Scaling of the Ectotympanic Tube and Tympanic Membrane Diameter among Catarrhine Primates

The ectotympanic portion of the ear is often used as a phylogenetic marker among primates. However, the factors influencing growth and adult structure of this region are not well studied. The ectotympanic tube is a C‐ or U‐ shaped tube that is fused to the petrosal part of the temporal bone. In catarrhine primates, the ectotympanic tube consists of a bony part between the tympanic membrane and the external auditory meatus and an external cartilaginous part. In ceboids and strepsirrhines the ectotympanic tube is almost completely cartilaginous. It has been suggested that the bony ectotympanic tube may be an adaptation to reduce noise produced during mastication. The present study suggests that the length of the ectotympanic tube is dictated by relative brain size and spatial packing, specifically the lateral expansion of the brain relative to the cranial base. This study examines the relationship between the length of the ectotympanic tube and other related structures, including brain volume, basicranial lengths and widths, and tympanic diameter. The sample includes CT data of adult primates from a wide range of catarrhine species. Two males and two females of each species were included where available. The results of this study show that ectotympanic tube length is positively allometric relative to endocranial volume, but negatively allometric when regressed on body mass. Tympanic diameter has a weaker correlation with brain size and body size, and is negatively allometric when regressed on either. Length of the ectotympanic tube was shown to be significantly correlated with basicranial breadth and endocranial volume, suggesting that these two factors may cause the ectotympanic tube to lengthen in some primates.