Cheetahs on the Run

Cheetahs have been making the rounds of science news lately. Famous for their speed, they have in the past been clocked at 29 meters per second (nearly 65 miles per hour), making them the fastest animals on land – but that’s old news. A new study by A.M. Wilson and colleagues delved deep into the mechanics of the hunt with five wild cheetahs in Botswana, and their findings were a little surprising.

The cheetah still holds onto its place as the fastest land animal – the researchers recorded an impressive top speed of 25.9 m/s, or about 58 mph – but it turns out that the cheetah’s dexterity and maneuverability, not its phenomenal speed, hold the secret to its hunting success.

Wilson, et al. outfitted five wild cheetahs with GPS tracking collars that also contained accelerometers, to capture and transmit data on the cheetahs’ movements. The researchers used the data to reconstruct the speed, acceleration, and maneuvering in each of 367 runs over the course of 17 months; they also tracked the terrain of each hunting attempt by overlaying the GPS data onto Google Earth.

When most people think of a running cheetah, they probably envision a high-speed chase through open grasslands. In reality, about half of the runs recorded in this experiment took place among shrubs or dense vegetation, and cheetahs hunted about as successfully in these environments as they did on open ground. Although each of the five cheetahs reached speeds of 20 m/s (about 45 mph) or higher at least once during the 17 month study, during most hunts they only got up to a leisurely 14.9 m/s (about 33 mph) – although that would still leave Olympic sprinters in the dust.

According to this study, a cheetah on the chase only runs at top speed for a second or two; once it pulls near its quarry, the cheetah decelerates, and this is where the chase reaches its critical moments. At a slower speed, the cheetah can employ its maneuvering prowess, using large claws, high-traction footpads, and a low posture to grip the ground and make sharp, fast turns, in order to subdue its agile prey. The deceleration phase of the chase is the most important – paradoxically, the researchers found that greater deceleration was correlated with greater likelihood of catching the prey in the end. Maneuverability is more important than speed in the final moments of a chase.

Cheetahs are undeniably powerful runners. The fibers in the running muscles of wild cheetahs shorten faster than fibers in similar-sized muscles in other running animals – the researchers mention racing greyhounds and horses, for comparison – giving cheetahs extraordinary power of acceleration. But in the end, the key to the cheetah’s chase isn’t in sheer power. It’s all in the control.

How Do Bird Lungs Work, Anyway?

Here’s a little more detail on how bird lungs work, compared to human lungs:

As I mentioned in the last post, avian “flow-through” lungs are much more efficient than human lungs. We humans pull in air and push it out of our lungs with every inhale and exhale, and in normal breathing, we only use a portion of our total lung capacity. During gas exchange, the fresh, oxygenated air we breathe in mixes with “old” air that’s been in the lungs for a while. Oxygen makes its way down to the alveoli (the little pockets where gas exchange between actually occurs); here, oxygen from the lungs diffuses out into the bloodstream, and carbon dioxide moves out into the lungs. Finally, carbon dioxide and a good deal of unused oxygen are expelled as we exhale.

This system works well enough, but it’s terribly inefficient. Birds, however, have evolved a much better system, illustrated below:

Bird Lungs Part 1

Bird Lungs Part 2

Step 1. Inhalation: Fresh air is inhaled into the first chamber

Step 2. Exhalation: Air is pushed across the lungs, where oxygen transfers over into the bloodstream (and carbon dioxide transfers into the lungs)

Bird Lungs Part 3

Bird Lungs Part 4

Step 3. Inhalation: Air, now deoxygenated, is pushed into the second chamber. Meanwhile, a new batch of fresh air is inhaled into the first chamber, as in Step 1.

Step 4. Exhalation: Deoxygenated air is exhaled out again. Meanwhile, the new batch of fresh air is pushed across the lungs, as in Step 2.

Bird Lungs Inhale

Bird Lungs Exhale

Step 5: The Cycle Continues…

Each “breath” of air actually remains in the bird’s lungs for two cycles of inhalation and exhalation, and is then fully expelled at the end. Fresh air flows across the lungs with every breath, and “old” air never mixes with new air (as it does in human lungs). Respiration is much more efficient, which allows birds to perform strenuous activities like flying that would be much more difficult with our paltry human lungs.

5 Reasons Birds Are Better Than Humans

I’ll admit, that last post was pretty heavy, so here’s a light little post on 5 ways that birds are more impressive than we humans:

#1: They are living dinosaurs, in the most literal sense.

