變溫動物如何演化為恆溫動物？目前認為最古老/最原始的恆溫動物是什麼？

01-08

變溫動物也稱冷血動物，體溫會隨著外界環境發生較大範圍波動；恆溫動物也稱溫血動物，體溫往往維持在略高且較穩定的範圍內，稍有程度大一些的波動就會有生命危險。兩種動物在生理結構上存在諸多區別，不過通識教育階段並未多做介紹，是否可以科普一下？
另外，更早出現在地球上的變溫動物是通過哪些改變，逐漸演化為了恆溫動物呢？大致發生什麼時期，發生於什麼物種？有哪些化石證據？

所謂變溫恆溫其實就調節自身體溫的能力，至於動物如何從變溫演化成恆溫，其實都是從低代謝往高代謝演化表現出的性狀，也就是區別兩者的一個特徵，其他的特徵都可以從高低代謝的角度找，比如高效的呼吸系統、四腔室的心臟、有骨髓的骨頭、發達的大腦諸如此類的高代謝特徵。

但也不是沒有例外，鱷魚是變溫卻有著恆穩動物才有的心臟和肺，於是有人猜想：鱷魚的三疊紀祖先其實像現在的鳥類哺乳類一樣活躍，才有高代謝恆溫的特徵，但因為環境的改變而被迫變成變溫適應食物來源不那麼穩定的水域。

演化是發生在什麼時候？

如今的恆溫動物除了某些魚類就數鳥類和哺乳類兩支，但這兩支生物分道揚鑣的時間太早，晚石炭紀就已經分化……（不廢話了）也就是說兩者的恆溫並非同源，而是趨同演化，從變溫到恆溫的演化從羊膜動物開始就沒有停止過（大概在二疊紀三疊紀之交中斷過一次）那兩者之間的生物是怎樣的呢？

中溫型，介於變溫與恆溫之間，調節體溫的能力比爬行動物好得多，但又不如鳥類哺乳類的生物是誰呢？有袋類和胎盤類的親戚，隱居在澳洲的老古董，只有兩種卻被立為亞綱，鴨嘴獸和針鼴，現存的中溫型羊膜動物（有鱗目的薩爾瓦托蜥在發情期會變身恆溫動物），針鼴的體溫不再像變溫動物那樣隨時間和氣溫變化，而是睡覺（尼克楊？？？）

至於那些已滅絕的生物就只能從生長率上找線索了

不過最近同位素的方法能研究出史前生物的體溫，研究的竊蛋龍和泰坦巨龍分別是32℃和38℃，泰坦38很好理解，如此巨大的體型體溫不可能低，那小型的竊蛋龍體溫依然高出環境溫度6℃說明當時的恐龍應該是群相當活躍的生物，而不是如今的人們眼裡的爬行動物那樣冷血無情。

上面的古生物例子里似乎沒有合弓，但哺乳動物之外的合弓似乎都沒有恆溫型？

放篇論文了事

Eggshells Tell Scientists About Dinosaur Body Temperatures

By Mike, October 13, 2015

Endotherms or Ectotherms the Debate Gets 「Egg-citing」

The debate as to whether dinosaurs were warm-blooded (endothermic) like mammals and birds or cold-blooded (ectotherms) like crocodiles and lizards, has taken a further twist with the publication of a study published in the journal 「Nature Communications」. Scientists have used a new method to chemically analyse dinosaur eggshells which has helped them gauge the long extinct animal』s body temperature. Armed with this data and an estimate of the temperature of the surrounding environment, the researchers can postulate about whether dinosaurs had high metabolic rates and active life-styles or whether like alligators and lizards they tended to be only capable of short bursts of activity.

The team』s findings suggest that dinosaurs were neither true endotherms or completely ectothermic, but somewhere in between – mesotherms. In addition, as two different types of dinosaur produced different results, this new study indicates that body temperature differed between dinosaur genera.

The conclusions drawn by the scientists are similar to those made by a team of American researchers who reported in 2014. Following an extensive analysis of the histology and ontogeny of fossilised bones (analysis of bone cells and growth) these scientists concluded that dinosaurs were probably mesotherms.

To read more about the 2014 research: Goldilocks and Dinosaurs the warm-blooded/cold-blooded Debate

Why All the Fuss?

The argument as to whether the Dinosauria were endothermic or ectothermic has raged for over 150 years. It was once thought that dinosaurs were slow-moving, sluggish, very lizard-like creatures. They had to bask in the sun or rely on other external sources of heat to help warm their bodies before they could become active. This meant that they were probably not very active at night, it also suggested that they had similar life-styles to extant reptiles such as crocodiles, snakes and lizards. These creatures can be very mobile, but only for short periods and they soon become tired. Thomas Henry Huxley (1825-1895), was one of the first scientists to challenge this view. He argued that birds and dinosaurs were closely related. In the 1960』s there was a revolution in thinking with regards to dinosaurs. John Ostrom』s work on the Theropod dinosaur Deinonychus led to dinosaurs being depicted as much more agile, fast-moving animals. Ostrom produced carefully argued and extremely detailed studies on dinosaur biology and anatomy and he depicted them as much more bird-like creatures capable of complex behaviour – more reminiscent of today』s mammals than like today』s reptiles.

Robert Bakker, a student under the tutelage of Ostrom developed these ideas further and portrayed dinosaurs as warm-blooded creatures, it was Bakker who produced the famous illustration of Deinonychus shown above and it was Bakker who influenced a lot of thinking regarding the Dinosauria with his ground-breaking book 「The Dinosaur Heresies」, which was published in 1986.

