Understanding functional performance in bird skulls: advanced computational modelling to investigate cranial biomechanics and kinesis

众所周知，鸟类的喙形状和大小千变万化；这一特征被广泛认为是它们进化成功的原因。不太为人所知的是，鸟类的嘴是非常灵活的：当它们张开嘴的时候，它们不仅会把下巴放低，还会用鼻孔后面的一个灵活的铰链区把上喙抬起。这个铰链呈现出一些难题，因为一个灵活的头骨应该不太擅长将肌肉力量转化为咬合力。我们的研究旨在测试几个基于力学的关于铰链进化原因的想法。也许铰接的头骨可以让鸟嘴张开得更大，而不会过度拉伸鸟类的下颌肌肉？或者允许在喙部产生的危险的高力在到达大脑之前被吸收？也许颅骨的运动改变了下颌肌肉的杠杆作用，使它们更有效率？或者铰链只是鸟类从笨重的恐龙祖先进化而来，为了减轻体重而使骨骼变薄的副作用？这些想法很少被探索，因为在不伤害鸟类的情况下很难看到运动的部分。我们的研究旨在通过建立鸟类进食时头骨运动的先进计算机模型来改变这一点，并利用它们来测试在活体动物身上测试是不可能或不道德的想法；例如，在计算机中，我们可以“关闭”某些肌肉，或者“锁定”铰链，使其无法移动。我们将使用博物馆捐赠的鸟类标本的3D x射线（“CT扫描”）在计算机中重建头骨和肌肉，并使用专为测试桥梁和汽车等人造结构性能而设计的工程方法（“有限元分析”和“多体动力学分析”）进行分析。我们还将制作一个鸟类骨骼、肌肉和铰链解剖特性的数据库，这些数据库将与模型一起在网上共享，供其他研究人员、博物馆和教师使用。这个过程的关键一步是验证计算机模型，以确保它们产生的数字是真实可靠的。在CT扫描仪内，我们将对博物馆捐赠的标本的喙施加力来模拟咬人。通过在施加力之前和之后进行扫描，我们将能够计算出骨骼发生了多大的变形，并将其与我们的计算机模型进行比较，以验证其准确性。使用CT扫描仪来测量这种变形是一个重大的进步，因为这是我们第一次能够通过整个结构来测量3D运动和变形，而不仅仅是在小区域的表面上。我们还将测量同一种鸟类的咬合力，方法是让它们咬一个小型压力传感器。同时，我们将拍摄鸟类咬人的过程，以跟踪鸟喙的运动和变形。这些运动和咬合力测量将用于验证多体动力学模型，然后将其反馈到有限元模型中，以模拟实际喂食过程中的头骨变形。经过验证的模型不仅允许我们测试上面概述的进化思想，而且它们将为我们提供关于如何最好地模拟几种鸟类特有的解剖特征的重要信息，但不是鸟类独有的，包括：超薄的骨头；复杂的骨骼形状；由多种材料制成的头骨；以及头骨内活动的关节。有了这些知识，我们不仅可以深入了解鸟喙的进化故事，还可以为计算机模型的最佳实践提供建议，这些模型可以减少，并有望最终取代侵入性动物试验的需要。

Birds are well known for having a huge variety of beak shapes and sizes; a feature that is widely believed to be responsible for their evolutionary success. What is less well known is that bird beaks are incredibly mobile: when opening their mouths, birds don't just lower their jaws, but raise the upper beak as well using a flexible hinge region just behind the nostrils. This hinge presents something of a conundrum, because a flexible skull should be less good at turning muscle force into biting force. Our research aims to test several ideas based on mechanics about why the hinge evolved. Perhaps a hinged skull allows the beak to open wider, without over-stretching birds' jaw muscles? Or allows dangerously high forces generated in the beak to be absorbed before they can reach the brain? Perhaps movement in the skull changes the leverage of the jaw muscles to make them more efficient? Or maybe the hinge is simply a side-effect of thinning down the bones to reduce weight as birds evolved away from their heavy, dinosaur ancestors? These ideas are rarely explored because the moving parts are difficult to see without harming the birds. Our research aims to change this by building advanced computer models of the movement in the skull during bird feeding, and using them to test ideas that would be impossible or unethical to test on living animals; for example, in the computer we can "turn off" certain muscles, or "lock" the hinge so that it can't move. We will use 3D x-rays ("CT scans") of bird specimens donated by museums to rebuild the skull and muscles inside a computer, and analyse them using engineering methods ("finite element analysis" and "multibody dynamics analysis") designed for testing the performance of man-made structures like bridges and cars. We will also produce a database of properties of bird bones, muscles, and hinge anatomy, which will be shared online along with the models for other researchers, museums, and teachers to use. A critical step in this process is validating the computer models, to make sure that they are producing numbers that are realistic and reliable. Inside the CT scanner, we will apply forces to the beaks of our museum-donated specimens to mimic biting. By taking a scan before and after the force is applied, we will be able to calculate how much deformation has occurred in the bones and compare this to our computer models to verify their accuracy. Using the CT scanner to measure this deformation represents a major advancement, because for the first time we will be able to measure the motion and deformation in 3D through the whole structure, instead of just on the surface over small areas. We will also measure the bite force of the same species of living birds by having them bite on a small pressure sensor. At the same time, we will film the birds biting to track the motion and deformation of the beak. These motion and bite force measurements will be used to validate the multibody dynamics models, which will then feed back into the finite element models to simulate skull deformation during actual feeding. Not only will the validated models allow us to test the evolutionary ideas outlined above, but they will give us vital information about how to best model several anatomical features that are characteristic of, but not exclusive to birds, including: ultra-thin bones; complicated bone shapes; skulls made from multiple materials; and moving joints within the skull. With this knowledge, we will not only gain insight into the evolutionary story of bird beaks, but we be able to advise others on best practices for computer models that can reduce, and hopefully ultimately replace, the need for invasive animal testing.