A few months ago I applied for, was offered, and quickly accepted an internship position within the Smithsonian Conservation Biology Institute. Thrilled by the prospect of working as part of the research program at the National Zoo, I relocated myself to Washington, D.C. one month later. This not only meant moving myself and a pair of obnoxiously large suitcases to a new city, but also suddenly living in one house with a handful of new people I’d never met; my own much more sane, and admittedly less glamorous, version of “The Real World”. Situations like this, where we are presented with change we must adjust to, are ones we are all familiar with on some level. It’s not always comfortable and usually not instant, but we find ways to individually settle into the new swing of things. It may be tempting to say we “adapt” to all of this, but that would be incorrect. One person, one animal, or one organism doesn’t adapt. Adaptation occurs over generations as evolution proceeds and genetic changes develop and those that are beneficial flourish. But one person, one animal, or one organism can acclimate; a gradual adjustment to a change in the surrounding environment.
Species tend to be fairly well adapted to the environment they normally live in. I’m saying this casually, but this is in part how evolution operates. Adaptation is not something that can happen to one individual; rather it’s the genetic shift of a population occurring over time as a response to natural selection, leading to changes in the way details of, and whole, organisms work. Evolution doesn’t yield just one solution and it doesn't create something new from nothing; it plays with what is already there. Think Darwin's famous finches. We can learn a lot by broadening our understanding of the numerous ways that species have adapted their biology to the challenges that exist.
A month or so ago, one of my five wonderful housemates got on a plane to Ecuador where he went to climb (some say) the world’s largest active volcano, named Cotopaxi. This peak in the Andes Mountain range has a summit recorded to be at 19,347 feet above sea level (about 5,900 meters). From the photographs shared it looks like a beautiful, glacial monster. A handful of us sat around the dinner table the night he returned to listen to his story, caught up in the crazy mental demands of a challenge so extreme. I started to think about biology; reminded that high altitude can do a lot of weird things to an animal’s physiology (our own included), but also provides a strong selective pressure. So while people who normally sip their coffee a few hundred feet above sea level feel the wicked effects of mountain sickness, many organisms sit cozy at altitude all the time. How? They can do things we can’t!
Before we dive in, I just want to make a brief note that this post is just grazing the very tippy-top of the iceberg in terms of high altitude physiology. There is so much more cool biology and are so many cool species that can teach us things, but for the purpose of this blog I’m sticking to just a couple. Here we go!
Some of the problems at altitude...
Low Pressure: As altitude increases, the air gets “thinner”. This translates to a decrease in pressure (hypobaric conditions, e.g. hypo = under) while the amount of oxygen in the air stays the same. What’s the result? A lower concentration of oxygen. Keep reading.
Hypoxia: Oxygen is one of the major things to keeping our bodies alive and thriving and we are adapted to exist in an environment rich with it. If someone asked you why your body actually needs those itty-bitty oxygen molecules could you answer it? This subject should be a blog post in itself (sounds like an excellent Molecule of the Month to tackle Sabah!) so I’ll keep it short here. Think of the “little people”! You need oxygen (O2) to stay alive because you’re made of cells and each cell in your body needs oxygen to survive. Oxygen is a must-have for many animals’ biochemistry to work, and it allows the body to make a ton of ATP through a process called oxidative phosphorylation. What’s ATP? Energy! Think of it like the fuel that powers all sorts of critical processes in each and every one of our tiny cells, ultimately keeping our physiology (and that’s the “big picture”) in shape.
The ability of the body to absorb all sorts of things is often dependent on the concentration of that particular thing both in, and outside, the body. A lower concentration of O2 in the environment ultimately leads to less O2 circulating in the blood. This condition is referred to as hypoxia, which quite literally means low oxygen. Normally after oxygen is taken in by the body it travels around on hemoglobin, a protein in red blood cells, through the bloodstream and gets dropped off at all the different tissues in the body that need it. But hemoglobin doesn’t grab as much oxygen when the concentration in the air is low, so the parts of the body that need it may suffer. Brain included. Bad news, I know. Additionally, the processes I mentioned earlier that make ATP are consuming less oxygen; at 5,000 meters it’s about 60% the amount at sea level and at the summit of Everest (8,848 meters) only 20%. Taken together, this reduction in energy produced affects the body’s ability to regulate all sorts of important things including the balance of ions in the body: sodium, potassium, and calcium to name a few big ones. Regulation of these ions is critical for signaling in the brain, the contraction of your skeletal muscles, and to keep your heart thumping.
