Mixed-Species Groups and Genetically Confirmed Hybridization Between Sympatric Phayre’s Langur (Trachypithecus phayrei) and Capped Langur (T. pileatus) in Northeast Bangladesh

Interspecific hybridization occurs almost in all major primate radiations. Intermediate phenotypes, sometimes seen in mixed-species groups, are often the first indications of hybridization. Beyond natural hybridization, human activities, such as habitat fragmentation or population depletion, can be important drivers for initiating or intensifying the formation of mixed-species groups and hybridization. As hybridization can lead to the cyto-nuclear extinction of species, it may pose additional threats to threatened primates. To assess the frequency of mixed-species groups and intermediate phenotypes between threatened Phayre’s (Trachypithecus phayrei) and capped langurs (T. pileatus), we conducted population surveys in six forests of northeast Bangladesh between 2018 and 2023. We also tested the hybrid status of one individual with intermediate phenotype genetically, by sequencing the maternally-inherited mitochondrial cytochrome b gene and fragments of two biparentally-inherited loci. During our surveys, we encountered 98 groups of langurs of which 41 were Phayre’s langur groups, 49 capped langur groups, and eight mixed-species groups (8%). In three mixed-species groups, we detected three individuals of intermediate phenotype, among them one adult female whose nipples were elongated indicating that she had an infant, and thus was likely to be fertile. Genetic analyses confirmed the hybrid status (most likely F1) of one individual with intermediate phenotype. To our knowledge, there are no reports of mixed-species groups and hybridization from other parts of the range where the species occur sympatrically. Hence, it seems likely that such groups occur mainly in the northern part of their common range where natural habitats are deteriorating fast. Therefore, we suggest monitoring of the populations, to find out whether the frequency of mixed-species groups increases, and a large-scale population genetic study to investigate whether hybridization is a recent phenomenon or occurred historically in the zone of range overlap in Bangladesh.