Varmblodig eller kaldblodig? Kjemiske ledetråder løser et av de eldste mysteriene i paleontologi

Paleontologer har diskutert i flere tiår om dinosaurer var varmblodige, som moderne pattedyr og fugler, eller kaldblodige, som moderne reptiler. Å vite om dinosaurene var varm- eller kaldblodige kunne gi oss ledetråder om hvor aktive de var og hvordan hverdagen deres var, men tidligere metoder for å bestemme deres varm- eller kaldblodige – hvor raskt stoffskiftet deres kunne gjøre oksygen til energi – var usikre. Imidlertid i en ny artikkel publisert i tidsskriftet Naturavduker forskere en ny metode for å studere dinosaurenes metabolske hastigheter, ved å bruke ledetråder i beinene deres som indikerte hvor mye de enkelte dyrene pustet i løpet av sin siste time av livet.

“Dette er veldig spennende for oss som paleontologer – spørsmålet om dinosaurer var varm- eller kaldblodige er et av de eldste spørsmålene innen paleontologi, og nå tror vi vi har en konsensus om at de fleste dinosaurer var varmblodige,” sier Jasmina Wiemann, papirets hovedforfatter og postdoktor ved California Institute of Technology (Caltech).

“Den nye proxyen utviklet av Jasmina Wiemann lar oss direkte utlede metabolisme i utdødde organismer, noe vi bare drømte om for bare noen få år siden. Vi fant også forskjellige stoffskiftehastigheter som karakteriserte forskjellige grupper, noe som tidligere ble foreslått basert på andre metoder, men aldri testet direkte, sier Matteo Fabbri, en postdoktor ved Field Museum i Chicago og en av studiens forfattere.

Folk snakker ofte om metabolisme når det gjelder hvor lett det er for noen å holde seg i form, men i kjernen er “metabolisme hvor effektivt vi omdanner oksygenet som vi puster inn til kjemisk energi som gir energi til kroppen vår,” sier Wiemann, som er tilknyttet Yale University og Natural History Museum of Los Angeles County.

Dyr med høy metabolsk hastighet er endoterme, eller varmblodige; varmblodige dyr som fugler og pattedyr tar i seg mye oksygen og må forbrenne mange kalorier for å opprettholde kroppstemperaturen og holde seg aktive. Kaldblodige, eller ektotermiske, dyr som reptiler puster mindre og spiser mindre. Livsstilen deres er mindre energisk dyr enn et varmblodig dyrs, men det har en pris: kaldblodige dyr er avhengige av omverdenen for å holde kroppen på riktig temperatur for å fungere (som en øgle som soler seg i solen), og de har en tendens til å være mindre aktive enn varmblodige skapninger.

Med fugler som var varmblodige og krypdyr som var kaldblodige, ble dinosaurer fanget midt i en debatt. Fugler er de eneste dinosaurene som overlevde masseutryddelsen på slutten av[{” attribute=””>Cretaceous, but dinosaurs (and by extension, birds) are technically reptiles — outside of birds, their closest living relatives are crocodiles and alligators. So would that make dinosaurs warm-blooded, or cold-blooded?

Scientists have tried to glean dinosaurs’ metabolic rates from chemical and osteohistological analyses of their bones. “In the past, people have looked at dinosaur bones with isotope geochemistry that basically works like a paleo-thermometer,” says Wiemann — researchers examine the minerals in a fossil and determine what temperatures those minerals would form in. “It’s a really cool approach and it was really revolutionary when it came out, and it continues to provide very exciting insights into the physiology of extinct animals. But we’ve realized that we don’t really understand yet how fossilization processes change the isotope signals that we pick up, so it is hard to unambiguously compare the data from fossils to modern animals.”

Another method for studying metabolism is the growth rate. “If you look at a cross-section of dinosaur bone tissue, you can see a series of lines, like tree rings, that correspond to years of growth,” says Fabbri. “You can count the lines of growth and the space between them to see how fast the dinosaur grew. The limit relies on how you transform growth rate estimates into metabolism: growing faster or slower can have more to do with the animal’s stage in life than with its metabolism, like how we grow faster when we’re young and slower when we’re older.”

