Observations of Albinism in Endemic Toque Monkeys Macaca sinica from Different Bioregions of Sri Lanka

Abstract

Albinism is an exceptionally rare congenital condition in wild non-human primates, typically arising from autosomal recessive mutations that impair melanin biosynthesis. In this study, we report the first photographic documentation of albinism in the endemic Toque Monkey (Macaca sinica) from different bioregions in Sri Lanka. Field observations conducted between January 2024 and July 2025 identified four albino individuals across distinct geographical units corresponding to the natural ranges of the three recognized subspecies: M. s. sinica, M. s. aurifrons, and M. s. opisthomelas. Diagnostic features included a complete absence of pigmentation in the fur, visibly pink skin, and red or pink eyes, hallmarks of oculocutaneous albinism. Observations from both captive and wild contexts revealed that albino individuals were socially integrated within their troops and exhibited no immediate signs of behavioral exclusion. These findings suggest that albinism, although phenotypically conspicuous and potentially maladaptive in terms of predation and sun sensitivity, may not drastically disrupt social cohesion in M. sinica. The spatial distribution of these sightings raises questions regarding potential genetic drift, inbreeding, or habitat-induced population fragmentation that may be increasing the expression of rare recessive traits. This study highlights the need for targeted genetic assessments and long-term ecological monitoring to better understand the evolutionary and conservation implications of albinism in island-endemic primate populations.