Park. Matilde had lugged heavy cameras, binoculars and even a telescope to the top in the hopes of spotting a Principe thrush. She didn’t see any there, but back at the campsite the next day, one of the birds approached her friend out of curiosity as she crouched in the undergrowth, peeing. Matilde told her friend not to move while she photographed the bird, much to her friend’s frustration.

After this encounter, Príncipe’s thrush held a “very special place in my heart,” said Matilde. “They seem to see no evil in the world.”

Banner image: A Principe thrush. Image courtesy of Fundaçao Príncipe.


Guedes, P., Dos Santos, Y., De Lima, RF, & Bird, TL (2021). The introduced mona monkey is a major threat to Príncipe’s Thrush, a critically endangered species. Oryx, 55(6), 809-809. doi: 10.1017 / s0030605321001198

Dallimer, M., Melo, M., Collar, NJ and Jones, PJ (2010). The Thrush Principe Turdus xanthorhynchus: A flagship species of the newly divided forest, “critically endangered”. International Bird Conservation, 20(4), 375-381. doi: 10.1017 / s0959270910000390

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Biodiversity, Birds, Conservation, Critically endangered species, Endangered species, Environment, Extinction, Hunting, Invasive species, Islands, Monkeys, Primates, Protected areas, Fauna, Wildlife conservation, Wildlife rehabilitation

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