The new method proposed by Wiemann, Fabbri, and their colleagues doesn’t look at the minerals present in bone or how quickly the dinosaur grew. Instead, they look at one of the most basic hallmarks of metabolism: oxygen use. When animals breathe, side products form that react with proteins, sugars, and lipids, leaving behind molecular “waste.” This waste is extremely stable and water-insoluble, so it’s preserved during the fossilization process. It leaves behind a record of how much oxygen a dinosaur was breathing in, and thus, its metabolic rate.

The researchers looked for these bits of molecular waste in dark-colored fossil femurs, because those dark colors indicate that lots of organic matter are preserved. They examined the fossils using Raman and Fourier-transform infrared spectroscopy — “these methods work like laser microscopes, we can basically quantify the abundance of these molecular markers that tell us about the metabolic rate,” says Wiemann. “It is a particularly attractive method to paleontologists, because it is non-destructive.”

The team analyzed the femurs of 55 different groups of animals, including dinosaurs, their flying cousins the pterosaurs, their more distant marine relatives the plesiosaurs, and modern birds, mammals, and lizards. They compared the amount of breathing-related molecular byproducts with the known metabolic rates of the living animals and used those data to infer the metabolic rates of the extinct ones.

The team found that dinosaurs’ metabolic rates were generally high. There are two big groups of dinosaurs, the saurischians and the ornithischians — lizard hips and bird hips. The bird-hipped dinosaurs, like Triceratops and Stegosaurus, had low metabolic rates comparable to those of cold-blooded modern animals. The lizard-hipped dinosaurs, including theropods and the sauropods — the two-legged, more bird-like predatory dinosaurs like Velociraptor and T. rex and the giant, long-necked herbivores like Brachiosaurus — were warm- or even hot-blooded. The researchers were surprised to find that some of these dinosaurs weren’t just warm-blooded — they had metabolic rates comparable to modern birds, much higher than mammals. These results complement previous independent observations that hinted at such trends but could not provide direct evidence, because of the lack of a direct proxy to infer metabolism.

These findings, the researchers say, can give us fundamentally new insights into what dinosaurs’ lives were like.

“Dinosaurs with lower metabolic rates would have been, to some extent, dependent on external temperatures,” says Wiemann. “Lizards and turtles sit in the sun and bask, and we may have to consider similar ‘behavioral’ thermoregulation in ornithischians with exceptionally low metabolic rates. Cold-blooded dinosaurs also might have had to migrate to warmer climates during the cold season, and climate may have been a selective factor for where some of these dinosaurs could live.”

On the other hand, she says, the hot-blooded dinosaurs would have been more active and would have needed to eat a lot. “The hot-blooded giant sauropods were herbivores, and it would take a lot of plant matter to feed this metabolic system. They had very efficient digestive systems, and since they were so big, it probably was more of a problem for them to cool down than to heat up.” Meanwhile, the theropod dinosaurs — the group that contains birds — developed high metabolisms even before some of their members evolved flight.

“Reconstructing the biology and physiology of extinct animals is one of the hardest things to do in paleontology. This new study adds a fundamental piece of the puzzle in understanding the evolution of physiology in deep time and complements previous proxies used to investigate these questions. We can now infer body temperature through isotopes, growth strategies through osteohistology, and metabolic rates through chemical proxies,” says Fabbri.

In addition to giving us insights into what dinosaurs were like, this study also helps us better understand the world around us today. Dinosaurs, with the exception of birds, died out in a mass extinction 65 million years ago when an asteroid struck the Earth. “Having a high metabolic rate has generally been suggested as one of the key advantages when it comes to surviving mass extinctions and successfully radiating afterward,” says Wiemann — some scientists have proposed that birds survived while the non-avian dinosaurs died because of the birds’ increased metabolic capacity. But this study, Wiemann says, helps to show that this isn’t true: many dinosaurs with bird-like, exceptional metabolic capacities went extinct.

“We are living in the sixth mass extinction,” says Wiemann, “so it is important for us to understand how modern and extinct animals physiologically responded to previous climate change and environmental perturbations, so that the past can inform biodiversity conservation in the present and inform our future actions.”

Reference: “Fossil biomolecules reveal an avian metabolism in the ancestral dinosaur” by Jasmina Wiemann, Iris Menéndez, Jason M. Crawford, Matteo Fabbri, Jacques A. Gauthier, Pincelli M. Hull, Mark A. Norell and Derek E. G. Briggs, 25 May 2022, Nature.
DOI: 10.1038/s41586-022-04770-6