Be thankful for your maxilloturbinals. The only time most of us are even dimly aware of these structures--thin curls of bone deep in the nasal cavity--is when they fail us. Ordinarily their mucus-coated surfaces filter out dust and bacteria, but when a cold virus attacks, the mucus coating swells up and clogs the nose. Yet according to Willem Hillenius, a physiologist at UCLA who is also a paleontologist, maxilloturbinals perform an even more fundamental task than acting as a filter: they permit mammals like us to be warm-blooded.
Researchers first recognized the value of turbinals three decades ago in kangaroo rats. Like other animals, kangaroo rats have to keep their lungs moist so that the oxygen they breathe can dissolve into their bloodstream. Continually exhaling this humid breath into the bone-dry air of the deserts they inhabit, kangaroo rats ought to become quickly dehydrated. Yet even though they don’t drink any water, they thrive--by trapping water before it leaves the body.
In 1961 biologist Knut Schmidt-Nielsen of Duke University discovered that a rat’s maxilloturbinals are the trap. As the animal exhales warm, moist air from its lungs, the moisture condenses onto the cool surfaces of the turbinals. The next breath of cool, dry air from the outside world cools the turbinals again and also dries them, carrying water vapor back to the lungs.
When biologists discovered that other desert mammals, such as camels, also conserve water in this way, they concluded that maxilloturbinals had evolved specifically as an adaptation to life in dry places. But there is a flaw in that hypothesis, says Hillenius: almost all mammals have intricate maxilloturbinals, whether they live in deserts or not. On the other hand, no reptiles have them--not even desert reptiles. What that suggests, Hillenius argues, is that turbinals aren’t adaptations to deserts; they’re adaptations to being warm-blooded. Mammals maintain a high body temperature because they burn food faster than reptiles do, which means they must take in more oxygen, which means they must breathe faster. As a result, dehydration is always a danger to them, even outside a desert.
To prove his point, Hillenius did an experiment on five types of mammal--rats, squirrels, ferrets, rabbits, and opossums--that are native to the area around Oregon State University, where he was earning his Ph.D. (Western Oregon, he notes, is about as far from desert conditions as a mammal can get.) First he measured the normal humidity and temperature of the animals’ exhalations. Then he deprived them of the use of their turbinals--by plugging their nostrils and thereby forcing them to breathe through their mouth--and repeated the measurements.
From the way the animals’ breath became more humid, Hillenius calculated that their turbinals ordinarily reclaim as much as 45 percent of the water from the air they exhale. As a result, while these five mammals consume up to 11 times more oxygen than reptiles of similar size, they lose only twice as much water. Kangaroo rats do even better--they recycle 88 percent of their water. But in Hillenius’s view, their turbinals are simply an improved version of an apparatus that first evolved when warm- bloodedness itself did, as a way of preventing dehydration from rapid breathing.
When might that have been? At what point did the reptilian ancestors of mammals start developing a warm-blooded metabolism? That question has kept paleontologists guessing for decades. In pursuit of an answer, they’ve tracked the way mammallike reptiles changed from a sprawling stance to a more upright, mammalian one, or the way the animals evolved teeth that allowed them to grind food more efficiently and thus eat more. But not one of these highly indirect clues to warm-bloodedness has produced a clear-cut description of its origins.
Hillenius thinks turbinals can. So far, he says, they are the first and only structures with a direct correlation to high ventilation rates, which are key to endothermy. The paper-thin turbinals themselves rarely survive fossilization, but the distinctive ridges along which the turbinals attach to the internal walls of the nose often do. Indeed, the fossils of the earliest known mammals, going back 210 million years, had been found to bear these ridges even before Hillenius did his work. Most of the time, though, paleontologists had ignored maxilloturbinals or confused them with other turbinals that both reptiles and mammals use for smelling rather than for recycling water.
Inspired by the success of his experiments with modern mammals, Hillenius decided to do a systematic search for turbinal ridges in the fossil skulls of mammallike reptiles. The oldest one he could find in a museum collection was 300 million years old. It showed no signs of turbinals. But in the 260-million-year-old skull of an animal named Glanosuchus--and in all the mammallike reptiles that followed Glanosuchus-- Hillenius saw distinct turbinal ridges.
His finding suggests that warm-bloodedness originated at least 30 million years earlier than researchers had estimated previously. Yet Glanosuchus, Hillenius believes, was only partially warm-blooded. A stocky six-foot-long carnivore that hunted smaller reptiles on the warm plains of what is now South Africa, it had a metabolism that was higher than a reptile’s but lower than a mammal’s. Like other reptiles, it had an opening in the front of its palate, so that when it inhaled, air stayed only briefly in its nasal cavity before passing into the mouth. As a result, Glanosuchus’s turbinals had little time to humidify the incoming air and return water to the lungs. Only over the course of tens of millions of years, says Hillenius, did the reptilian palate seal shut and the nasal cavity expand into an efficient water trap. Like the evolution of mammals themselves, the evolution of warm-bloodedness was apparently a very gradual process.
That raises the question of why warm-bloodedness evolved at all. The ability to maintain a constant body temperature is a great advantage: it allows animals to search for food even in cold regions or at night. But to stay warm and active in the cold, an animal has to produce a lot of heat--at least five times more than a modern reptile does, for instance. As mammallike reptiles gradually evolved from a reptilian metabolism toward warm-bloodedness, they wouldn’t have accrued the benefit of a constant body temperature for tens of millions of years. There must have been a more incremental benefit along the way.
The incremental benefit, according to Hillenius--and to his former adviser at Oregon State, physiologist John Ruben, who first suggested the idea--was increased stamina. Although a cold-blooded animal would need a much higher metabolism to thermoregulate, even a slightly higher one would give it more energy and stamina. According to Ruben and Hillenius, the ancestors of mammals evolved warm-bloodedness initially as a way of moving around more to find more food. The ability to maintain their body temperature and thus to thrive in cold environments was an incidental luxury that came much later.
Hillenius is now taking his research on turbinals into far more controversial territory: the metabolism of dinosaurs. Some researchers have argued that dinosaurs were as warm-blooded as mammals; others claim the great beasts had a metabolism closer to that of a cold-blooded reptile. Both sides have presented only ambiguous evidence.
In principle, turbinals could settle the matter. If turbinals are direct evidence of warm-bloodedness, as Hillenius claims, then birds--which most paleontologists consider the warm-blooded descendants of dinosaurs-- should use their turbinals to conserve water, as mammals do. Hillenius plans to test that prediction by plugging up some bird nostrils. Then he’ll make CT scans of fossil skulls of ancient birds and dinosaurs to see when turbinal ridges, and presumably endothermy, first appeared. It will take me three years just to get a start on this, he says. But once other people are aware of the importance of these ridges, more of them may start looking as well.