Archive for the 'Physiology' category

>Dinosaurs and the Mystery of Body Temperature III: Intertial Homeothermy

Jan 28 2007 Published by under Animals, Evolution, Links, Paleontology, Physiology

>Body size is an important factor in the debate over whether dinosaurs were cold or warm blooded (or something in between). When you have a land animal 42 feet long weighing nearly as much as a blue whale, temperature models tend to break down. If the dinosaurs were ectotherms, relying on the environment for heat, they may lack the surface area to sufficiently heat the blood pumping directly beneath the skin. If dinosaurs are endotherms, and internally heated by its own metabolism, it may not have enough surface area to expel excess heat from the depths of its massive body.

The following chart shows this principle a little more clearly.


As you can see, the second two cubes have the exact same volume (body size), but the surface areas are vastly different. Large animals like dinosaurs and blue whales are like the middle cube with the smaller ratio; it becomes difficult to use surface area to heat/cool its insides. Also, the more massive an animal is, the more heat it produces/requires, generally speaking.

The reason blue whales get away with being the most massive animal to ever live (so far) is that temperature exchange with their environment is rapid. The ambient temperature of the ocean is on average much lower than ambient temperatures on land, allowing the whale to circulate heat through the thinner parts of its body and allowing the cold water to carry away the excess. Plus, the whale's 100 tons is spread out along 100 feet of body as well.

You can see how a creature on land weighing as much as the blue whale, compacted into 40 or 50 feet and lacking the might present a particular problem for scientists to figure out, especially in the absence of direct evidence.

But, the creature did exist. We're just now picking up the pieces, so to speak.

And recently, scientists put those pieces to good use. By simulating the ontogenetic development of eight different dinos using data from recent bone analyses, they were able to determine that the internal temperature of dinos depended on size. Smaller dinosaurs maintained a lower body temperature and probably grew at a rate consistent with extant reptiles, while the larger dinos maintained a higher body temperature, like today's birds and mammals.

The largest animal studied, Sauroposeidon proteles, was estimated to have an internal temperature of 48 degrees Celsius (120 degrees Fahrenheit), a few degrees higher than what was thought to be the upper limit of temperature tolerance for animals. Because of this extremity, the authors believe that temperature may have been the ultimate cap on body size.

Ultimately, this study was transposing a state called "inertial homeothermy," which is observed in ectotherms like crocodiles and the Galapagos tortoise that can maintain their internal temperatures by adjusting their internal physiological conditions, much like endotherms. The researchers performed the same tests on crocodiles of similar size (when they could; there are no crocs alive today to compare with the larger dinos):


Perhaps, if time allows in the near future, I'll detail a bit more about all the thermies: poikilo, homeo, hetero, ecto and endo.

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>Dinosaurs and the Mystery of Body Temperature II: The Evolution of Endothermy

Jan 19 2007 Published by under Animals, Birds, Evolution, Links, Paleontology, Physiology

>There's a fairly significant problem with the evolution of endothermy from ectothermy, a paradox that has no satisfactory conclusion as of yet: How could well-insulated animals with high metabolic rates producing lots of heat from within their bodies evolve from animals with low metabolic rates and poor insulation, expertly absorbing heat from the surrounding environment?

If these characteristics evolved independently what purpose would they serve? An ectotherm has no use for insulation like feathers or hair since heat exchange needs to be rapid with its surroundings, just as it has no use for a heat producing high metabolism without the necessary insulation.

Raymond Cowles' experiment showed that by putting little fur coats on lizards wasn't keeping heat in, it was keeping heat out. The lizards couldn't warm up.

That's the paradox, the catch-22. The ticket out, however, is the exaptation: An adaptation of a structure that becomes useful for one biological purpose that originally evolved for another.

Feathers are are thought to be derived from the long scales of ancient archosaurs. These reptiles could lift their scales and expose their skin directly to the source of heat, or orient them so that they could block heat absorption. Just as the marine iguanas of the Galapagos Islands are able to trap a layer of air within their scales, these reptiles are thought to have done the same, retaining more metabolic heat, leading to a more active life.

Another theory concentrates on our reptilian ancestors from the synapsid lineage. The synapsids were steadily becoming more active (illustrated by changes in bone structure), and those morphological changes could have been accompanied by higher metabolic rates, leading to more heat in the body. The more hair on the body, the better heat retention.

So which came first, the dino or the egg?

It's apparent that endothermy evolved at least twice; once beginning with an exaptational insulator in the case of birds, and once beginning with exaptational skeletal changes and a needed increase in activity for foraging, in the case of mammals.

Largely, however, the jury is still out. (I heard that there is also evidence that pterosaurs might have been endothermic, leading to a third origin of endothermy; please link research if you know of any.)

