Chemical clues reveal dinosaur metabolisms
From: Field Museum
May 25, 2022 -- For
decades, paleontologists have debated whether dinosaurs were warm-blooded, like
modern mammals and birds, or cold-blooded, like modern reptiles. Knowing
whether dinosaurs were warm- or cold-blooded could give us hints about how
active they were and what their everyday lives were like, but the methods to
determine their warm- or cold-bloodedness-- how quickly their metabolisms could
turn oxygen into energy-- were inconclusive. But in a new paper in Nature,
scientists are unveiling a new method for studying dinosaurs’ metabolic rates,
using clues in their bones that indicated how much the individual animals
breathed in their last hour of life.
“This is really
exciting for us as paleontologists-- the question of whether dinosaurs were
warm- or cold-blooded is one of the oldest questions in paleontology, and now
we think we have a consensus, that most dinosaurs were warm-blooded,” says
Jasmina Wiemann, the paper’s lead author and a postdoctoral researcher at the
California Institute of Technology.
“The new proxy
developed by Jasmina Wiemann allows us to directly infer metabolism in extinct
organisms, something that we were only dreaming about just a few years ago. We
also found different metabolic rates characterizing different groups, which was
previously suggested based on other methods, but never directly tested,” says
Matteo Fabbri, a postdoctoral researcher at the Field Museum in Chicago and one
of the study’s authors.
People sometimes talk
about metabolism in terms of how easy it is for someone to stay in shape, but
at its core, “metabolism is how effectively we convert the oxygen that we
breathe into chemical energy that fuels our body,” says Wiemann, who is
affiliated with Yale University and the Natural History Museum of Los Angeles
County.
Animals with a high
metabolic rate are endothermic, or warm-blooded; warm-blooded animals like
birds and mammals take in lots of oxygen and have to burn a lot of calories in
order to maintain their body temperature and stay active. Cold-blooded, or
ectothermic, animals like reptiles breathe less and eat less. Their lifestyle
is less energetically expensive than a hot-blooded animal’s, but it comes at a
price: cold-blooded animals are reliant on the outside world to keep their
bodies at the right temperature to function (like a lizard basking in the sun),
and they tend to be less active than warm-blooded creatures.
With birds being
warm-blooded and reptiles being cold-blooded, dinosaurs were caught in the
middle of a debate. Birds are the only dinosaurs that survived the mass
extinction at the end of the 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 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 afterwards,” 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.”
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