~ The Heartrate
~ Cellular membranes and free radicals
~ Brain size
~ Telomeres
~ Genetic defects
~ Conclusion
This essay will explore the average lifespan of a typical dragon, bringing into play a discussion on its heartrate compared to comparable animals and also the structure of telomeres and their effects on cellular and individual ageing. Some of this essay concerns genetics, however if you have no prior knowledge, all the genetic terms mentioned are explained.
The 'typical' dragon used for this essay is the Western-type dragon, as it is probably the most famous and the one that normally springs to mind when the word 'dragon' is mentioned.
As this is an anatomy essay and describes the dragon's anatomy through purely scientific means, any form of supernatural longevity has been disregarded. It defeats the purpose of an anatomy essay and provides an all too-easy excuse to explain away some of the dragon's more outrageous traits. However, if you prefer to think of a dragon's lifespan as being extended through these means, you are more than welcome to. In that case, merely regard this essay as an interesting five-minute diversion.
The Heartrate:
It has been stated in countless fictional dragon stories that the beasts live for many eons; many thousands of years. Unfortunately, romantic and appealing though this view is, it is nigh-impossible for the scientifically-minded dragon. Here is why:
Dragons are flying creatures. This requires an enormous amount of energy and results in a fast metabolism (Fox, 2003). During the strenuous ritual of flight, the heart of even the fittest dragon would be beating extremely fast in order to pump oxygen around the body, especially the flight muscles. As dragons aren't the massive, house-sized animals they are often viewed as, as this would make at least powered flight impossible, their heartrate (governed by metabolism, activity level, etc) would be comparable to that of an active mammal of similar size (possibly a horse).
Generally speaking, larger animals have slower heartbeats than smaller animals (Fox, 2003). Mice have far faster heartbeats than us, and we have faster heartrates than a elephant. Disregarding medical interventions and other lifestyle changes in the case of humans, the smaller animals have shorter lifespans than larger animals (Fox, 2003). Mice live for a maximum of five years, whereas an elephant isn't even sexually mature at this age. It has been shown that a heart is only "good" for a certain number of beats, that is, there is potential for a certain number of heartbeats before an animal dies. Obviously it follows that a smaller animal with a faster heartrate would use up its beats faster than a larger creature.
From the above, a dragon may therefore live only about as long as a horse, (maybe a bit extra as it has a more sedentary lifestyle, as do most predators). This gives a maximum age of around thirty years, far from the historical, centuries-old dragon1*. However, there is more evidence to be examined before we can conclude with this figure....
Cellular membranes and free radicals:
Recent research has shown that not only do larger animals have comparatively lower metabolisms, their cellular membranes are far more resistant to the degrading effects of free radicals released from breakdown of foods (Fox, 2003, see also Achenbach, 2003). As Pamplona et al. (2000, p. 169) state: "free radical damage is currently considered a main determinant of the rate of aging", with unsaturated fatty acids (such as those in cell membranes) most vulnerable to oxidisation.
Small animals have far less watertight cell membranes than larger ones, allowing much faster exchanges of gases and nutrients with the bloodstream, but this also means that their cells are damaged easily (Fox, 2003).
This research also discovered that certain components in cell membranes are more or less at risk of being oxidised by free radicals (Fox, 2003). It has been long known that birds enjoy far longer lifespans than mammals of similar size and metabolic rate (Herrero and Barja, 1998). This may be due to the makeup of their cell membranes. The fatty acids found in avian membranes contain less omega-3 PUFA (of an extremely unsaturated type) than mammalian membranes (Fox, 2003). As this fatty acid "tail" is unsaturated, it is quite prone to oxidation by free radicals. As birds have less of this substance, the life of the cells and of the individual may be prolonged, due to the cells suffering less damage as a by-product of normal metabolism (Fox, 2003).
Studies into some cage bird species have shown that the mitochondria (the cellular organelle in animals that is responsible for respiration) leaks fewer free radicals than those of mammals and subsequently produce less hydrogen peroxide (Herrero and Barja, 1998). As a result of less free radical production by the mitochondria, the DNA is exposed to these harmful substances relatively infrequently, and is therefore more protected from damage (Barja,1998). This could lead to the very slow ageing found in birds as compared to mammals, and has been suggested to be more important in determining longevity than metabolism (Herrero and Barja, 1998).
