Fossil tooth analysis confirms Tyrannosaurus rex was warm-blooded

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Scientists have suspected for nearly 60 years that Tyrannosaurus rex was warm-blooded.

When a fossilized footprint of T. rex turned up in Alaska in 2022, it added to the evidence that these predators could thrive in frigid environments, where most cold-blooded reptiles could not. Now, researchers have put a number on just how warm tyrannosaurs likely ran: about 97 degrees Fahrenheit, or 36 degrees Celsius, similar to humans.

A new study published Wednesday in the journal Science Advances provides the first direct estimate of T. rex’s body temperature, using a technique that analyzes chemical signatures preserved in fossil teeth. The finding adds a crucial piece of evidence to the decades of research suggesting the giant predator was warm-blooded and capable of maintaining an active metabolism even in cold climates.

“It’s probably the most direct and least complicated constraint on the actual body temperature of a T. rex that’s been possible before,” said study coauthor Robert Eagle, a geobiologist and associate professor at the University of California, Los Angeles.

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Having an estimated body temperature for T. rex provides a better glimpse into what life was like for the dinosaur 69 million to 66 million years ago. “This was a warm-bodied organism, which really places new constraints on its behavior, its physiology, how it interacted with other species and how much food it needed, for example,” Eagle added.

So far, the technique has been used on a handful of ancient species, including other dinosaurs , woolly mammoths and megalodons , giving scientists a clearer picture of how extinct animals regulated their body temperatures in ancient climates.

The temperature is unusually high for a reptile, but Eagle said the number itself was not surprising . Dinosaurs, while classified as reptiles, share an evolutionary link with birds. The new number reveals that T. rex sits between the two — hotter than reptiles but colder than modern avians — and allows scientists to form new hypotheses about how exactly the prehistoric predator might have moved and lived.

Eagle and a team of scientists used two small portions of teeth from Thomas the T. rex — an approximately 70% complete Tyrannosaurus rex skeleton held at the Natural History Museum of Los Angeles County — to estimate the dinosaur’s body temperature from chemical bonds preserved in its tooth enamel.

The researchers spent more than a decade perfecting the method to make the measurement possible. Over the years, their work reduced the amount of fossil material needed by roughly 90%, enabling the museum to provide tiny samples of two teeth that weren’t on display.

“Nobody can measure a T. rex tooth unless you can show them you only need a few milligrams,” said senior study author Aradhna Tripati, a climate scientist and professor of geochemistry at UCLA.

“For an animal this famous, it is remarkable how little we actually knew. We had guesses about T. rex metabolism, mainly from bone and biomechanics. We did not have a temperature,” Tripati added in an email. “The enamel chemistry recorded the temperature at which it formed, and 66 million years later we can read it.”

courtesy Dinosaur Institute/Natural History Museum of Los Angeles County

The researchers essentially used the T. rex teeth as a prehistoric thermometer. Inside tooth enamel are different forms of the elements carbon and oxygen, called isotopes. These isotopes form bonds with one another at rates that depend on temperature — cooler temperatures produce more bonds, while warmer ones produce fewer. By drilling out a few milligrams of enamel and measuring those bonds, scientists could determine the temperature at which the enamel formed, revealing the T. rex’s body temperature.

This technique can be used on any part of the skeleton, but teeth work best because their hard enamel is less likely to change over millions of years, Eagle explained.

“We have wondered about whether dinosaurs were warm-blooded or not literally since the first time dinosaurs got their name ,” said vertebrate paleontologist Thomas Holtz Jr., referring to Richard Owen’s 1842 paper, which introduced the name Dinosauria and suggested that dinosaurs might have had characteristics of warm-blooded animals.

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Holtz added that while there has been various evidence supporting the idea that dinosaurs were endothermic, or warm-blooded, this new method offers another way to examine the question. Comparing T. rex’s body temperature with those of cold-blooded crocodiles and clams from the same time and place “gives high confidence that the values they find for Tyrannosaurus really show a high body temperature,” said Holtz, a principal lecturer in the department of geology at the University of Maryland. He was not involved in the study.

In the future, Holtz said it would be interesting to explore the question on whether all dinosaurs were warm-blooded. Using this method next on groups such as Triceratops, Stegosaurus and Brachiosaurus, which have sometimes been considered “cooler-blooded,” could help answer the question, he added.

Although T. rex was a reptile, its estimated body temperature of 97 F falls within the range seen in modern warm-blooded animals. Warm-blooded species vary considerably in body temperature. Some mammals, including sloths and anteaters, can have temperatures in the low 90s Fahrenheit (around 32 C), while small, high-metabolism birds can exceed 104 F (40 C), Eagle said.

