In some ways, mammals aren't the most advanced lifeforms on the planet, no matter how much we'd like to think we are. Sure, we got that whole endothermy thing down, and we've managed to colonize every continent on the planet (and are making tentative plans to colonize other planets), but face it — we're only barely number one, and many might argue that we're not on top at all, despite our dubious achievements in the technical, philosophic and artistic spheres.

Our endothermic metabolisms are great when there's plenty of food around, because it allows us to expand into environments that may otherwise be inhospitable to ectotherms. However, when food is scarce, starvation hangs over our furry little heads like a spectre, taunting us while the cockroaches and lizards grow fat and contented on our gaunt and withered corpses. Endothermy is great, but it's only one adaptive mechanism out of countless many, and when the environment gets harsh enough, even endotherms perish (while ectotherms may survive, or even thrive, in the same environments).

One thing in particular we mammals ought to feel cheated from is our cardiopulmonary system. This is one instance where birds have it much, much better than we do. Avians are about the only critters on this Earth who can give us mammals a metabolic run for our money. The act of flight (which, of course, is not the sole province of avians) is extremely energy-intensive, and requires a whole slew of physiological adaptations to support it, including:

  • an enlarged, keel-shaped sternum to anchor the pectoral muscles, in order to allow wingbeats powerful enough to attain flight. Our puny simian sternums are flat, our pectoral muscles pathetically weak and our machine-assisted powers of flight are but a barbaric parody of that of birds.

  • an increase in the size and density of certain brain regions responsible for the complex ballet of motions required for controlled flight. Watch a slow-motion video of bird flight, and you'll find out soon enough that it's a whole lot more than simply flapping your wings up and down. Upstroke and downstroke require their own different motions, and to keep the center of gravity in line with the center of lift, the entire body needs to shift somewhat at each phase of the wingbeat. Hardly simple, but birds manage it just fine, while we hairless apes shuffle about on our grossly-oversized hindquarters, angling our heads skywards with a sad longing to join our distant avian cousins up there amongst the clouds.

  • an increase in reliance on eyesight, which spurs a further increase in brain size to process the data gathered. Compare the size of avian eyes and mammalian eyes; relative to the rest of their skulls, birds have enormous eyes, often exceeding the size of their already-enlarged braincases. Birds in general have amazing eyesight, which is helpful, since that's about the only sense that'll be of use in the fast and furious world of flight (though bats and certain birds get along fine with echolocation). And to think, some mammals are still largely dependent on scent to obtain information about their surroundings! How barbaric in the extreme!

  • hollowed bones and a loss of pretty much anything extraneous, in order to reduce weight. Birds lost their heavy mineral-based teeth very early on and replaced them with lightweight keratinous beaks. Most of the bones in a bird's body are hollowed-out and braced with a latticework of honeycombed struts and beams, making them very light and strong, unlike our brutishly dense mammalian bones.

  • and finally — a more robust cardiopulmonary system to support not only the increased brain size and muscle mass requisite for flight, but also the very heavily aerobic act of flight itself. Bryan Allen, the pilot of the Gossamer Albatross, who flew the 22 miles across the English Channel in two hours and forty-nine minutes on a pedal-powered airplane, averaged about 400 watts, which is near the top end of what a human being is capable of. Your humble author is only capable of sustaining about 200 watts in the gym on a rowing machine (maybe 275 or 300 for short anaerobic bursts), and he considers himself to be a very fit Homo sapiens indeed. Also, it is worth noting that a typical bird could have made the same crossing in less than an hour.

The avian cardiopulmonary system is the envy of the animal kingdom. While the heart and lungs of a mammal may be compared to the V-6 under the hood of your buddy's Camaro, the circulatory and respiratory systems of a bird may be compared to the V-12 under the hood of that Lamborghini that cut you off on the Interstate. At least we mammals aren't as poorly off as the reptiles, whose cardiopulmonary systems are more like the pathetic 3-cylinder engine of a Geo Metro.

rootbeer277 notes that this analogy can be extended to one of efficiency in addition to brute power: that Lamborghini may be able to top 200 miles per hour on a straightaway, but you'll run out of gas quickly. Your Camaro might only be able to reach 130 or so, but you'll probably be able to drive a couple of hundred miles before you need to refill it. But a Geo Metro gets a jaw-dropping 40-50 miles per gallon (making up for its dismal engine power), and by analogy, reptile metabolisms are such that a typical reptile only needs to feed every few days, weeks or even months, while birds need to eat almost constantly to maintain life processes.

Basically, avian lungs allow them to take in air during both inhalation and exhalation. In short, they breathe in a loop, and their lungs maintain a constant volume with fresh air coming in ceaselessly, while mammals' lungs are filled about half the time with a stale and spent miasma. The constant volume of the avian lung also allows them to maintain a consistent partial pressure equal to that of the outside environment, allowing for more efficient gas exchange.

In addition to the lungs, birds have a set of air sacs that aren't directly involved in gas exchange, but serve as bellows to keep fresh air coming into the lungs during exhalation (they also serve to further reduce body weight). Typically, a bird will have about nine of these, though the number may vary. During inhalation, about half of the fresh air inhaled is shunted to these sacs, while the other half is shunted directly to the lungs. During exhalation, the lungs are emptied of the stale air by the contraction of these air sacs, forcing in fresh air in their place in the very same step. The cycle then begins again. Because of this, a bird's lungs are afforded a constant supply of fresh, oxygen-rich air. The effect of all these adaptations is somewhat like installing a turbocharger on the aforementioned V-12 Lamborghini, forcing oxygen very efficiently into the bloodstream and gracing avians with the ability to sustain the very energetic act of flying.

So, basically, if you're into Intelligent Design, go and ask your god/gods why you've been cheated so. If you're not of that persuasion, then ponder why natural selection so cruelly and callously withheld such an awesome set of hearts and lungs from us, leaving all but the bats earthbound.

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