Boundary zone between northern and southern pig-tailed macaques and their morphological differences

Area of Science:

• Primatology
• Zoology
• Taxonomy

Background:

• Conflicting reports exist regarding the taxonomic status of northern and southern pig-tailed macaques (Macaca nemestrina).
• Previous research suggested a potential boundary zone for these macaque forms within Thailand.
• The precise species, subspecies, or form classification of pig-tailed macaques remains debated.

Purpose of the Study:

• To investigate the distribution range of pig-tailed macaques in Thailand.
• To compare the morphological characteristics of northern and southern pig-tailed macaque forms.
• To resolve taxonomic ambiguities concerning pig-tailed macaque populations in Thailand.

Main Methods:

• Conducted a nationwide questionnaire survey across 7,410 subdistricts in Thailand.
• Performed field surveys in 72 subdistricts reporting macaque presence.
• Measured and compared morphological characters of wild northern pig-tailed macaques with southern forms from Sumatra, Indonesia.

Main Results:

• Identified a partially sympatric boundary zone for northern and southern pig-tailed macaques in the Surat Thani-Krabi depression, Thailand.
• Documented significant morphological distinctions between the two forms, including crown patch, facial features, pelage color, and tail carriage.
• Observed sex-specific differences in morphological characters and female sexual skin swelling patterns.

Conclusions:

• The northern and southern pig-tailed macaques exhibit distinct morphological characteristics and partially overlapping distributions in Thailand.
• These findings provide strong evidence to support the classification of northern and southern pig-tailed macaques as distinct forms or subspecies.
• The study contributes to resolving the long-standing taxonomic debate surrounding pig-tailed macaques.