#2: Their lungs are way, way better than ours. Human lungs are essentially sacks that we partially fill and partially empty with fresh air with every inhalation and exhalation, which isn’t very efficient for gas exchange. Birds, on the other hand, have a “flow-through” lung structure, which creates a constant flow of fresh air across the lungs, and prevents fresh air from mixing with older, deoxygenated air (which is what happens in human lungs). This allows for a much more efficient transfer of oxygen and carbon dioxide between the air and the bloodstream.

#3: Magnetoreception. Birds can detect magnetic fields and frequently use them for navigation. Many birds probably detect magnetic fields using small, magnetite-containing, compass-like structures inside their heads. Not only do birds detect the earth’s magnetic field (useful for migration), but they can even detect local magnetic fields, which allows them to accurately navigate an area even without visual cues about their location.

#4: Most of them can fly. Some can run. Some can swim underwater. Some can hover in midair. Many can sing (and some can do much more). A few can even dance. The point being, they would definitely come out on top in a talent competition, hands down.

#5: They have super-excellent vision. Let me count the ways:

  • They can see in ultraviolet! Some birds have feather patterning in ultraviolet-colored pigments that are invisible to the human eye (and putting UV-blocking sunscreen on their feathers can mask these pigments to the birds).
  • Some birds have two foveae (centers of vision) in each eye. We humans only have one per eye.
  • The American Woodcock has a visual field of 360 degrees.
  • Bird motion perception is so sharp that they could potentially navigate by the movement of the stars. No, not the placement of the stars – they can see the stars moving across the night sky as the earth rotates, where the human eye would only see motionless points of light.

The inescapable conclusion? We humans may think we’re special, but birds are truly amazing.



(As a side note, bird hearts are also pretty impressive. They are generally larger than mammal hearts relative to body size, and have more muscle fibers, with more mitochondria per muscle. As a result, avian hearts have more power in pumping blood. This didn’t make the list, however, because it comes with a major downside: such powerful hearts can lead to very high blood pressure.)

Reference:

Gill, Frank B. Ornithology: Third Edition. New York: W. H. Freeman and Company, 2007. Pp. 31-36, 146-150, 184-191.

How is a Fish Gill Like an HVAC System?

Countercurrent exchange is one of my favorite physiology tidbits. Creatures across the animal kingdom use countercurrent exchange to increase the efficiency of functions ranging from a fish collecting oxygen in its gills to a wolf conserving valuable body heat in cold weather. We humans also employ countercurrent exchange in architecture and engineering – it’s one way to make a building’s heating and air conditioning systems more efficient.

So, what is countercurrent exchange? Imagine that you have any two fluids (either liquid or gas) flowing through two adjacent conduits, and you want to transfer either molecules or heat energy from one fluid to the other as they flow alongside each other. Flow in the same direction is called concurrent exchange; flow in opposite directions is called countercurrent exchange. For reasons that I’ll explain below, the transfer of molecules or heat is much more efficient if the fluids are flowing in opposite directions than in the same direction.

Concurrent Exchange Demo Countercurrent Exchange Demo

But how does it work? Let’s start with a few basic concepts:

First of all, molecules and energy like to spread out, and will tend to move from a more crowded area to a less crowded area. A drop of bright red food coloring will spread out to color a whole glass of water pink; an ice cube dropped in a hot cup of tea will melt and make the whole cup of tea cooler (and weaker); and a dollop of spaghetti sauce on your plate will somehow find a way to stain the tablecloth, the placemat, and the white shirt you’re wearing (or maybe that’s just me.)
molecules
Why does this happen? Keep in mind that molecular movement is very haphazard, and molecules crash into and bounce off each other pretty much at random. Now, say you have two “rooms” with a door connecting them, like in the picture on the right; one room contains a lot of high-energy molecules and the other just a few. In the crowded room, molecules are bouncing around a lot and will likely shove a number of molecules over into the emptier room. Meanwhile, very few of the molecules from the empty room are likely to get pushed back into the already-crowded room. This process continues until eventually, both rooms contain approximately the same number of molecules, although they are still passing from one room to the other. At this point, the two rooms have reached a state of “dynamic equilibrium”.

The tendency of molecules to move from more crowded to less crowded is a useful property when transferring molecules (or energy) from one substance to another. Imagine a fish moving oxygen from oxygen-rich surrounding water to the oxygen-starved blood in its gills: the oxygen already has a tendency to move from the water to the blood, as long as it can pass through whatever barrier lies in between, such as the permeable walls of blood capillaries in a fish’s gills.

