Warming Up


How a Crinkle in Cellular Metabolism Gave Mammals, Birds,

and (Maybe) Dinosaurs a Major Competitive Advantage—at a Cost

By this point in the school year, between a colossal workload and a meager money supply, you have probably begun wondering exactly how often you have to eat. It is a good question: eating is a hassle. It drains time and money. Unfortunately, not eating, even for a seemingly reasonable stretch of time, takes a heavy toll on just about everything.

    Taking this into account, it would appear that an evolutionary adaptation is in order. At the microscopic level, energy is gleamed from food during cellular respiration, the bogeyman of high school life science. This process has been around since time immemorial—and you’ve probably been quizzed on it for about as long. We would have hoped that, in terms of producing energy, it would have evolved to be a little more efficient by now. In actuality, for mammals and birds, it has become markedly less efficient. This is a consequence of an evolutionary trade-off that turned the Mesozoic world on its head and still causes your stomach to rumble today.

The Mesozoic was the crucible for many of the mammals’ and the birds’ most enduring evolutionary characteristics. It would also have been a great time to be a painter, providing that you had a dinosaur-proof cage to sit inside.  (Source)

Lighting a Fire

The Mesozoic world was practically on its head to begin with. A bit of background: Mesozoic means “middle-life” in a language that is almost as extinct as the awesome creatures that lived during this time. Squeezed in the middle of the Proterozoic Eon—which encompasses all the time that complex life has been around—the “Age of Reptiles” was, in a sense, the middle school of the geologic timescale. Mammals and dinosaurs both emerged at the beginning, fresh from the fallout of a mass extinction that obliterated most of their elementary school classmates (paleobiology probably has the toughest exit exams). Dinosaurs became the cool peaked-in-middle-school kids who were @#$%@$& amazing until the era ended and they weren’t able to graduate. Mammals became the wimpy kids who had a pretty awful time for 150 million years but were able to squirm through. They—our own ancestors—got by on stealing dino eggs and guzzling milk, hoping that doing so would make them larger, although not large enough to be noticed by a cool kid and devoured.

    Among infinitely many other reasons, the Mesozoic was interesting because it saw the evolution of warm-blooded species: the mammals and the birds, an offshoot of the dinosaurs that diverged mid-era. Most people are familiar with the concept of warm-blooded animals; we sweat to cool off, shiver to warm up, and actively maintain our body temperatures via automatic responses. This contrasts with cold-blooded animals, like lizards and frogs, which modify their temperature via behavior, usually by scurrying into the sun when it is cold or into the shade when it is hot. You might know that these terms have absolutely nothing to do with the actual temperature of our blood, only with how it is maintained. The more accurate jargon, in that regard, is endothermic and ectothermic. An endotherm generates its own heat from within its body, while an ectotherm is reliant on external heat sources. You’ve probably heard these terms in high school biology, and hopefully you haven’t gotten them confused with what your chemistry professor writes on the board before setting off the fire alarm.

    Endotherms include all mammals and their feathered compadres. Ectotherms, on the other hand, are the fish and amphibians and reptiles that held back. Of course, there was a definite advantage to holding back; as you might have surmised, your endotherm-wired metabolism is what forces you to go about finding food on a nearly-continuous basis. If we were an ectotherm, then UC San Diego’s Dining Dollar program would probably last long enough for your grandchild to buy a Goody’s burrito. The mechanism behind this trade-off is at the root of endothermy itself, and might be mistaken for a glitch in your metabolism.

A simplified diagram of cellular respiration, the principal engine of life on Earth. Oxidative phosphorylation, which takes place inside the mitochondria at the end of the process, provides the bulk of energy.  (Source)

A Subcellular Spark

Let’s talk about your mitochondria, who are about to make this blog all about them just like they managed to make your ninth grade biology class all about them. Your mitochondria, as you’ve been told too many times, are the engines of every one of your cells. Inside them, the Krebs cycle eviscerates sugar to create carbon dioxide. In doing so, it musters electrons that, during oxidative phosphorylation (that’s a big word!), are used to build a proton gradient; in short, your mitochondrial membrane acts as a kind of dam. Protons flow through that dam, turning a motor to produce ATP, the type of chemical-energy-currency that your cells use to do everything that cells do.

    Here’s the problem with you, me, your cat that you forgot to feed, and every other poor, hungry endotherm out there: your dam is filled with holes. Specifically, your proton gradient dissipates much faster than it is supposed to. Uncoupling agents spring leaks through which protons can slip out of the gradient. This does nothing to produce the critical ATP but does release a lot of heat—it is exothermic, as your chemistry professor would be dying to point out. By bleeding protons in this way, your mitochondria also act as the furnaces that warm your body. They are still your cells’ engines, however, and in order to be the engine and furnace simultaneously, your mitochondria need a whole lot of fuel. This is why endotherms must consume up to ten times as much food as ectotherms, the reason that you experience stomach pangs even though it hasn’t been that long since your last meal, and the reason you have to feed your dog every day but your snake only once a week.

