The term species defines the unit of classification of organisms related by common characteristics (1).
Members of the same species have the ability to interbreed and pass on their genes to further generations. Usually, when two members of different species breed, the resulting offspring is infertile, lacking the ability to reproduce. With regards to evolution, this is not the favored outcome as it prevents the population from increasing and introduces the risk of extinction.
Thus, hybridization between species doesn’t occur unless there is an external factor such as stress brought upon by human intervention or environmental factors such as drought or climate change (2).
The ‘Mystery Monkey’ that was encountered in Borneo six years ago is now thought to be a hybrid of a male proboscis monkey, or Nasalis larvatus, (see Figure 1) and a female silvered leaf monkey, also known as Trachypithecus cristatus (see Figure 2).
The primate was first noticed by Brenden Miles who is a tour guide and a member of the Sabah Tourist Guide Association (3), as he was traveling along the Kinabatangan River (4). He first thought that “it could be a morph of the silvered leaf monkey,” meaning that it could simply be a silvered leaf monkey with a rare color variation. However, at a closer look, he noticed some odd details that did not match with the species as he noted down:
At first, I thought it could be a morph of the silvered leaf monkey. Its nose was long like that of a proboscis monkey, and its tail was thicker than that of a silvered leaf [monkey].
After its picture was posted on social media (see Figure 3), the mystery monkey was long-forgotten.
Figure 1: a family of proboscis monkeys
Figure 2: a group of silvered leaf monkeys
Figure 3: The Mystery Monkey of Borneo
Found in a recent journal article published by the International Journal of Primatology, research suggests that rather than being a morph, the monkey is the result of the rare hybridization between two distantly related primate species that share the same habitat: the proboscis monkey and the silvered leaf monkey(5). Both of these species live in groups including a dominant male and multiple females along with offspring. Similar to most primate species, the males born in the social groups leave once they mature to start or take over another group. Due to habitat decline, the habitats of these species have started to overlap, and it has been observed that “there are mixed groups where female proboscis monkeys even take care of silver langur babies (silvered leaf monkeys)."
This was quite surprising for researchers as primate species had been accepted as reproductively isolated. Considering that these two species inhabit different ecological niches in overlapping habitats, the hybridization was unexpected due to its unlikeliness. Therefore, the hybridization that resulted in the birth of the mystery Monkey has distressed scientists. The area around the Kinabatangan River lost 40 percent of the forest in the span of 37 years, with logging and palm oil plantations being the most distinct causes (6).
Nadine Ruppert, a primatologist at the Universiti Sains Malaysia stated that “severe habitat loss, fragmentation and degradation caused by expanding palm oil plantations along the Kinabatangan River could explain how the possible hybrid came to be.”
Although the unusual hybrid has charming qualities, scientists hope that the case of hybridization is the last of its kind as it is an indication that the parent species are in danger. What makes the hybrid more unusual is that it was recently documented while nursing an infant (see Figure 4), serving as an exception to the notion that hybrids are infertile. While also serving as the introduction of a possible new species that challenges scientific notions, the hybrid is a canary in a coal mine, suggesting that the parent species are endangered.
Figure 4: Mystery Monkey 6 years later, holding an infant