The more efficient the transfer of oxygen from water to gills, the better for the fish. Generally speaking, organisms that spend less energy to do the same things will out-compete their less efficient rivals. They will be more likely to survive, produce more offspring, and be overall more successful; this leads to huge evolutionary pressure in favor of efficiency. Thus, the high-efficiency benefits of countercurrent exchange have led to its widespread adoption in biological systems.

So how exactly do concurrent and counter-current systems work? Let’s look at them in the model of a fish gill:

On a basic level, fish gills contain a lot of small, thin blood vessels. Deoxygenated blood is pumped in after circulation through the body to be re-oxygenated by the surrounding water that gets pushed through the gills. The freshly oxygenated blood then continues on to bring oxygen to the rest of the body.

Cocurrent NumbersIn a concurrent exchange setup, the blood vessels are oriented in the same direction as the flow of the water through the gills. This is not a very efficient setup – only half of the concentration of oxygen at most can transfer from the water to the blood. For simplicity’s sake, let’s stay that the fish starts out with an adjacent stream of 100% oxygen-saturated water flowing through the gills containing 0% oxygen-saturated blood, and that both water and blood can hold equal amounts of oxygen. Oxygen will tend to flow from the 100% oxygen-saturated water into the deoxygenated blood when they first come into contact with each other. Further down the line, as the oxygen continues to transfer, you might get 90% saturated water, 10% saturated blood; then 70% saturated water, 30% saturated blood. So far, so good – oxygen is still flowing from water to blood. The transfer of oxygen continues until you finally have 50% oxygen-saturated water and 50% saturated blood – and now, no more oxygen can transfer from water to blood. True, the fish has gained oxygen in its bloodstream, but half of the oxygen remains in the water flowing out of the gills again. All that oxygen, wasted! If only there were a way for gills to harvest almost all of the oxygen from the water passing through…

And that’s where countercurrent exchange comes in to play. Simply by switching the direction of blood flow so that the water and the blood are flowing in opposite directions, efficiency of oxygen transfer increases dramatically. How? Well, may seem counterintuitive, but here’s an image to illustrate the basic difference between countercurrent and concurrent exchange:

Cocurrent DotsCountercurrent Dots

In countercurrent exchange, the water and blood are flowing in opposite directions. At the point where the blood first begins to flow alongside the water (at the right side of the figure above), the water has already traveled the length of the section where water and blood are adjacent. Countercurrent Numbers As you can see in the image, there is a higher concentration of oxygen in the water than in the blood for almost the entire length where they interact, which means that there is a consistent tendency for oxygen to move from the water into the blood. Even at the far right end of the image, when there is hardly any oxygen remaining in the water, this flow from water to blood still continues; although there is very little oxygen remaining in the water, there is none at all in the deoxygenated blood entering the gills.
In short, transfer of oxygen in a countercurrent exchange system can be almost 100% efficient – twice the maximum efficiency of a concurrent exchange system – with a simple reversal of the direction of flow for one of the fluids.

Countercurrent exchange is used for more than just oxygenation of gills. Many endothermic (“warm-blooded”) animals use it to prevent heat from their extremities by conserving heat in their core body.

Imagine a wolf walking in the snow: with body-temperature blood continuously flowing through thin vessels in the wolf’s legs and feet, a lot of body heat will be lost unnecessarily. Fortunately, countercurrent exchange prevents some of this heat loss – in fact, much of the heat carried through the bloodstream will never reach the foot. The artery that carries blood to the foot runs right next to the vein that carries blood back from the foot, allowing countercurrent exchange of heat along the length of the leg. As warm blood travels down the artery, heat continuously leaches over into the cooler blood carried back up by the vein, which is thus warmed up as it travels back to the body. As a result, by the time the blood in the artery reaches the wolf’s foot, it is considerably cooler, and therefore less body heat is lost from the foot in the snow. Interestingly, the foot operates at a lower temperature than the core of the wolf’s body.

So how is a fish gill like an HVAC system? Many air conditioning and heating systems use the same principle of countercurrent exchange when ventilating buildings with outside air. The air intake and output pipes run adjacent to each other, in opposite directions, so as outside air is brought into the building, it is made warmer or cooler by capturing heat from, or dumping heat into, the building-temperature air flowing out the ventilation system. Countercurrent exchange can save us energy, too!

Countercurrent exchange is a simple and elegant solution for a variety of problems in nature and engineering. What other applications can you think of?

