Cranial allometry and geographic variation in slow lorises (Nycticebus)

Area of Science:

• Primatology
• Evolutionary Biology
• Morphometrics

Background:

• The genus Nycticebus (slow lorises) comprises several species and subspecies with debated taxonomic classifications.
• Understanding craniodental variation is crucial for resolving taxonomic relationships and evolutionary patterns in primates.
• Previous research has suggested distinct species within the Nycticebus genus, but detailed morphological analyses are needed.

Purpose of the Study:

• To analyze craniodental variation in Nycticebus coucang (common slow loris) and Nycticebus pygmaeus (pygmy slow loris).
• To investigate patterns of subspecific and specific variation using multivariate statistical methods.
• To test hypotheses regarding clinal variation, Bergmann's Rule, ontogenetic scaling, and character displacement in slow lorises.

Main Methods:

• Acquisition of 20 craniodental measurements from specimens of N. coucang and N. pygmaeus.
• Application of multivariate analysis of variance (MANOVA) to adult craniodental data.
• Comparison of geometric means of cranial dimensions with latitudinal data to assess clinal variation.
• Analysis of ontogenetic series to compare relative growth patterns between species and subspecies.

Main Results:

• MANOVA results support the classification of Nycticebus into two species (N. coucang and N. pygmaeus) with subspecies.
• Cranial size variation in N. coucang is largely clinal, following Bergmann's Rule, with larger forms found at higher latitudes.
• N. pygmaeus exhibits smaller skull size in more northern populations, potentially indicating character displacement due to sympatry with N. c. bengalensis.
• Ontogenetic scaling of skull proportions was observed in most comparisons, with exceptions in the masticatory complex of N. pygmaeus, suggesting dietary adaptations.

Conclusions:

• Craniodental morphology provides strong support for the current taxonomic classification of Nycticebus.
• Size variation within N. coucang is primarily driven by clinal factors, while N. pygmaeus may show evidence of character displacement.
• Differential extension of relative growth patterns and potential dietary shifts contribute to skull form variation between the two species.