T. rex teeth indicate it ran as warm as an elephant
T. rex teeth indicate it ran as warm as an elephant
霸王龙牙齿研究表明：其体温与大象相当
For most of the last century, T. rex was pictured as a sluggish, tail-dragging reptile that had to warm up in the sun before it could go anywhere. Then, further research changed that view, replacing it with the active, bird-like animal shown in the movie Jurassic Park. But whether T. rex used warm blood to power that activity remained a question. Now we might have gotten closer to an answer. 在过去的一个世纪里，霸王龙大多被描绘成一种行动迟缓、拖着尾巴的爬行动物，必须在阳光下晒热身体才能活动。后来，进一步的研究改变了这一观点，将其重塑为电影《侏罗纪公园》中那种活跃的、类似鸟类的动物。但霸王龙是否利用温血来维持这种活动能力一直是个谜。现在，我们可能离答案更近了一步。
A team of researchers led by Randon J. Flores and Robert A. Eagle, geochemists at the University of California, Los Angeles, has measured the T. rex body temperature by analyzing its teeth. This dental thermometer read about 36° Celsius—roughly the body temperature of a modern elephant. 由加州大学洛杉矶分校的地球化学家 Randon J. Flores 和 Robert A. Eagle 领导的研究团队通过分析霸王龙的牙齿，测量了其体温。这种“牙齿温度计”显示其体温约为 36 摄氏度，这大致相当于现代大象的体温。
Dental thermometry
牙齿测温法
Paleontologists have long argued about dinosaur physiology based on indirect evidence like bone microstructure, growth rates, and where fossils turn up on the map. Some studies suggested many dinosaurs were endotherms, generating their own body heat like birds and mammals. Others argued that each lineage may have had its own thermal strategy. 长期以来，古生物学家一直根据骨骼微观结构、生长速度和化石分布地点等间接证据，对恐龙的生理机能争论不休。一些研究表明，许多恐龙是内温动物，像鸟类和哺乳动物一样能产生自身体温。另一些人则认为，每个恐龙谱系可能都有其独特的体温调节策略。
Earlier attempts to measure T. rex’s temperature relied on oxygen isotopes in bones and teeth. The problem was that those ratios depend not only on temperature but also on the composition of the water present in the animal’s body, which was something nobody could pin down for an extinct creature. 早期测量霸王龙体温的尝试依赖于骨骼和牙齿中的氧同位素。问题在于，这些比例不仅取决于温度，还取决于动物体内水分的成分，而对于已灭绝的生物来说，这一点无人能够确定。
Flores, Eagle, and their colleagues got around that with a technique called clumped isotope thermometry, which Eagle introduced to dinosaur research over a decade ago in work on Jurassic sauropods. Carbonate minerals in tooth enamel contain rare, heavy, heavy isotopes of carbon and oxygen (carbon-13 and oxygen-18). How often these heavy atoms bond with each other, or clump together, depends on the temperature at which the mineral formed. Since tooth enamel forms inside a living body, the number of those clumped bonds records the animal’s body temperature, regardless of the water content in its body. Flores、Eagle 及其同事通过一种称为“团簇同位素测温法”的技术解决了这个问题。Eagle 在十多年前研究侏罗纪蜥脚类恐龙时，首次将该技术引入恐龙研究。牙釉质中的碳酸盐矿物含有稀有的重同位素——碳-13 和氧-18。这些重原子相互结合（即“团簇”）的频率取决于矿物形成时的温度。由于牙釉质是在活体内部形成的，这些团簇键的数量记录了动物的体温，且不受其体内水分含量的影响。
The teeth
牙齿分析
The team analyzed three T. rex teeth provided by the Natural History Museum of Los Angeles County, all from Montana’s Hell Creek Formation, which preserves the final period of time before the asteroid impact. Two of the teeth belonged to a young adult T. rex that likely weighed over 3 tons. The third one was an isolated partial tooth from another individual. For comparison, the researchers also analyzed five teeth of crocodilians that shared the same rivers and floodplains. 研究团队分析了由洛杉矶县自然历史博物馆提供的三颗霸王龙牙齿，它们均来自蒙大拿州的地狱溪组（Hell Creek Formation），该地层保存了小行星撞击地球前的最后一段时期。其中两颗牙齿属于一只体重可能超过 3 吨的年轻成年霸王龙，第三颗则是来自另一只个体的孤立残齿。为了进行对比，研究人员还分析了与霸王龙生活在同一河流和洪泛区的鳄类动物的五颗牙齿。