Recent Research: Biodiversity

These days, biodiversity loss is one of the primary hot-button issues in conservation, and loss of biodiversity has been an issue of concern for quite some time. So, today I’m going to talk about it a little bit, in light of a recent research article published in Ecology Letters.

Biodiversity is important. It’s a major indicator of the health of an ecosystem, and it’s also an important contributor to the ecosystem’s well-being. Ecosystems with low biodiversity – either inhabited by only a small number of species, or dominated by a few species, with other species being relegated to marginal roles – tend to be more susceptible to other environmental threats, like pollution and temperature changes. Ecosystems that lose biodiversity become less stable and less resilient against further damage.

When trying to understand the current state of biodiversity and gauge the effectiveness of current conservation efforts, it’s important to know how things have been in the past, and what changes have taken place over the long term. To that end, researchers Carvalheiro, et al. conducted a broad analysis of insect and plant biodiversity, using data collected over the course of six decades.

Carvalheiro, et al. looked at data on the biodiversity of plants and flower-visiting insects (bees, hoverflies, and butterflies) in Great Britain, the Netherlands, and Belgium, from 1950 to 2009. The study examined biodiversity over a large spectrum of spatial ranges, from 10 x 10 kilometer chunks at the smallest to entire countries at the large end of the scale. This allowed the researchers to compare patterns of change at local, regional, and nation-wide levels.

Great Britain, the Netherlands, and Belgium have instituted fairly rigorous conservation efforts in the past few decades, and the findings in this study reflect positively on those efforts. They indicate that biodiversity fell most precipitously from about 1950 to 1990, coinciding with a period of rapid habitat loss. However, since about 1990, an upturn in public awareness and the institution of good conservation measures slowed the loss of biodiversity in these countries. That’s not to say that biodiversity isn’t still declining – it is – but this analysis tells us that careful conservation efforts can be successful in slowing the loss of biodiversity.

Results aside, the methods used for analysis in this study were pretty interesting in and of themselves. The researchers didn’t go out and collect data themselves. Instead, they collated a huge tangle of existing data – gathered with a variety of different methods, sample sizes, and areas of interest – and performed a variety of statistical tests to decipher patterns of change over 20-year chunks of time 1950 to the present. Despite the challenges of working with non-standardized data, the researchers were able to glean some meaningful and highly robust patterns. Nevertheless, they do note that consistent data collection over time is important for long-term ecological study.

The researchers conclude on a note of hope. Biodiversity loss has become less severe in response to well-managed conservation efforts in Great Britain, the Netherlands, and Belgium. Conservation efforts are valuable, worthwhile endeavors; instituting them in other countries, and continuing them where they already exist, could further help counteract today’s rapid biodiversity loss.

Carvalheiro, Luisa Gigante, William E. Kunin, Petr Keil, Jesus Aguirre-Gutiérrez, William Nicolaas Ellis, Richard Fox, Quentin Groom, Stephan Hennekens, Wouter Van Landuyt, Dirk Maes, Frank Van de Meutter, Denis Michez, Pierre Rasmont, Baudewijn Ode, Simon Geoffrey Potts, Menno Reemer, Stuart Paul Masson Roberts, Joop Shaminée, Michiel F. WallisDeVries, Jacobus Christiaan Biesmeijer. “Species richness declines and biotic homogenization have slowed down for NW-European pollinators and plants.” Ecology Letters. Published online May 21, 2013. Doi: 10.1111/ele.12121

Book Look: Hallucinations by Oliver Sacks

Last year, Oliver Sacks, prolific writer of weird tales about the brain and perception, brought us a new book on hallucinations. I’ve only read one of Sacks’ books before, The Man Who Mistook His Wife for a Hat – a sampling platter of strange brain-related case studies– so when I picked up Hallucinations, I suppose I was expecting something similar. However, if The Man Who Mistook His Wife for a Hat is like a collection of neurological short stories, then Hallucinations is a positive encyclopedia (albeit a short one, with plenty of personal touches). In his introduction, Sacks calls his book a “natural history or anthology of hallucinations,” which pretty much sums it up.

Sacks takes his readers through a museum’s worth of hallucinatory experiences and personal descriptions written by people who experienced them, including one memorable chapter focusing on his own extensive dabbling in hallucinogenic drugs. The book is an interesting blend of the scientific and the personal, with a thread of social science running throughout. Underlying the descriptions of hallucinations and misperceptions – and the sheer number and variety of hallucinations catalogued in this book were enough to hold my interest – there is a somewhat surprising message. Sacks demonstrates that hallucinations and misperceptions, far from being rare and exceptional harbingers of madness, are actually quite common quirks of the brain. Take Charles Bonnet Syndrome, for example – as Sacks describes in the opening chapter, simple visual or auditory hallucinations are very widespread among completely sane individuals who have undergone loss of vision or hearing, as if the brain is compensating for a lack of sensory input by creating its own original material.