Its important to realize that there are in-between states of thermoregulation, and the progression from ectothermy to endothermy (and back again, in some cases) took place in baby steps across millennia. Time and again we're shown that organisms tend not to fit our definitions and molds. It's not like flipping a switch.

More on dinos and thermoregulation for part III.

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>Dinosaurs and the Mystery of Body Temperature I: Endothermy vs. Ectothermy

Jan 19 2007 Published by under Evolution, Links, Paleontology, Physiology

>This is repost from September of '06. I never had the chance to finish the series because of school, so I will be finishing in three parts this week.

Why have the dinosaurs been relegated to little kid stuff? Why can't I find a purple triceratops t-shirt in XL?

Were the dinosaurs warm-blooded or cold?

I want to talk about dinosaurs and body temp for a couple of posts, but I think the best place to start is with a little discussion of why this issue is important and the evolutionary implications of endothermal/ectothermal states.

Ectothermy is the state of commonly referred to as "cold-bloodedness" (an inaccurate term; it's more about precise regulation) which is observed in most reptiles (though not all), most fish (not all) and basically every animal that is not a bird or a mammal (though not all). Ectotherms rely on heat from outside sources to maintain their body temperature. The sun is the main source of this heat energy (infrared), which can be transferred directly through absorbing the sun's rays, or indirectly through convection - movement of heat between objects (including organisms) and the air.


When a lizard suns itself, its blood vessels just below the skin open up, allowing more blood to flow next to the skin and the source of heat (the sun's radiation). The lizard's blood warms and transmits heat to the rest of the body.

Endotherms, on the other hands, are animals that regulate their own heat, like us. We don't need external sources because heat production is built in to our metabolic machinery. In fact, most endotherms spend more time trying to dissipate heat than acquire it. As you might expect, there is a formal range of classifications (lower lethal, lower critical, thermoneutral, upper critical, etc.) that biologists can use to determine the tolerances of certain endotherms.

The most important thing to remember is the necessity of temperature regulation in animals, the "why." Metabolism is all about burning calories, and the processes that burn those calories are driven by chemical reactions. Chemical reactions within the body are finicky; they depend on enzymes to catalyze which only work within a relatively narrow band of temperatures.

Endotherms have an advantage in this respect. A constant high body temperature increases the activity of the central nervous system, and subsequently neurotransmitter and enzymatic activity. Ectotherms do not have this advantage; on cool days/nights, they lose the ability to be as active. Keep in mind that this does not mean that endotherms are better, just different.

So what does this have to do with dinosaurs?

The main problem in assessing the body temperature of dinosaurs is that we have no direct evidence. There are no extant dinos, so scientists have to look to their descendents, birds and reptiles. But, as we covered earlier, temperature regulation is far from uniform in these animals.

There is one other problem: temp regulation is a special problem in the case of such huge animals.

Next time we'll look into the evolution of endothermy and how it might have arisen from the dinosaurs.

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Why Spiders Aren't Insects IV: Molecular Homology

Photo by: Gerald Yuvailos


This is the fourth part in an ongoing series discussing the distinction and evolution of spiders and other arthropods.
Part I, part II and part III have led up to this point:

In the last post of this series, we established that spiders descended from marine arthropods called the eurypterids, distinct and separate from insects, appearing in the fossil record in the late Silurian/early Devonian, about 425 million years ago.

The cladogram we used to analyze the spider's history was based on the organism's morphological characteristics, that is, visible structures like chelicerae and book lungs that can be tied to other organisms that possess the same structures. Limulus (the extant horseshoe crab) has both of these structures and predates the spiders, placing them further back in the chelicerates' evolutionary history.

Homologous bones from human (I), dog (II), pig (III), cow (IV), tapir (V) and horse (VI):
r — Radius, u — Ulna, a — Scaphoid, b — Lunare, c — Triquetrum, d — Trapezium,
e — Trapezoid, f — Capitatum, g — Hamatum, p — Pisiforme

Paleontologists call this comparison of physical characteristics homology (coined by Richard Owen an anatomist and, ironically an opponent of Darwin). The mouth parts of a spider are homologous to the mouth parts of Limulus because of the cherlicae's exact form and function. This is a different designation than analogy; analogous structures may function in the same way, but they are different in form because of their different lineage. For this reason, scientists call analogy an artificial classification system.

A good example of analogous structures are wings from bat and bird. They perform the same function in varying degrees, but they have evolved very different forms. A bat's wing is basically a modified mammal hand, while the bird wing is a modified tetrapod arm.

Homology is essential in building an organism's phylogeny (evolutionary history). More recently, geneticists have employed this classification technique to analyze and find similarities among the less visible traits in life, RNA (ribonucleic acid) and the building blocks of proteins, amino acids.