Circulatory Stress: People recognized very early on that strange stuff happens to the heart during high altitude climbs. Animals tend to need the ability to move around and mobility is often essential to an animal’s survival in the wild. Exercise creates a higher demand for oxygen to support muscle contractions, and a higher demand for O2 puts a higher demand on the circulatory system to get oxygenated blood to the that muscle. As a result of continued activity at heights where O2 is limited, heart rate is often seen to increase within the first day an animal begins acclimating to altitude. Great, problem solved right? Well something else may happen simultaneously. Blood thickens. Yes, it actually gets thicker. This happens in some animals because the body tries to deal with the internal oxygen deficiency by making more things that can collect and carry it: red blood cells chock full of hemoglobin. More red blood cells floating around in the original volume of blood increases density. So now your heart is beating faster and cardiac muscle being forced to actually work harder to pump a thicker liquid. Double whammy. The extent of this occurrence is species dependent but if this does happen, the heart can be under quite a bit of strain and cardiac failure is a real risk.
Some of the Answers...
Here I highlight some ways that some species tackle some of the issues.
Let’s start with bar-headed geese, Anser indicus. They may not look like anything special, but these birds soar over the Himalayan Mountains (reaching heights of 9,000 meters), breeding at incredibly high plateaus in Asia. The heights they reach are reason enough to give those birdies some props, but the fact they can also maintain high levels of exercise during these lengthy migration patterns should be more highly praised. Muscle contraction (heart-beat included) puts a heavy demand on the body to provide those valuable ATP energy molecules I mentioned earlier; about 10-20 times more than rest. And remember oxygen is needed to make a ton of it very quickly. Bar-headed geese have acquired adaptations over evolutionary time to be able to maintain high rates of O2 consumption (therefore energy production) in extremely low oxygen environments. This means their skeletal muscles (important for flying) and heart (important for…everything) can function normally. They have increased lung mass than similar low-altitude species, suggesting more surface area and therefore increasing ability to absorb oxygen. Their hearts have more capillaries, smallest of the body’s blood vessels, which allow for easier diffusion, or movement, of oxygen from the blood to the muscle cells of the heart. If we delve a bit deeper, these birds are shown to have a unique genetic mutation that’s altered the last step (cytochrome c oxidase) in the ATP-producing process of oxidative phosphorylation. This change, to a gene otherwise neatly conserved across all vertebrates, is proposed to result in a structural adaptation that increases enzyme activity and betters ATP production, but these hypotheses need further research. Check out this sweet story of one researcher, Jessica Meir, and her unique work with these birds here!
But genetic adaptations to high altitude are not a new thing. A study that pre-dates my existence took a look at the genes of the deer mouse, Peromyscus maniculatus, a tiny animal with a huge altitudinal range (sea level to 4300 meters), the broadest, in fact, of any North American mammal! The interest was in the genes for their hemoglobin proteins to which oxygen binds in the blood. They found that certain variations in the genes were favored at high altitude vs. low and that these variations allowed the hemoglobin circulating in the blood to let go of oxygen more easily at the various parts of the body that need it, thus allowing cells there to use it to produce ample energy. Current technology now enables scientists to explore a huge number of genes and their expression patterns all at once, which may give a better idea of what’s happening overall in a functional sense. A 2012 research letter in Nature summarized findings that yaks, Bos grunniens, a significant animal to Tibetan life and culture, have specialized expression of genes related to hypoxia and energy metabolism. Closely related “lowland” cattle don’t show these types of patterns.
Sometimes animals living in totally different parts of the world can end up with adaptations that are very similar to one another as a result of living in an environment that provided similar selective pressure (this is called convergent evolution). So many of the above methods to dealing with high altitude stress are used by a lot of different species. While the selective pressures involved in evolution don’t operate randomly, the mutations in genes that arise over time in individual organisms do. A Google Scholar search of high altitude physiology will highlight that there are many, many…seriously many, other ways to getting around the same biological challenges as a result of this. If you’re interested in a looking at human populations adapted to high altitude check out this easy-to-read summary of work by Emilia Huerta-Sánchez here!
I’ve found that science sometimes has a way of beautifully weaseling its way into scenarios you may not always expect it to. At times, all it takes is a good conversation or a story of an adventure to spark a little curiosity. Keep your mind as open as your eyes because I promise, cross my oxygen-craving heart, the questions are everywhere.