There is another, closely related theme threading through the book, uniting the often-meandering narrative. Hallucinations, Sacks argues, may play a much more significant role in human cultures and mythology than we realize. Hallucinogenic substances hold an important place in many cultures, and hallucinations are not as rare as most people think. Could stories of monsters, elves, aliens, and mystical beings originate from the actual experiences of people who didn’t realize it was “all in their head?” I was intrigued by the hints at this possibility, and I would have liked for Sacks to explore the cultural value of hallucinations – so heavily stigmatized in modern America – in more detail.

What did I think of the book overall? I liked it, though naturally, as a science lover, I would have enjoyed more detail into the neurological hows and whys behind the intriguing hallucinations he describes. The book suffers from some repetition (Sacks does touch on the subject of déjà vu a couple of times…) and the narrative ambles along without a strong directional flow. This is not necessarily a bad thing, however; I think the purpose of the book is to take the reader on a journey into the bizarre world of hallucinations in all their variety and strangeness, and that Mr. Sacks accomplishes very well indeed.

I would recommend Oliver Sacks’ books to anyone interested in an engaging glimpse into how the mind works – and what strange things can happen when brain function goes awry.

Sacks, Oliver. Hallucinations. New York: Alfred A. Knopf, 2012.

What’s That Science? Eurypterids

Orchids? You can buy some of those at the nearest Home Depot. Squids? Most people have probably at least heard of calamari. But what on Earth is a eurypterid?

Well, it isn’t an organism you will find living on Earth any more. Eurypterids, also known as “sea scorpions,” were common underwater predators from about 470 to about 250 million years ago. Long before the rise of the dinosaurs, these chelicerates – relatives of modern-day spiders, scorpions, and horseshoe crabs – lived abundantly in a variety of aquatic habitats, preying upon fish, other arthropods, and each other. Some larger species could reach almost two meters in length – that’s longer than the average American is tall!

Eurypterids were diverse organisms. Some swam, some walked, some crawled and burrowed, and some did all of these things. This variety in lifestyle is clearly visible in the diversity of body plans eurypterids display in the fossil record; some have paddle-like limbs useful for swimming, while others had appendages better suited for walking or gathering food.

Sea scorpions seem to have lived in a wide variety of habitats, ranging from solidly marine ocean floor to freshwater lakes and rivers to brackish estuaries and bays (where freshwater meets and mixes with saltwater). Alas, eurypterids reached their peak during the Silurian Period, around 440 – 420 million years ago, when plants were still in the early stages of moving onto land. If you want to see a eurypterid these days, you’ll have to look into the fossil record. Eurypterids are most commonly found in North American and European fossil assemblages; the rest of the world has a fairly sparse record of these creatures.

Why should you care about eurypterids? Well, who wouldn’t care about a two-meter-long sea scorpion that could have been swimming around a lake or river near you – if you had lived 250 million years ago, that is!

References:

Benton, Michael J., and David A. T. Harper. Introduction to Paleobiology and the Fossil Record. Chichester, West Sussex: Wiley-Blackwell, 2009. Pp 375-8; 482-6; 550. Provided information on eurypterids, and on the colonization of land by plants.

McDowell, M. A., C. D. Fryar, C. L. Ogden, K. M. Flegal. “Anthropometric reference data for children and adults: United States, 2003-2006.” National Health Statistics Reports: No 10. Hyattsville, MD: National Center for Health Statistics (2008). http://www.cdc.gov/nchs/data/nhsr/nhsr010.pdf – see pages 14 and 16. Provided height statistics of men and women in the United States.

Selden, P. A., “Autecology of Silurian Eurypterids.” Special Papers in Palaeoecology Issue 32, 39-54 (1984). Provided information on ecology of eurypterids. http://www.paulselden.net/uploads/7/5/3/2/7532217/autecology.pdf

Tetlie, O. Erik. “Distribution and dispersal history of Eurypterida (Chelicerata).” Palaeogeography, Palaeoclimatology, Palaeoecology Vol. 252, Issue 3-4: pp. 557-574 (2007). Provided information on distribution of eurypterid fossils. http://dx.doi.org/10.1016/j.palaeo.2007.05.011