Species boundaries could become blurred: The loss of habitat can lead to problematic hybridization in endangered primates, a study shows. This development is evident in two species of langurs in Bangladesh. In some groups, crossbreeding types of both species were spotted and subsequently genetically confirmed. There is also evidence that these mixed individuals are capable of reproduction themselves. If the hybridization process continues, it could lead to the loss of one or both species, the researchers say.
As with the branching of a growing tree, over the course of the evolutionary history of animals, species split into new ones: certain subgroups initially developed increasingly distinctive characteristics until separate species eventually emerged. The fact that the newly emerged species avoided each other when looking for a partner contributed to further differentiation. However, it is known that occasional flirtations across species boundaries can still occur. This can then result in hybrid forms, which, if the populations are healthy, do not have any problematic effects on the identity of the two animal species.
Such occasional hybridizations are also known from primates when the distribution ranges of related species overlap. But in some cases, intersections could also become problematic. The concern is with species whose populations have been severely decimated by human activities and are limited to small habitats. There is evidence that they sometimes form communities with related primates there. It is feared that this could lead to increased hybridization, which is problematic due to the small population sizes. Because through ongoing integration, one species could eventually merge into the other. Under certain circumstances, however, a mixed form could arise that no longer corresponds to either of the two original types. Hybridization can therefore lead to the loss of one or even the two affected species.
Two endangered monkey species in sight
The researchers led by Tanvir Ahmed from the German Primate Center – Leibniz Institute for Primate Research in Göttingen have now documented a case in which such a development is apparent. The focus of their study was the Phayre’s spectacled langur (Trachypithecus phayrei) and the capped langur (Trachypithecus pileatus), which live in northeastern Bangladesh. Both species have suffered greatly from persecution and habitat loss. It is estimated that there are fewer than 500 Phayre’s-faced langurs and 600 crested langurs left in Bangladesh, some of which rely on shared habitats. For their study, the researchers examined the populations of these two species over five years.
As the team reports, their observations showed that Phayre and Cape langurs live together in around ten percent of the monkey groups recorded. The researchers found that such mixed groups occur particularly frequently in habitats that are comparatively heavily affected by forest loss. Accordingly, the animals there probably joined together to form interspecies communities due to the particularly small number of individuals. In some of these mixed groups, the researchers also noticed specimens that had a combination of the external appearance characteristics of both species. For example, they had the white eye rings of the Phayre’s spectacled langurs and the golden brown chest hair of the capped langurs.
Problematic hybridization
To provide more clarity, the researchers then analyzed a genetic sample from such a specimen. The suspected hybridization was confirmed: The results showed that the animal had a crested langur mother and a Phayre’s spectacled langur father. As the researchers also report, they also spotted a female hybrid specimen that was apparently nursing a young animal. This is therefore an indication that interspecies crosses do not lead to sterile individuals, as is known with other animals – for example in the case of donkeys and horses. At least female hybrids of crested and Phayre’s-spectacled langurs appear to be fertile and can give birth to young, the observations suggest.
“The existence of fertile hybrids is particularly alarming because it suggests that gene flow between these two endangered species could irreversibly influence their future genetic composition,” says senior author Christian Roos from the German Primate Center. He emphasizes the overarching importance of the discoveries, because other primates and other animal species could also be affected by the problem. “This is not just a local problem. When habitats are destroyed, animals often form mixed-species groups and hybridization can occur. This could even mean the extinction of one or both species,” says Roos.
In the case of the two langur species, forest protection appears to be fundamental to their survival. But the team says further research is needed to better understand the effects of hybridizations and develop appropriate protection strategies. “This study is a wake-up call. We need more data to develop effective long-term conservation strategies for affected species. Further research will help us understand the extent of hybridization and the impact of human activities on it and prevent the worst consequences,” says co-author Dietmar Zinner from the German Primate Center.
Source: German Primate Center, specialist article: International Journal of Primatology, doi: 10.1007/s10764-024-00459-x