Variability in the number of infraorbital foramina in rhesus macaques (Macaca mulatta) and cynomolgus macaques (Macaca fascicularis)

This study aimed to determine the number of infraorbital foramina in monkeys of the Papionini tribe. The authors performed a mu CT analysis of the morphology of the infraorbital foramina. A total number of 52 simian skulls belonged to two macaque species: Macaca mulatta and Macaca fascicularis were used in the study. The number of infraorbital foramina was counted macroscopically and with the use of a magnifying glass. Next, the skull representing the most common morphological type was selected and scanned by micro-computed tomography (mu CT). The Shapiro-Wilk normality test was used in the study. To compare the differences in the number of infraorbital foramen between species, sex and sides, the Mann-Whitney U test was applied. Three infraorbital foramina were present in most individuals from the test group. The Mann-Whitney test revealed no statistically significant difference between the number of foramina on the right- and left-hand side. Likewise, no statistically significant differences between the numbers of infraorbital foramina across sexes were observed. Volumetric reconstructions revealed the presence of separate infraorbital canals for each infraorbital foramen. Craniofacial innervation in macaques is formed by complex branching patterns of cranial nerves. Variability in the number of infraorbital foramina suggests a variable maxillary innervation pattern in these animals. Based on the analysis of volumetric projections, the presence of two labial branches and a single nasal branch of the infraorbital nerve is suggested. Detailed descriptions are supported by quantitative data and mu CT evidence.