Growth pattern of the maxillary sinus in orang-utan based on measurements of CT scans

The postnatal growth of the maxillary sinus was analyzed in 20 male and 20 female skulls of orang-utan (Pongo satyrus borneensis) in 5 age steps. Coronal CT scans were carried out of all skulls using a computed tomograph Siemens SOMATOM DR. The distances between the CT scans were always 4 mm. The outline of the maxillary sinus was surrounded on the CT scans with a planimeter and then the volumes were calculated. Using a growth model introduced by Fanghänel (1974), growth functions for the volume measurements for male and female animals were calculated separately. The growth curves showed a particular course, which was characterized by a quick rise until the age of 15 years. After the age of 20 years the curves for females orang-utan became gradually flat. At the age of 21 to 23 years the confidence intervals of the growth curves for male and female orang-utan began to drift apart, indicating the appearance of a sexual dimorphism. Using the basicranial length as an indicator of skull size, it becomes evident, that the sexual difference in the maxillary sinus' volume is probably based on the fact, that the maxillary sinus' volume of the male orang-utan increases further on following a common growth pattern.