A Study of Rare Isotopes

The research team utilised a pioneering procedure which allows the internal temperature of female dinosaurs to be plotted by analysing the behaviour of two rare isotopes found in calcium carbonate, a key ingredient in eggshells. A total of nineteen dinosaur eggs were included in the study, they represent two very different types of dinosaur. Eggshells from Argentina came from a large Titanosaur (Sauropoda), the eggshells collected in the Gobi desert, Mongolia, represent fossils from a small Theropod dinosaur, an oviraptorid. The isotopes oxygen-18 and carbon-13 tend to cluster together at colder temperatures. The amount of clumping enabled the scientists to calculate the body temperature of the mother. Chemical analysis of the surrounding rock matrix permitted the researchers to estimate the temperature of the palaeoenvironment at around the time the eggs were laid.

The analysis of these eggshells, all of which were collected from Upper Cretaceous strata, suggest that the Titanosaur』s body temperature had been around 38 degrees Celsius at the time of laying. This is a little higher than our own internal body temperature (37 degrees Celsius). The implication is that the Titanosaur had an internal body temperature similar to that of a endotherm, although it has to be considered that the huge gut of this giant herbivore, essentially an enormous, heat-producing digestive vat would probably have influenced the internal temperature. The sheer size of the animal, volume compared to surface area, would have probably helped it to maintain a relatively constant body temperature. Thermo-regulation for such large animals is not so much as trying to keep warm, heat loss from a large body is much less severe than with a small animal, for example. In the case of the Titanosaurs, once they warmed up they probably stayed warm for a very long time, keeping cool could have been more of a problem (that』s why African elephants have big ears).

Picture Credit: Everything Dinosaur

The results from the much smaller oviraptorid paint a different picture. These dinosaurs were very closely related to birds, but the body temperature plotted is much lower than an endotherm. The scientists calculated that the Theropod had an internal temperature of around 32 degrees Celsius, but this was at least six degrees warmer than the environment. This suggests that oviraptorids, dinosaurs that were closely related to birds were able to raise their body temperatures above that of their surroundings.

Commenting on these results, lead author Robert Eagle (University of California Los Angeles) stated:

「The temperatures we measured suggest that at least some dinosaurs were not fully endotherms [warm-blooded] like modern birds. They may have been intermediate, somewhere between modern alligators and crocodiles and modern birds」.

This study supports the idea that dinosaurs could produce heat internally and raise their body temperature higher than their surrounding environment, but they were unable to maintain this temperature at a consistent level like a mammal or a bird (endotherms). The scientists conclude that if the dinosaurs were mesothermic, they were probably more active than alligators but not as active as birds or mammals. The international research team state that they could not observe any strong evidence either for ectothermy or endothermy in the species examined. The body temperatures calculated for the Titanosaur and the oviraptorid indicate that variable thermoregulation likely existed among non-avian dinosaurs and that not all dinosaurs had body temperatures in the range of that seen in extant birds.

Link to an article published in June 2015, that postulates that the Dinosauria were endothermic: Dinosaurs were Warm-blooded Debate Hots Up

補充：

@蕭楚提到哺乳動物的祖先，事實上現在的內溫動物是多源的（polyphyletic），也就是說哺乳動物和鳥類的溫度調節方式並不是從一個共同祖先得到的。哺乳動物和鳥類的共同祖先是羊膜動物，但是在早期就已經分開，所以他們的共同祖先很可能並沒有內溫動物的特徵。而哺乳動物最早的內溫動物祖先是什麼形態的這個也並不清楚，因為已經滅絕了所以沒有辦法研究生理。鳥類和爬行動物在代謝上有更多的相似之處，所以我們可以想像出一種介於鳥類和爬行動物之間的似內溫動物。

（圖 from wikipedia commons）

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動物學已經基本忘光的某人來試答一下......

首先有這個問題內溫動物、外溫動物、恆溫動物、變溫動物之間有什麼關係？ - 生物

事實上在動物學課上提到過冷血動物和溫血動物並不是在科學上使用的概念......然後恆溫和變溫都不是絕對的，因為沒有一個明確規定的溫度變化的範圍。比較適用的概念是外溫和內溫動物，區別在於用不用體內的熱源來調節體溫。

爬行綱（Reptila）和鳥綱（Aves）是脊索動物門親緣關係比較近的兩個類，爬行動物是外溫動物，而鳥類是內溫動物，所以它們的最近共同祖先（Most recent common ancestor）就可能位於外溫動物和內溫動物的交界線上。

至於這個最近共同祖先是什麼似乎還沒有定論......畢竟是已經滅絕物種。

（圖Archaeopteryx lithographicafrom wikimedia commons）

問題挺好的。

這正好是一個進行時的大熱點。

正好也是通過古組織學，研究二三疊各種四足動物的生長發育，血壓，骨血管流量等等。

想知道哪個是最早的內溫動物肯定需要這種研究，安心等幾個大實驗室給結果吧。出來結果也不用來找(′?_?`)我，會在nature或者science上看到的。

其實沒有任何質的變化，所謂諸多區別都是無關痛癢的小差異，主要變化就是呼吸加快，心室更封閉，新陳代謝更快，都是小突變。

最早的恆溫動物一般認為是運動活躍的羊膜動物，恐龍、獸孔類、迅猛鱷類，等等。但是某些大型海洋魚類也可能用肌肉活動產熱

最古老的恆溫動物是鳥

是因為恆溫（而且溫度較高）令一般化學反應更有效率。

你有沒有留意，家中的壁虎（檐蛇／蜥蜴）行動時總是突然移動幾步然後又停下來？

而恆溫的老鼠行動非常快，完全不凝滯？