Bones of contention: The functional morphology and biomechanics of the mammalian os penis and os clitoridis

点击翻译按钮获取中文摘要

When Darwin developed his theory of natural selection, he considered that individuals with characteristics most suited to their environment were more likely to survive and reproduce. A moth that is neatly camouflaged against a tree trunk is less likely to be eaten than the moth that is garishly coloured. However, Darwin also realized that some animals have features that do not appear to help them adapt to their environment. The elaborate tail feathers of male peacocks, for example, do not seem to be particularly advantageous when navigating the jungles of India. But Darwin suggested these characteristics are due to sexual selection; individuals with traits making them more likely to win a mate (through combat or display) will go on to produce more offspring. Deer antlers, lions' manes and dancing cranes are all examples of sexual selection at work.The genitals of animals are especially likely to experience sexual selection as they play an essential role in the delivery and reception of sperm; an individual who cannot deliver sperm effectively will produce less offspring. In several groups of mammals (including monkeys, rats, dogs, moles and bats) the males have a bone located inside their penis, known as a baculum. Humans are the only apes who do not possess a baculum. Several lab-based studies on mice have shown that males with longer or wider bacula produce more offspring. Yet exactly how the baculum improves the mating success of males remains unknown. In some species, females also possess an equivalent bone located in the clitoris, known as the baubellum. Very little is known about this structure, and its function is even more enigmatic than that of the males.In this project, I will investigate the function of these mysterious structures in the mammal skeleton. I will test several theories about how the baculum improves the mating success of males, including, i) stiffening the penis to help with insertion, ii) straightening the urethra to ensure sperm can be delivered, and iii) stimulating females to trigger ovulation. I will test whether the possession of a baubellum in females is related to how much competition they might experience to find a mate.I will work with the National Museum of Scotland in Edinburgh to collect carcasses of many different species of deceased mammals, including both wild and zoo animals. Genitals will be removed from carcasses and will be CT scanned at Manchester's X-ray Imaging Facility. The CT scanning process is similar to technology used in hospitals to image broken bones, and allows us to visualize 3D internal structures of genitals without cutting into (and damaging) the objects. In some instances when the specimens are not valuable however, I will remove samples of soft tissue and bone and conduct tests to determine their strength and the arrangement of fibers inside the penis. I will also conduct a unique experiment in which the penis of a rat will be removed and then artificially inflated. The penis will be placed in a loading rig inside an X-ray machine, and will be compressed and bent during a CT scan. This will be the first time the internal structure of the penis will be visualized whilst at the same time experiencing bending. This experiment will allow us to understand more about how the baculum bone behaves, and which regions of the penis are most stressed, during sex. Based on the X-ray scans, 3D digital models of penes (both with- and without bacula) will be created, and I will run virtual computer simulations of the penis to investigate the effect the bacula has on penis stiffness. Finally, I will travel to numerous museums in the UK and abroad to gather more data on the mysterious female bone, the baubellum.By combining aspects of traditional anatomy with high-resolution x-ray scans and detailed 3D digital models, I hope to finally resolve one of the most puzzling enigmas in mammalian biology: What is the benefit of a penis bone?