Montilla said he became obsessed with knowing everything about night monkeys, and to his surprise found there wasn\u2019t that much in the existing literature. This gap led him to become a biologist and now a leading expert on night monkeys. Over almost two decades, he has followed these monkeys through the nights and discovered differences in activity patterns<\/a> depending on the moonlight and the places where they prefer to live. He has assessed their diet<\/a>, reported the first case of albinism in a night monkey<\/a>, and helped create the first management plan for the conservation of the Andean night monkey<\/a> (Aotus lemurinus) in Quind\u00edo.<\/p>\n Montilla recently published a study<\/a> in the International Journal of Primatology that suggests it\u2019s the Magdalena River, and not the Andean massif, that could be the true barrier separating two identical-looking night monkey species on the border of the departments of Antioquia and Santander. That finding could shift conservation approaches for the five species of night monkeys found in Colombia, which are losing habitat to cattle ranching, oil palm plantations and mining, and are traded as pets.<\/p>\n Identifying night monkey species is a challenge for taxonomists. Widely distributed from Panama to northern Argentina, the monkeys are all visually almost indistinguishable.<\/p>\n \u201cThey are complex twin species. A night monkey in the Amazon, in the Caribbean, [and] in the Andes looks exactly the same,\u201d Montilla told Mongabay.<\/p>\n Night monkeys are an example of cryptic species, which are genetically different but almost indistinguishable physically. Natural barriers such as rivers, mountains and deserts isolate the species, limiting their dispersal and giving rise to multiple taxa. This is a phenomenon that\u2019s been seen in amphibians, bats, birds and other nocturnal primate species<\/a>; the most iconic cryptic species example is Astraptes fulgerator, a skipper butterfly that turned out to be not one, but 10 different species<\/a>.<\/p>\n In 2010, biologists trying to identify the habitat limits for each species of night monkey suggested a natural barrier at 1,000 meters (about 3,300 feet) in the central and eastern Andes. Under this hypothesis, the night monkeys found in Colombia\u2019s north and the Magdalena Valley would fall under the species Aotus griseimembra; in the Pacific lowlands, they would be Aotus zonalis; and above 1,000 meters, Aotus lemurinus. That would mean both A. griseimembra and A. lemurinus could be found on either side of the Magdalena River, with the monkeys\u2019 hair the only physical trait setting them apart: those in the mountains are hairier than those in the lowlands.<\/p>\n \u201cIt\u2019s a bit ambiguous to think that there\u2019s an imaginary line at 1,000 meters, and that a monkey above 1,000 meters is going to say, \u2018Oh no, I\u2019m not going to go past 999 meters,\u2019\u201d Montilla said. \u201cSo there was always this thought that there might be more out there.\u201d<\/p>\n Not convinced by this hypothesis, Montilla has dedicated his doctoral studies to researching how night monkeys are genetically distributed throughout Colombia. Between 2022 and 2025, he followed night monkeys every night, collecting droppings and extracting DNA from 92 different locations across the country.<\/p>\n To his surprise, the results showed that in the middle Magdalena Valley, two groups of monkeys belonging to the same species, A. griseimembra, and living just 100 m (330 ft) apart, were genetically very different. Notably, they were separated by the Magdalena River. Montilla found that the pattern repeated itself several times in the upper and middle sections of the Magdalena Valley, indicating that the river, rather than the mountains, played the central role in limiting the dispersal and connectivity of night monkey populations.<\/p>\n Emerging at an elevation of almost 3,700 m (12,100 ft) in the Paramo de las Papas, in Popay\u00e1n, Cauca department, the Magdalena River is perhaps Colombia\u2019s most important waterway. It flows some 1,540 kilometers (957 miles) and carries on average more than 8,000 cubic meters (2.1 million gallons) of water per second until it reaches the Atlantic Ocean at Bocas de Ceniza.<\/p>\n The river is not only a historic route for human settlements, culture, economic development and transport between the Andes and the Caribbean. It\u2019s also a richly biodiverse wildlife corridor<\/a>, home to more than 120 fish, 630 bird, 120 reptile and 4,000 plant species. The region also hosts endemic and threatened mammals such as the blue-billed curassow (Crax alberti), the brown spider monkey (Ateles hybridus), jaguar (Panthera onca), Magdalena striped catfish (Pseudoplatystoma magdaleniatum), and of course night monkeys.<\/p>\n According to Sebasti\u00e1n Garc\u00eda, president of the Colombian Primatological Association (APC), Montilla\u2019s findings open the possibility of redefining the current distribution maps for A. griseimembra and A. lemurinus. \u201cThese kinds of research will impact where conservation efforts are located throughout the country,\u201d Garc\u00eda told Mongabay by phone.<\/p>\n For Garc\u00eda, being able to correctly identify each species and where it lives is crucial for improving action plans to protect wildlife and for initiatives such as Colombia\u2019s National Primate Conservation Program. A set of strategies designed by experts to address the main threats for the country\u2019s 38 primate species, including the five night monkey species, is currently pending approval from the Ministry of Environment and Sustainable Development.