First, though, the team had to make sure that 66 million years underground hadn’t changed the chemistry of the teeth. They focused on enamel, which is far more resistant to alteration than bone or dentin, and ran multiple checks. It turned out the enamel and dentin of the same teeth had different isotopic signatures, which would be unlikely if their chemical composition had been changed after burial. Infrared spectroscopy showed that the fossil enamel looked a bit like that of the modern alligator, and its carbonate content matched that of modern reptilian enamel. Carbon isotopes even showed the dietary signal expected from predators. Once the team was sure that the millions of years underground did not change the teeth, they used their method to infer the body temperature. 首先，研究团队必须确保 6600 万年的地下埋藏没有改变牙齿的化学成分。他们重点研究了比骨骼或牙本质更耐腐蚀的牙釉质，并进行了多次检查。结果发现，同一颗牙齿的牙釉质和牙本质具有不同的同位素特征，如果化学成分在埋藏后发生过改变，这种情况是不太可能出现的。红外光谱分析显示，化石牙釉质看起来有点像现代短吻鳄，其碳酸盐含量与现代爬行动物的牙釉质相符。碳同位素甚至显示出了捕食者应有的饮食特征。在确认了数百万年的地下埋藏没有改变牙齿后，他们利用该方法推断出了霸王龙的体温。
A warm reptile
一种温热的爬行动物
The two teeth from the juvenile T. rex yielded temperatures of 37.3° and 35.9° Celsius, while the tooth from the second individual came in at 34.7°. Averaged together, T. rex had a body temperature of 36.3°, give or take 2.5°. That lines up well with Indian and African elephants, which run at around 36°, and falls within the margin of error of large flightless birds like ostriches and emus. It’s a bit cooler than smaller flying birds, which average above 41°. 来自那只幼年霸王龙的两颗牙齿测得的温度分别为 37.3°C 和 35.9°C，而来自另一只个体的牙齿测得的温度为 34.7°C。平均计算得出，霸王龙的体温为 36.3°C（误差范围为 ±2.5°C）。这与体温约为 36°C 的印度象和非洲象非常吻合，也处于鸵鸟和鸸鹋等大型走禽的体温误差范围内。这比小型飞行鸟类（平均体温在 41°C 以上）要低一些。
The crocodilians the team used as a comparison came in at an average of 30.9°—in line with the preferred range of modern crocodilians, which keep themselves at around 30° to 35° by shuttling between the water and basking spots on the riverbank. The gap between the T. rex and contemporary crocodiles also mirrors the one measured today between large mammals and crocodilians. 研究团队用作对比的鳄类动物平均体温为 30.9°C，这符合现代鳄类动物的偏好范围——它们通过在水中和河岸的晒太阳点之间穿梭，将体温保持在 30°C 到 35°C 之间。霸王龙与同时代鳄鱼之间的体温差距，也反映了当今大型哺乳动物与鳄类动物之间的体温差异。
The next thing to check was whether the T. rex could maintain stable body temperature or was simply as warm as its surroundings. To answer that, the scientists looked at clumped isotopes in fossil freshwater mussels from the same area in the Hell Creek Formation. Mussels mostly record summer water temperatures, and those turned out to average about 26°. The team also ran a new high-resolution climate model of the late Cretaceous, with a grid of roughly 60 kilometers, under two scenarios: colder and hotter. Even in the hotter scenario, the warmest summer months at Hell Creek peaked at around 33°, with mean annual temperatures near 21°. So T. rex was consistently warmer than the world it lived in. But body temperature, the authors note, is not an unambiguous indicator of metabolism. 接下来要验证的是，霸王龙是能够维持稳定的体温，还是仅仅与周围环境温度相当。为了回答这个问题，科学家们研究了来自地狱溪组同一区域的淡水贻贝化石中的团簇同位素。贻贝主要记录的是夏季水温，结果显示平均约为 26°C。研究团队还运行了一个新的晚白垩世高分辨率气候模型（网格约为 60 公里），模拟了寒冷和炎热两种情景。即使在炎热的情景下，地狱溪地区最热的夏季月份气温峰值也仅在 33°C 左右，年平均气温接近 21°C。因此，霸王龙的体温始终高于其所处的环境。但作者指出，体温并不是新陈代谢的明确指标。
Room to roam
漫游空间
A very large animal can stay warm simply because its bulk loses heat slowly, a phenomenon called inertial homeothermy, or gigantothermy. The juvenile T. rex’s temperature was higher than what body size scaling models predict for a cold-blooded animal of its weight. Although those models are debated, the team behind the new work argues that its results add to a growing body of evidence that T. rex was a homeothermic endotherm, an animal that kept a steady body temperature using its own metabolic heat, while acknowledging that the question is not yet settled. 体型巨大的动物仅靠其庞大的身躯缓慢散热就能保持温暖，这种现象被称为“惯性恒温”或“巨型恒温”。幼年霸王龙的体温高于体型缩放模型对同等体重冷血动物的预测值。尽管这些模型尚存争议，但这项新研究的团队认为，他们的结果为“霸王龙是恒温内温动物”（即利用自身代谢热量维持稳定体温的动物）这一观点提供了更多证据，同时也承认这个问题尚未最终定论。