Think of these cellular chemicals this way: If DNA is the blueprint of life, RNA is the builder and its materials are amino acids. When these amino acids are placed in the correct sequence by RNA, they become proteins, the framework of our body. And, since the genetic code for protein constructions is nearly universal*, geneticists can compare entire swaths of RNA from one organism to those of another and find homology at the molecular level.

Here's an example (sequences are greatly abbreviated for the sake of our sanity):

Organism 1: ACGC-CCCCC
Organism 2: ACGC-CCCUC
Organism 3: ACGU-CUCUC

Basically, from noting the differences in each RNA sequence, and determining the homologous sequences (such as the ACGU sequence above), a cladogram can be constructed that shows common ancestry without the murky distinctions that sometimes cloud the comparison of bones to bones, or mouth parts to mouth parts.

The problem with this molecular system of analysis is that it often provides vastly different cladograms than the ones crafted through morphological analysis. This is not necessarily the case between the spiders and and Limulus, the molecular evidence supports the fossil record's interpretation of ancestry, but it calls into question the descent of insects from chelicerates like spiders.

In short, the molecular evidence agrees with the morphological evidence: spiders are more closely related to horseshoe crabs than insects. But where and when did the insects arise?

Next time we'll tackle the more recent movements to elucidate the phylogeny of arthropods, including a discussion on the significance Hox genes and evolutionary-developmental biology (evo-devo).

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Why Spiders Aren't Insects III: The Evolutionary Advantages of Mass Extinction

So far we have established that spiders are distinct from insects for two reasons: physiology (mouth parts, body plan, respiratory structures) and more importantly, evolutionary history (or phylogeny, as scientists call it).

But where did spider's come from? How did they crawl out of the water as euryterids and speciate (become a distinct organism that cannot interbreed)?

The answer, like many in invertebrate paleontology, is cloudy. Organisms without hard, thick shells rarely become fossilized. In fact, for any organism's parts to become fossilized, even vertebrates, is a profound rarity, as Bill Bryson illustrates in A Short History of Nearly Everything:

Only about one bone in a billion, it is thought, ever becomes fossilized. If that is so, it means that the complete fossil legacy of all the Americans alive today - that's 270 million people with 206 bones each - will only be about fifty bones, one quarter of a complete skeleton.

Needless to say, invertebrate paleontologists are having a heck of a time piecing things together from such a paltry fossil record. But that doesn't mean there's no evidence.

According to morphological and geological evidence, and therefore directly observable comparison, spiders and their brethren descended from the eurypterids, many of which were sea-going creatures. The eurypterids arose in the Ordovician, a period that began with the decimation of perhaps 60% of all marine life, and consequently ended with another more devastating cataclysm, which which some paleontologists rank as the second most destructive extinction event in the history of the world (by extinction of family). This has become known, quite appropriately, as the end-Ordovician event.

Mass extinctions make room for the evolution of unique characteristics as dictated by an organism's environment, and the environment changed drastically for the eurypterids at the end of the Ordovician. Glaciers began to creep down from the upper latitude, as the greenhouse gas carbon dioxide was depleted from the atmosphere, reducing the Earth's ability to trap the sun's heat energy. As the glaciers encroached, sea levels dropped and global temperatures cooled. This rapid progression decimated habitats, and destroyed a species' equilibrium with its environment.

But the end-Ordovician event was comprised of two parts: glaciation and then a period melting, an interglacial. Temperatures warmed once more, glaciers melted, flooding the land, and raising sea levels once more. The world had completely lost almost 50 percent of the families of life, but the ancestors of the spiders had survived. The Silurian period had begun, and new ecological niches were available for exploitation, a habitat opportunity that eventually would produce the spider.

That's about how it stands from a morphological perspective. But more recently scientists have been delving into molecular evidence and crafting very different explanations of not only the rise of the spider, but the vast diversification of arthropods in general.

Next time we'll address the new cladograms produce by this molecular evidence, and what ramifications it might have in interpreting the diaspora of the most abundant creatures on the planet.

*Interestingly enough, we are in the middle of an interglacial right now, the Holocene. Much like the success of the spider, our current interglacial, which began about 16,000 years ago, may have contributed to the ultimate "success" of Homo sapiens.

Resources:

Pechenik, J. A. (2000). Biology of the Invertebrates. : McGraw Hill Companies.

Gradstein, Felix, James Ogg, and Alan Smith, eds., 2004. A Geologic Time Scale 2004 (Cambridge University Press)

Baez, J. (2005). Temperature. Retrieved July 18, 2006, from http://www.math.ucr.edu/home/baez/temperature/

Webby, Barry D. and Mary L. Droser, eds., 2004. The Great Ordovician Biodiversification Event

University of Bristol. (2004). Fossil chelicerates and evolution. Retrieved July 18, 2006, from http://palaeo.gly.bris.ac.uk/Palaeofiles/Fossilgroups/Chelicerata/fossils.html

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