<\/p>\n Nonetheless, studies<\/a> have shown that although knowing the distribution and habitat the limits of a cryptic species is one of the first steps toward protecting it, many conservation action plans fail because cryptic species are not detected or are data-deficient.<\/p>\n \u201cWe need these types of studies and more government support that can be applied directly or indirectly to the conservation of primates in our country, because this is one of many primates in Colombia that requires this type of study,\u201d Garc\u00eda said.<\/p>\n \u201cThe thing is that since Aotus is strictly nocturnal, it\u2019s very difficult to study,\u201d Eduardo Fernandez-Duque, a professor of anthropology at Yale University in the U.S., who wasn\u2019t involved in the study, told Mongabay by video call.<\/p>\n In the case of night monkeys, there are still many gaps to fill<\/a>. Many of the night monkey species are data-deficient or listed as vulnerable on the IUCN Red List because there\u2019s no basic information about their population in the wild, social dynamics, reproduction, diet, response to fragmentation, or vulnerability to climate change. For Garc\u00eda and other experts in the field, this makes Montilla\u2019s findings a valuable contribution toward filling these knowledge gaps and supporting efforts to protect all night monkey species.<\/p>\n \u201cWe conducted a year-long study of Aotus in Ecuador, and it was very difficult,\u201d Fernandez-Duque said. \u201cWhat our Latin American colleagues are doing is impressive. They\u2019ve accomplished things that, of all the nocturnal Aotus study projects, are undoubtedly the most advanced in Colombia.\u201d<\/p>\n Montilla said he aims to consolidate his findings with more evidence, by comparing the number of chromosomes between species, identifying differences in their vocalizations, and comparing morphological data to establish the genetic and physical differences between the various night monkeys.<\/p>\n This would not only help draw the lines of where these cryptic species are found on each side of the Magdalena River, but also along other large rivers such as the Amazon, Caqueta and Putumayo that could be having the same barrier effect on other groups of night monkeys.<\/p>\n \u201cOnly time will tell to what extent that idea withstands the test of time, the scrutiny of other groups, contradictory evidence, and supporting evidence,\u201d Fernandez-Duque said. \u201cThat\u2019s what doing science is all about.\u201d<\/p>\n Banner image: Night monkeys (genus Aotus). Image courtesy of Manuel Fonseca.<\/p>\n Citations:<\/p>\n Link,\u00a0A., Mu\u00f1oz-Delgado,\u00a0J., & Montilla,\u00a0S.\u00a0O. (2023). Nocturnality and activity budgets of owl monkeys in tropical ecosystems.\u00a0Developments in Primatology: Progress and Prospects, 353-373. doi:10.1007\/978-3-031-13555-2_12<\/a><\/p>\n Montilla,\u00a0S.\u00a0O., Mop\u00e1n-Chilito,\u00a0A.\u00a0M., Murcia,\u00a0L.\u00a0N., Triana,\u00a0J.\u00a0D., Ruiz,\u00a0O.\u00a0M., Montoya-Cepeda,\u00a0J., \u2026 Link,\u00a0A. (2021). Activity patterns, diet and home range of night monkeys (Aotus griseimembra\u00a0and\u00a0Aotus lemurinus) in tropical lowland and mountain forests of central Colombia.\u00a0International Journal of Primatology,\u00a042(1), 130-153. doi:10.1007\/s10764-020-00192-1<\/a><\/p>\n Montilla,\u00a0S.\u00a0O., & Link,\u00a0A. (2022). Albinism in a wild Caribbean night monkey (Aotus griseimembra) in a fragmented landscape in Colombia.\u00a0Therya Notes,\u00a03, 14-17. doi:10.12933\/therya_notes-22-62<\/a><\/p>\n Montilla,\u00a0S.\u00a0O., Arango-Lozano,\u00a0J., Toro-Cardona,\u00a0F.\u00a0A., Ram\u00edrez-Chaves,\u00a0H.\u00a0E., Di Fiore,\u00a0A., & Link,\u00a0A. (2026). Phylogeography and niche modeling of night monkeys (Aotus) in northwestern South America.\u00a0International Journal of Primatology. doi:10.1007\/s10764-025-00532-z<\/a><\/p>\n Hending, D. (2025). Cryptic species conservation: A review.\u00a0Biological Reviews,\u00a0100(1), 258-274. doi:10.1111\/brv.13139<\/a><\/p>\n Hebert,\u00a0P.\u00a0D., Penton,\u00a0E.\u00a0H., Burns,\u00a0J.\u00a0M., Janzen,\u00a0D.\u00a0H., & Hallwachs,\u00a0W. (2004). Ten species in one: DNA barcoding reveals cryptic species in the neotropical skipper butterfly\u00a0Astraptes fulgerator.\u00a0Proceedings of the National Academy of Sciences,\u00a0101(41), 14812-14817. doi:10.1073\/pnas.0406166101<\/a><\/p>\n Wolovich,\u00a0C.\u00a0K., Shanee,\u00a0S., Maldonado,\u00a0A.\u00a0M., M\u00e9ndez\u2010Carvajal,\u00a0P.\u00a0G., Perea\u2010Rodriguez,\u00a0J.\u00a0P., Tabares,\u00a0S., \u2026 Evans,\u00a0S. (2023). A call\u2010to\u2010action to assist in efforts to protect owl monkeys (Aotus\u00a0spp.).\u00a0American Journal of Primatology,\u00a086(3). doi:10.1002\/ajp.23501<\/a><\/p>\n![\"\"](\"https:\/\/www.newsbeep.com\/au\/wp-content\/uploads\/2026\/04\/DSCN0051.jpg\")
Night monkeys are an example of cryptic species, which are genetically different but almost indistinguishable physically. Image courtesy of Sebastian Montilla.<\/p>\n![\"\"](\"https:\/\/www.newsbeep.com\/au\/wp-content\/uploads\/2026\/04\/IMG-20250726-WA0101.jpg\")
Sebastian Montilla has been passionate about studying night monkeys since early childhood. Image courtesy of Sebastian Montilla.<\/p>\n![\"\"](\"https:\/\/www.newsbeep.com\/au\/wp-content\/uploads\/2026\/04\/DSC03358.jpg\")
According to Montilla\u2019s research, the Magdalena River, and not the Andean massif, could be the true barrier separating two identical-looking night monkey species on the border of the departments of Antioquia and Santander. Image courtesy of Sebastian Montilla.<\/p>\n
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