Modern cold-blooded reptiles typically have body temperatures around 82 F to 86 F (28 C to 30 C). Eagle added that these temperatures align with animals’ energy levels — reptiles are often more idle, whereas birds have to expend a lot of energy while in flight.

A higher, regulated body temperature also would have helped T. rex stay active for longer periods.

“I don’t think T. rex is thought to be a particularly fast sprinter, but the implications of this are maybe that it could maintain an energetic performance over longer periods, probably, than a cold-blooded organism would be able to,” Eagle said. For example, “crocodiles can run over short distances, but they’re not able to keep it up for a long time.”

A warm-bodied T. rex supports the idea that tyrannosaurs were active predators with fast metabolisms. The dinosaur’s ability to maintain a high body temperature also would have allowed it to survive in cooler climates where cold-blooded animals could not.

“What the teeth tell us is that T. rex was warmer than the environment around it,” Tripati said. “A warm-blooded T. rex is a T. rex that can go almost anywhere on the continent, including the Arctic. That is a very different animal from a sun-basking reptile.”

To explore where the dinosaur could have lived, the researchers used paleoclimate models to reconstruct what temperatures were like across North America about 66 million years ago during the late Cretaceous. The researchers then considered those conditions alongside T. rex’s estimated body temperature. Their models suggested that the dinosaur could have inhabited a broad stretch of North America, from what is now Mexico to Alaska.

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The high body temperature also implies that the T. rex would have had to pursue prey and eat more than other reptiles, such as the cold-blooded giant crocodiles of the Mesozoic, to keep up with their high metabolism, Holtz added.

Eagle said that researchers hope eventually to apply the temperature-measuring technique to other ancient species, potentially reaching even further back in evolutionary history.

“I think the ancestors of mammals are particularly exciting to look at. There are not great constraints about when warm-blooded mass occurred in the evolution of mammals,” Eagle said. “That’s in itself an extra challenge because then we’re getting even further back in time. But that would be really exciting if that works.”

Taylor Nicioli is a freelance journalist based in New York.

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近60年来，科学家们一直怀疑霸王龙是温血动物。

2022年，在阿拉斯加发现了霸王龙的化石足迹，这进一步证实了这些掠食者能够在大多数冷血爬行动物无法生存的严寒环境中繁衍生息。如今，研究人员已经确定了霸王龙可能适宜的温度范围：大约97华氏度（36摄氏度），与人类的体温相近。

周三发表在《科学进展》（Science Advances）杂志上的一项新研究，利用分析化石牙齿中保存的化学特征的技术，首次直接估算了霸王龙的体温。这一发现为数十年来关于这种巨型掠食者是温血动物，即使在寒冷气候下也能维持活跃新陈代谢的研究增添了关键证据。

“这可能是迄今为止对霸王龙实际体温最直接、最简单的限制，”该研究的合著者、加州大学洛杉矶分校的地质生物学家兼副教授罗伯特·伊格尔说。

Eric Lalmand/Belga Mag/法新社/Getty Images

为什么霸王龙的胳膊那么短？一项新的研究或许能给出答案。

估算霸王龙的体温能让我们更好地了解6900万至6600万年前这种恐龙的生活状况。“霸王龙是恒温动物，这确实对其行为、生理机能、与其他物种的互动方式以及所需的食物量等方面提出了新的限制，”伊格尔补充道。

到目前为止，这项技术已经应用于少数古代物种，包括其他恐龙、猛犸象和巨齿鲨，使科学家们更清楚地了解已灭绝的动物如何在古代气候中调节体温。

对于爬行动物来说，这个温度异常高，但伊格尔表示，这个数字本身并不令人惊讶。恐龙虽然被归类为爬行动物，但它们与鸟类有着进化上的联系。新的数据显示，霸王龙的温度介于两者之间——比爬行动物高，但比现代鸟类低——这使得科学家们能够提出新的假设，来探讨这种史前掠食者究竟是如何移动和生活的。

伊格尔和他的科学家团队利用来自霸王龙托马斯（一具保存约 70% 完整的霸王龙骨架，现藏于洛杉矶县自然历史博物馆）的两小块牙齿，通过保存在牙釉质中的化学键来估算恐龙的体温。

研究人员花费十余年时间完善测量方法。多年来，他们的工作使所需的化石材料减少了约90%，使得博物馆能够提供两颗原本未展出的牙齿的微小样本。

“除非你能向他们证明你只需要几毫克，否则没有人能测量霸王龙的牙齿，”该研究的资深作者、加州大学洛杉矶分校的气候科学家兼地球化学教授阿拉德纳·特里帕蒂说道。

“对于如此著名的动物来说，我们对它的了解竟然如此之少，这真是令人惊讶。我们之前对霸王龙的新陈代谢只有一些推测，主要依据骨骼和生物力学数据。我们甚至不知道它的体温，”特里帕蒂在邮件中补充道。“牙釉质的化学成分记录了它形成时的温度，6600万年后，我们终于可以解读这些信息了。”

