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

当达尔文提出他的自然选择理论时，他认为具有最适合其环境特征的个体更有可能生存和繁殖。一只飞蛾巧妙地伪装在树干上，比一只色彩艳丽的飞蛾更不容易被吃掉。然而，达尔文也意识到一些动物的特征似乎并不能帮助它们适应环境。例如，雄孔雀精致的尾羽在印度丛林中航行时似乎并不是特别有利。但达尔文认为，这些特征是由于性选择；具有使其更有可能赢得配偶（通过战斗或展示）的特征的个体将继续产生更多的后代。鹿角、狮子的鬃毛和跳舞的鹤都是性选择的例子。动物的生殖器尤其可能经历性选择，因为它们在精子的传递和接收中起着至关重要的作用；不能有效地提供精子的个体将产生更少的后代。在一些哺乳动物群体中（包括猴子、老鼠、狗、鼹鼠和蝙蝠），雄性的阴茎内有一块骨头，被称为阴茎骨。人类是唯一没有阴茎骨的类人猿。几项以老鼠为基础的实验室研究表明，杆状体更长或更宽的雄性能生出更多的后代。然而，阴茎骨究竟是如何提高雄性交配成功率的，目前尚不清楚。在一些物种中，雌性在阴蒂处也有一块类似的骨头，被称为baubellum。我们对这种结构所知甚少，它的功能甚至比雄性的更神秘。在这个项目中，我将研究这些神秘结构在哺乳动物骨骼中的功能。我将测试几种关于阴茎骨如何提高雄性交配成功率的理论，包括：1)使阴茎变硬以帮助插入；2)使尿道变直以确保精子可以输送；3)刺激雌性触发排卵。我将测试女性拥有baubellum是否与她们在寻找配偶时可能经历的竞争程度有关。我将与爱丁堡的苏格兰国家博物馆合作，收集许多不同种类的死亡哺乳动物的尸体，包括野生动物和动物园动物。生殖器将从尸体上取下，并在曼彻斯特的x射线成像设施进行CT扫描。CT扫描过程类似于医院中使用的骨折成像技术，它允许我们在不切割（和损坏）物体的情况下可视化生殖器的3D内部结构。然而，在某些情况下，当样本没有价值时，我会取出软组织和骨骼样本，并进行测试，以确定它们的强度和阴茎内纤维的排列。我还会做一个独特的实验，将一只老鼠的阴茎取出，然后人工膨胀。阴茎将被放置在x光机内的装载装置中，在CT扫描期间被压缩和弯曲。这将是第一次看到阴茎的内部结构，同时体验弯曲。这个实验将使我们更多地了解阴茎骨的行为，以及在性交过程中阴茎的哪个区域受到的压力最大。基于x射线扫描，将创建阴茎的3D数字模型（包括有和没有杆状体），我将运行阴茎的虚拟计算机模拟来研究杆状体对阴茎硬度的影响。最后，我将前往英国和国外的众多博物馆，收集更多关于神秘女性骨骼——baubellum的数据。通过将传统解剖学的各个方面与高分辨率x射线扫描和详细的3D数字模型相结合，我希望最终解决哺乳动物生物学中最令人费解的谜团之一：阴茎骨有什么好处？

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?