科学美国人Scientific American EP370 |回答一个古老的谜团:鸟类究竟是如何飞行的？

This is Scientific American’s  60-Second Science, I'm Emily Schwing.


Have you ever looked up to see a  hawk soar overhead, or a small chickadee flit by and wondered: How do they do that?


Believe it or not, scientists never really knew either⁠—until now. 


Talia Lowi-Merri: I looked at the relationship between form and function in the most basic sense.


Talia Lowi-Merri is a Ph.D. student at the University of Toronto in Canada. She says bird flight has everything to do with the shape and size of a bird’s sternum, or breastbone. Bird sternums have a projection from the middle called the keel, and this is where the flight muscles are attached. 


Lowi-Merri: It's plausible to think that this element is important for flight. But why does it vary so much in shape and size relative to the body? There are all these questions about it that haven't been answered in the past.


So, Merri set out to find some answers using a database of CT scanned sternums from 105 different bird species, like the Red-capped lark, Leach’s Storm petrel and the Southern cassowary. She also included two extinct birds: the Dodo and the Great auk. The scans combine a series of x-rays to create three dimensional images.


Lowi-Merri: There have been newer technologies come out recently to look at shape in three dimensions. And so because the sternum is a complex element in three dimensions, it's not just a 2-D bone, it's got projections about the middle and its sides. Looking at it in three dimensions is the best way to quantify the shape and analyze it in a statistical framework. So more recently, those methods have become more accessible. And I guess because of that, I was able to do it now and maybe 10 or 15 years ago, it wasn't possible.


Merri and colleagues used the scans to create computerized 3D models. 


Lowi-Merri: And so when you do that, you can move it around. You can place points on it in the important spots. And so that's what I was doing. I was putting these dots that are called landmarks, and the landmarks basically quantify in 3-D computer space where the important points are on the element.


The findings, published in BMC Biology, show that sternum size and shape has a direct impact on the way a bird flies. [Talia M. Lowi-Merri et al., The relationship between sternum variation and mode of locomotion in birds]


[Sound of an eagle]


Lowi-Merri: So an eagle would be soaring and wouldn't be moving its wings as much. It just has its arms stretched out, and it has very intricate structures in its wings and at its shoulder to hold the wings out, but it doesn't have to use as much flapping power.


But compare the majestic eagle with a frantically flapping duck…


[Sound of ducks flying]


Birds with a deep sternal keel fly more slowly, those with long sternums are associated with running birds. Merri also looked at foot-propelled, underwater diving birds. These are species like the cormorant, the loon and the grebe. 


Lowi-Merri: They have this streamlined sternum with lower sternal keels. Everything is kind of compact and flattened, but you actually see something really similar in birds that are wing propelled divers.


Those wing-propelled divers include small birds you might find on the ocean - puffins, common murres and penguins. Merri says whether wing or foot-propelled, the sternums of these birds are similar in shape. 


All other sorts of factors are possibly most likely contributing to the shape of the sternum, not just locomotion, things like birds that dance to attract a mate or how big their egg is relative to their body size. We've just scratched the surface, looked at one aspect of variation, but there's so much more.” (00:33)


Merri believes that the shape and structure of the sternum impacts how different species of birds breathe. She also says different methods of flight mean different resource demands for individual species. 


Diving deep into how birds fly today can tell scientists a lot about how they evolved over millions of years.


Lowi-Merri: So, birds evolved from dinosaurs… and we don't know exactly which fossil birds and which dinosaurs were capable of flight. But, gaining a better understanding of how birds fly today is the key to completing that picture of how the dinosaurs were moving through the world.


Merri plans to dig into fossil birds next, in part to learn more about the origins of flight.


Lowi-Merri: The thing about fossil birds is that a lot of them are flattened into rock slabs. But there are so many amazing bird fossils, especially from China …. And so they will have to be studied a little bit differently because they may not be able to put them in a three dimensional context, as they did with the modern bird sterna.


For now, though, Merri says she’s looking differently at the small, passerine birds that flit by her windows and dominate the tree branches in her backyard in Ontario. 


Lowi-Merri: They're mostly continuous flapping birds. They're flapping pretty quickly. They're moving from branch to branch. They're trying to keep away from predators and get some food, whether it's insects or berries. it made me think about how their skeletons are structured and also how their muscles are working much differently than, let's say, a hawk that's soaring above. And so they would require different metabolism and different food sources and how they use that in their body would be very different.


[Bird wing flapping]


For 60-Second Science, I’m Emily Schwing.

