Night Monkeys in Colombia

Grey-legged Night Monkeys, Aotus griseimembra
La Camelia, Victoria, Caldes, Colombia. November 2025

We saw three currently-recognised species of night monkey in Colombia last November. Night monkeys are variable in coloration and it is case of knowing what lives where in order to identify what one is seeing. The  three species are separated by high ranges of the Andes and major river systems, i.e. they are parapatric.

The first small group was seen by torchlight high in the trees on the track through the Otún Quimbaya Sanctuary on the western slope of the Cordillera Central. These were Lemurine Night Monkeys (Aotus lemurinus). A few days later were headed north west to cross the Cordillera Central and descend into the Magdalena Valley. Near the small town of Victoria we saw another species, the Grey-legged Night Monkey (Aotus griseimembra) by night. The next day in a coffee plantation we saw the same species by day. Two, the mother and father of the small group, were looking out of their tree hole. Then, after driving west to Bogota and on to emerge from the Cordillera Oriental we saw the third species, Brumback’s (Aotus.brumbacki) in the re-wilded grounds of Rancho Camaná, a popular lodge for wildlife trips. The small family group lives by day in a particular bamboo thicket. A new baby had arrived a few weeks earlier and could be seen being carried by the mother.

Mother and baby (top right centre) Brumback’s Night Monkey, Aotus.brumbacki
Rancho Camaná, near Restrepo, Meta, Colombia. November 2026

'Baby in the scope!'

Up until the 1970s there was only one species recognised. That was the Douroucouli, Aotus trivirgatus. Then, a series of discoveries showed that the number of chromosomes varied across their range across South and Central America from 46 to 58. How such changes in number came about and how they became fixed in a population has been the subject of much debate. The number of chromosomes—the karyotype—can have a number of different effects on hybrids between two forms, from  death at an early stage of development, to sterility (as in mules), to viability but gross chromosomal disturbance to, should say a split chromosome line up sufficiently well with the DNA of a single chromosome during fertilization, normal development.

These differences in karyotype together with differences in coloration of the neck and differences in susceptibility to malaria infection led Philip Hershkovitz (1909-1997) of the Field Museum of Natural History in Chicago, in a paper published in 1983, to split the douroucouli into nine species.

‘Owl monkey’ was used extensively as an alternative to douroucouli before and after the initial splitting of that species. I do not now when the use of ‘night monkey’ became to be accepted as the common name.

Research continues on the karyotypes and on the number of species, the latter complicated by adherents to particular ‘species concepts’. However, it does seem that the differences in karyotype have been generally accepted as constituting a barrier to reproduction and that differentiation by such means does result in ‘good’ biological species.

One of the reasons the karyotype of these monkeys has been investigated is that they are susceptible to Plasmodium falciparium, the parasite that causes malaria in the human population. Moreover, the time-course and pathological changes are the same. Therefore laboratory colonies of night monkeys have been established and have played a vital role in the development of antimalarials and to the development of vaccines. Those responsible for setting up and maintaining these colonies realised that karyotyping their stock was essential in  order to ensure that genetically compatible individuals were obtained and paired up. As far as I have read, the lab night monkeys are Grey-legged, A. griseimembra).

I have combined the geographical distributions for the three species shown in the IUCN Red Data List onto one map. The places we saw the animals are indicated by three red dots.

The night monkeys we saw by day were clearly looking back at us. However, their view of the world is very different from ours. Their eyes can see only in monochrome. The size of their eyes though shows their clear adaptation to life in the dark. By day, their pupils are tightly constricted.

I was particularly interested to see night monkeys by day. I have written before on the history of the discovery of Vitamin D and the particular requirement of South American monkeys for Vitamin D3 obtained either from the diet and/or by synthesis in the skin induced by the ultraviolet rays in sunlight. The question of course is how do the only nocturnal monkeys get their Vitamin D. As an experiment I asked a so-called AI agent the question. The answer came back that night monkeys must obtain their Vitamin D from their diet since they are nocturnal. Well that bit of AI sloppiness did not take into account of where and how night monkeys spend their day. At both the tree hole site, where bare areas of skin are exposed to sunlight, and in the bamboo thicket, where the tropical sun would have provided ultraviolet rays, there was clearly plenty of opportunity—up to 12 hours a day in fact—for synthesis of Vitamin D. In fact I can think of no other reason, other than early warning of approaching predators, for the tree-hole inhabitants to sit in full view. Indeed, depending on the size of the entrance hole a whole family group can be seen seemingly peering out at the outside day-lit world.

I will return to one of the species we saw in a future post since its description was an early and important event in the biology of night monkeys but one that left the man who did the work completely unaware of what had happened next…and the story ended in tragedy.