Tuning the Temperature

Put this into the perspective of the Mesozoic. Endothermy provides a plethora of advantages to a furry, wimpy mammal; it can stay snug and warm inside its burrow when the climate turns nippy, it can scurry around in the middle of the night without any external energy source, and, with a higher body temperature, it can sustain activity for a much longer time. A 1979 paper argued that this last benefit—namely, the promotion of aerobic activity—was the driving force behind the evolution of endothermy. Ectothermy was better for short anaerobic spurts—think of the lizard that you spooked on the sidewalk—but it was crummy for powering an active daily routine. In short, the reason we are now warm-blooded was because it allowed a rodent that lived a long time ago to jog off with a dinosaur egg at a strong, steady pace. Of course, being warm-blooded meant that rodent needed to steal a lot of dinosaur eggs just to stay alive and to feed its offspring.

A recent study suggests that dinosaurs fell in between the endothermic and ectothermic extremes. So-called “mesotherms” exist today but are uncommon.  (Source)

    Despite how energetically expensive it is, endothermy was profitable enough to be evolved, not once, but at least twice. Warm-bloodedness is an example of convergent evolution, meaning that both mammals and birds—two separate lineages, subjected to similar environments—evolved the same adaptation. With birds, the question is how early did they evolve endothermy, or, put more directly: were dinosaurs also warm-blooded? If science succeeds in cloning an Othnielia in the near future, how often will you have to feed it?

    This has long been something about which paleontologists like to bicker. I’d like to discuss it since, if nothing else, it would have been very relevant to our ancestors that lived in these beasts’ shadows. When dinosaurs were first unearthed in the 1800’s, the answer seemed obvious; something as huge as a Tyrannosaurus would never have been able to eat enough food to sustain both its mass and its internal temperature. Ectothermic it was. Later, however, more fossils were discovered and naturalists became aware that birds were descended from dinosaurs. The revelation that your pet canary is only distantly related to a Velociraptor must have been mildly disconcerting to experts at the time. It would likely have made them more open to the idea that, maybe, these beasts—and in particular their metabolism—had been a bit more bird-like and a little less lizard-like.

    A study published in 2014 finally offered a solution. Biologists from the University of New Mexico came up with a formula that linked body mass to growth rates. Both could be determined from looking at fossilized bones. The results prompted the researchers to term the dinosaurs mesotherms, meaning that they had intermediate metabolisms. This isn’t so far-fetched; a few modern-day species—like great white sharks, leatherback sea turtles, and sociology professors—fall into a similar category. A mesothermic metabolism would have cut the larger dinosaurs some slack; they wouldn’t have needed nearly as much food as, say, a school-bus-sized housecat. Of course, there are still scientists who object to this model; one paleontologist in particular challenges the methodology of the 2014 study and claims that dinosaurs were as hot-blooded as you and me. But it is an intriguing solution to the question of how these creatures lived and what the world back then was like.

Mounting evidence that birds are descended from dinosaurs flames the debate as to whether the famous reptiles were warm-blooded. We still haven’t arrived at an undisputed answer.

Putting it Together

Mesotherms or not, dinosaurs were the undisputed monarchs of the Mesozoic. However, once the end of the Mesozoic hit—and it literally hit, off the coast of Mexico and with the force of 100 million megatons of TNT—that mantle soon passed to the absolute endotherms: birds and mammals. Endothermy allowed these species to prowl their territory and forage for food around the clock. In terms of macroevolution, it opened up numerous niches, which in turn led to the whopping diversity of mammals and birds that we know today. A small price to pay, possibly, for global predominance—but you can decide that for yourself the next time your stomach begins to rumble in biochemistry class.

In summary for this week:

  1. Endotherms first evolved during the Mesozoic Era. They were able to sustain their internal body temperatures but had to eat much more food than ectotherms did. The advantage, paleontologists think, was that endotherms could sustain aerobic activity for much longer than ectotherms could.
  2. The basis of endothermy is that your mitochondria leak protons during cellular respiration. This generates a lot of heat at the expense of churning out less energy. The end result is that you stay warm and can be active, but need to compensate for that energy loss by consuming a lot more fuel.
  3. Mammals and birds evolved endothermy separately, but scientists aren’t sure about dinosaurs. A 2014 study made the suggestion that dinosaur metabolism was intermediate between endothermic and ectothermic. The case isn’t closed, however, regarding the metabolism of the Mesozoic’s signature clade.

Some sources where you can read more about this:

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