【参考译文】

这里是科学美国人的60秒科学，我是Emily Schwing。

你是否曾经抬头看到一只老鹰在头顶翱翔，或者一只小鸡仔鸟飞过，并想知道。它们是如何做到的？

信不信由你，科学家们从未真正了解过，直到现在。

Talia Lowi-Merri：我从最基本的意义上研究了形式和功能之间的关系。

Talia Lowi-Merri是加拿大多伦多大学的一名博士生。她说，鸟类的飞行与鸟类胸骨的形状和大小有很大关系，也就是胸骨。鸟类的胸骨有一个从中间突出的部分，称为龙骨，这就是飞行肌肉连接的地方。

Lowi-Merri：认为这个元素对飞行很重要，这是合理的。但是，为什么它的形状和大小相对于身体有如此大的变化？关于它的所有这些问题，在过去都没有得到答案。

因此，梅里开始利用一个数据库，从105种不同的鸟类，如红顶云雀、利奇风暴海燕和南沙鹦鹉的CT扫描胸骨来寻找一些答案。她还包括两种已灭绝的鸟类：渡渡鸟和大鸟。这种扫描结合了一系列的X射线，形成了三维的图像。

Lowi-Merri：最近有一些新的技术出现，可以在三维空间中观察形状。因此，因为胸骨在三维中是一个复杂的元素，它不只是一个二维的骨头，它有关于中间和两侧的投影。在三维空间中观察它是量化形状和在统计框架中分析它的最好方法。所以最近，这些方法变得更容易获得。我想正因为如此，我现在能够做到这一点，也许10或15年前，这是不可能的。

梅里和他的同事们用这些扫描来创建计算机化的三维模型。

洛维-梅里：所以当你这样做时，你可以移动它。你可以在它的重要位置上放置点。所以这就是我所做的。我把这些点称为地标，地标基本上在三维计算机空间中量化了元素上的重要点。

发表在《BMC生物学》上的研究结果表明，胸骨的大小和形状对鸟类的飞行方式有直接影响。[Talia M. Lowi-Merri等人，胸骨变化和鸟类运动方式之间的关系] 。

[鹰的声音]

Lowi-Merri：所以老鹰在翱翔时不会过多地移动它的翅膀。它只是伸出手臂，它的翅膀和肩部有非常复杂的结构来支撑翅膀，但它不需要使用那么多的拍打力量。

但是，将雄伟的老鹰与疯狂拍打的鸭子相比......

[鸭子飞行的声音]

胸骨深的鸟儿飞得更慢，那些胸骨长的鸟儿则与奔跑的鸟儿有关。梅里还研究了用脚推动的、在水下潜水的鸟类。这些是鸬鹚、泥鳅和鸊鷉等物种。

洛维-梅里：它们有这种流线型的胸骨，胸骨下部有龙骨。所有的东西都是紧凑和扁平的，但你实际上在那些用翅膀推动的潜水者身上看到了非常类似的东西。

那些用翅膀推进的潜水员包括你可能在海洋上找到的小鸟--海雀、普通海鸥和企鹅。梅里说，无论是用翅膀还是用脚推进，这些鸟类的胸骨形状都很相似。

所有其他各种因素都可能最有可能促成胸骨的形状，而不仅仅是运动，比如鸟类通过跳舞来吸引配偶，或者它们的蛋相对于它们的身体尺寸有多大。我们只是触及了表面，看了变异的一个方面，但还有很多东西。" (00:33)

梅里认为，胸骨的形状和结构影响着不同种类鸟类的呼吸方式。她还说，不同的飞行方式意味着对个别物种的资源需求不同。

深入研究鸟类今天的飞行方式可以告诉科学家很多关于它们在几百万年中如何进化的信息。

洛维-梅里：所以，鸟类是从恐龙进化而来的......而我们并不确切知道哪些化石鸟类和哪些恐龙能够飞行。但是，更好地了解今天的鸟类是如何飞行的，是完成恐龙如何在世界范围内活动的关键。

梅里计划接下来挖掘鸟类化石，部分是为了了解更多关于飞行的起源。

洛维-梅里：关于鸟类化石的事情是，它们中的很多都被压成了石板。但是有很多令人惊叹的鸟类化石，特别是来自中国的....。因此，对它们的研究将不得不有点不同，因为他们可能无法把它们放在一个三维的环境中，就像他们对现代鸟类立体的研究那样。

不过现在，梅里说她正在以不同的方式看待那些从她的窗户边飞过、在她安大略省后院的树枝上占主导地位的小型过路鸟。

Lowi-Merri: 它们大多是连续拍打的鸟。它们拍打的速度非常快。它们在树枝之间移动。这让我想到了它们的骨骼结构，以及它们的肌肉工作方式与在上面翱翔的鹰有很大不同。因此，它们需要不同的新陈代谢和不同的食物来源，它们在体内的使用方式也会非常不同。

[鸟翼拍打]

60秒科学》栏目，我是Emily Schwing。