Phayre’s langur. Credit: tontantravel, CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons.

A new danger has emerged for endangered langur primates; forest loss leading to hybridisation across two species.

Research from the German Primate Center – Leibniz Institute for Primate Research shows a threatening development for two endangered primate species in Bangladesh: Phayre’s langurs (Trachypithecus phayrei) and capped langurs (Trachypithecus pileatus).

In mixed groups of these two species, hybrids have been found and genetically confirmed. If hybridisation continues, it could mean the extinction of one or both species.

Over five years (2018-2023), an international team of researchers led by Tanvir Ahmed, a PhD student at the German Primate Center, studied the langur populations in north-eastern Bangladesh.

They found that eight of the 98 langur studied groups consisted of Phayre’s and capped langurs, and in three groups, some individuals appeared as a mixture of the two species.

Later, researchers analysed genetic samples of the species in the lab of the German Primate Center, and confirmed one case of hybridisation. This langur had a capped langur mother and a Phayre’s langur father.

Another female with a hybrid appearance showed signs of motherhood, indicating that at least female hybrids are fertile and give birth to young.

Capped langur. Credit: Lonav Bharali, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons.

A rare but growing problem worldwide

Hybridisation is generally rare among primates but occurs in areas where the distribution ranges of related species overlap.

Human activities such as deforestation, habitat fragmentation, hunting and trapping of primates, leading to a thinning of populations and restricting the movements of individuals between local populations, can increase the risk of such hybridisation.

Ahmed, lead author of the study, said: “The existence of fertile hybrids is particularly alarming because it suggests that gene flow between these two endangered species could irreversibly affect their future genetic composition.”

Christian Roos, lead scientist of the study, emphasised the global relevance of the research results: “This is not just a local problem. When habitats are destroyed, animals mix unnaturally and form mixed groups and hybridisation might occur.

“This can even mean the extinction of one or both species.”

A juvenile hybrid langur in Rema-Kalenga Wildlife Sanctuary, Bangladesh. One can easily recognise the mixture of morphological characters. For example he white eye rings of the Phayre’s langurs and the golden-brown breast hair of the capped langurs. Credit: Auritro Sattar.

Urgent need for action

The study also showed that well-protected forests have a higher population density of langurs compared to less protected ones.

However, these forests are often too small, too isolated and fragmented to ensure the long-term survival of the species.

Ahmed said: “Forest conservation must become a national priority.

“If we don’t act now, we risk losing not only two monkey species but also an important part of Bangladesh’s biodiversity.”

In addition to forest protection, ongoing research is needed to better understand the effects of hybridisation and to develop suitable conservation strategies.

Dietmar Zinner, co-author of the study, summarised: “This study is a wake-up call. We need more data to develop effective long-term conservation strategies.

“Further research will help us understand the extent of hybridization, the impact of human activities and how to prevent its worst effects.”

With less than 500 Phayre’s langurs and 600 capped langurs in north-east Bangladesh, time is running out for these species.

Their survival depends on immediate action, including both forest conservation and creating dispersal corridors for the species between the forests.

The research is published in the International Journal of Primatology.