Bat Development: Interspecies Differences in Baculum Ossification Patterns

Developmental studies are fundamental to understanding specialized morphologies. The baculum (os penis) is a heterotopic bone in the glans penis, which is probably subject to sexual selection and may play a role in reproductive isolation between closely related species. In this ongoing study, the pre‐ and postnatal development of the reproductive organs will be compared between species of the bat genera Pipistrellus , Eptesicus , and Cynopterus histologically and with 3D X‐ray microtomography (microCT) imaging. We aim to show how bacula with different shapes start to calcify at different ossification centers and to compare soft tissue development of bat reproductive organs. Bat organ system development has mainly been studied using histological methods, dissection, or clearing and staining. The skeletal system has also been studied with x‐ray techniques including microCT. Here, we will use 1% (w/v) elemental iodine in 100% ethanol (I2E) to study the development of reproductive organs in a sample of 14 Cynopterus brachyotis (Chiroptera: Pteropodidae) embryos from the collection of the Museum Zoologicum Bogoriense (Indonesia), and several juvenile and subadult bats of the species Cynopterus sphinx (Indonesia), Pipistrellus pipistrellus (Naturalis Biodiversity Center Leiden, Netherlands), and Eptesicus serotinus (Netherlands and Ukraine). Iodine contrast staining has been used successfully in a number of studies on fetal and adult vertebrates, because soft tissues can thus be differentiated in microCT images. Preliminary results show different patterns of ossification. In Pipistrellus pipistrellus the distal part of the baculum reaches its adult shape before the proximal part. The different states of medullary cavity development found in our samples suggest the medullary cavity first forms from the ventral side of the baculum, where the branches of the base meet the shaft and that it is later replaced by a secondary medullary cavity. The baculum of Eptesicus serotinus calcifies from both lateral sides, while four different calcification centers can be shown in a developing baculum of Cynopterus sphinx .