图片由洛杉矶县恐龙研究所/自然历史博物馆提供

研究人员实际上是将霸王龙的牙齿当作史前温度计来使用。牙釉质内部含有不同形式的碳和氧元素，称为同位素。这些同位素相互结合的速率取决于温度——温度越低，结合的同位素就越多；温度越高，结合的同位素就越少。通过钻取几毫克牙釉质并测量这些同位素的结合情况，科学家可以确定牙釉质形成时的温度，从而推断出霸王龙的体温。

Eagle解释说，这种技术可以用于骨骼的任何部位，但牙齿效果最好，因为牙齿坚硬的牙釉质在数百万年的时间里不太可能发生变化。

“自从恐龙被命名以来，我们就一直在思考恐龙是否是温血动物，”脊椎动物古生物学家托马斯·霍尔茨二世说道。他指的是理查德·欧文在 1842 年发表的论文，该论文提出了“恐龙”这个名称，并认为恐龙可能具有温血动物的特征。

北卡罗来纳州自然科学博物馆

“恐龙决斗”化石迫使人们对霸王龙遗骸进行彻底的重新思考

霍尔茨补充说，虽然已有各种证据支持恐龙是恒温动物的观点，但这种新方法提供了另一种检验该问题的方式。霍尔茨是马里兰大学地质系的高级讲师，他表示，将霸王龙的体温与同时期和同一地区的冷血鳄鱼和蛤蜊的体温进行比较，“可以高度确信，他们发现的霸王龙体温值确实表明其体温很高。” 他并未参与这项研究。

霍尔茨表示，未来研究恐龙是否都是温血动物将是一件很有趣的事情。他补充说，接下来将这种方法应用于三角龙、剑龙和腕龙等有时被认为是“冷血”的恐龙类群，或许有助于解答这个问题。

虽然霸王龙是爬行动物，但其估计体温为97华氏度，与现代温血动物的体温范围相符。温血动物的体温差异很大。伊格尔说，一些哺乳动物，包括树懒和食蚁兽，体温可以达到90华氏度左右（约32摄氏度），而小型、新陈代谢旺盛的鸟类体温则可以超过104华氏度（40摄氏度）。

现代冷血爬行动物的体温通常在华氏82度到86度（摄氏28度到30度）之间。伊格尔补充说，这些温度与动物的能量水平相符——爬行动物通常比较安静，而鸟类在飞行时需要消耗大量能量。

较高的、受控的体温也有助于霸王龙保持更长时间的活跃状态。

“我认为霸王龙的短跑速度并不快，但这或许意味着它能比冷血动物更长时间地保持高能量状态，”伊格尔说。例如，“鳄鱼可以短距离奔跑，但它们无法长时间保持这种速度。”

一只体温正常的霸王龙支持了暴龙是活跃的掠食者且新陈代谢快的观点。这种恐龙维持高体温的能力也使其能够在冷血动物无法生存的寒冷气候中存活下来。

“牙齿告诉我们，霸王龙的体温高于周围环境，”特里帕蒂说。“温血霸王龙几乎可以遍布北美大陆的任何地方，包括北极。这与喜欢晒太阳的爬行动物截然不同。”

为了探究这种恐龙可能的生存区域，研究人员利用古气候模型重建了大约6600万年前白垩纪晚期北美洲的气温。然后，研究人员将这些气温与霸王龙的估计体温结合起来进行分析。他们的模型表明，这种恐龙可能生活在北美洲的广阔区域，从现在的墨西哥一直延伸到阿拉斯加。

乔丹·托文/路透社

罕见霸王龙化石在拍卖会上以创纪录的5010万美元成交。

霍尔茨补充说，高体温也意味着，为了维持其高代谢率，霸王龙必须比其他爬行动物（如中生代的冷血巨鳄）追逐猎物并吃得更多。

Eagle表示，研究人员希望最终能将温度测量技术应用于其他古代物种，甚至有可能追溯到更久远的进化史。

“我认为研究哺乳动物的祖先特别令人兴奋。关于温血动物体型在哺乳动物进化过程中出现的时间，目前还没有太多限制，”伊格尔说。“这本身就是一个额外的挑战，因为这意味着我们要追溯到更久远的过去。但如果成功的话，那将非常令人兴奋。”

泰勒·尼乔利是一位居住在纽约的自由撰稿人。