So what implications does this have for dragon longevity? Dragons are essentially composite creatures, and many avian features are incorporated into their anatomy and physiology. It is possible they may share the advantageous cell membrane structure outlined above. Birds live about twice as long as a similar mammal at any given metabolic rate (Fox, 2003). As dragons are partly avian, this could add an extra 10-20 years to their lifespan (as a rough estimate), bringing it up to 40-50 years. "Wear and tear", such as the inevitable injuries sustained while hunting, may whittle this down slightly (though no more than it should for a predatory bird), however it is still far longer than a comparatively sized mammal. Another offset to this long lifespan (as compared to a mammal) is that the dragon must ingest more food than a carnivorous mammal in order to provide enough energy for flight. This would in turn create even more free radicals in the body, which in turn may be limited by the avian-like mitochondria in the cells.
Brain size:
The carnivorous tendencies of the dragon are well-known; and it has already been established in the Western dragon anatomy essay that the creature also is an avid flyer and possesses excellent senses suitable for its role as a top predator. All of these factors would contribute towards a large brain relative to its size. It is probable the dragon would have evolved a brain (the degree of brain development during evolution is termed "encephalisation" [Hofman, 1983]) as large as that of a wild dog (eg. a wolf Canis lupus), which are generally considered to be very intelligent.
Hofman (1983) has theorised that brain size, coupled with metabolism (covered earlier) and body weight, have a limiting effect on the lifespan of mammals. Mammals have the largest brains relative to their size of all animals, and although dragons are not strictly mammals per se, they do share the feature of a large brain and so are comparable in this way more than other vertebrate classes. Hofman's research, when applied to the high degree of encephalisation in the dragon, may limit its lifespan somewhat. This could mean that the 40-50 years stated above should be reduced slightly to take this into account. Perhaps an average dragon would live a couple of years shy of this age? Hofman's statements concerning metabolism rate as a longevity factor in mammals have not been applied to the dragon, as Herrero and Barja (1998, p. 133) say: "birds have a maximum longevity... much higher than mammals of similar body size in spite of their high metabolic rates", and dragons resemble birds with regards to this feature much more than they do mammals.
Telomeres:
Now we come to the theory proposed by geneticists, concerning the effect/s that telomeres may have on ageing.
Telomeres are repeating sequences of DNA that occur at the ends of chromosomes (Achenbach, 2003). Every time our somatic cells divide through mitosis, the DNA needs to be replicated so that there is two copies, enough to go into each of the daughter cells. For the purposes of this anatomy essay, we can say that the telomeres at the ends of the chromosomes become shorter with each cell division (Achenbach, 2003).
It is thought that these telomeres protect the ends of the chromosomes, because without them, the chromosomes tend to start sticking together. This can create problems during mitosis ([somatic] normal cell division) and meiosis ([gametic] reproductive cell division). In fact, it has been shown, as stated by Kanungo (1994), that "critical deletion (of telomeres) causes cell death" (p. 239). Therefore, older or mature individuals would have shorter telomeres than younger individuals, as they have undergone more cell divisions. It is thought that the length of the telomeres has an effect on ageing; as individuals with unnaturally short telomeres (eg. Dolly the cloned sheep) tend to age and die quicker than 'normal' individuals2* (Knight, 2003).
The point is this: the theory is that telomeres have a limiting effect on cell division and therefore the lifespan of the cell and of the individual.
Telomeres may be continually lengthened by the enzyme telomerase. This enzyme is present in reproductive cells (if not, then every time the reproductive cells divided to produce sperm or ova, the telomeres would keep shortening, resulting in offspring with progressively shorter telomeres through successive generations).
Telomerase is also present in stem cells, which give rise to other cell types, and cancer cells. It is the presence of telomerase that helps cancer continue to spread without check where a normal cell would 'suicide' after a while (apoptosis, or programmed cell death).
How could a dragon possibly live for millennia if its cell division was limited by the length of its telomeres? It is very simple: it couldn't. You may argue that dragons may have the telomerase enzyme in all of their cells, but that raises two points:
1) Why dragons? What makes them more special than other creatures? Ignoring their mythological origins, they are the same as any other animal;
2) The incidence of cancer in a species whose cell divisions know no boundaries would be much higher than other beasts (Achenbach, 2003). There are many checkpoints a cell must breach before becoming cancerous, still, inherent unlimited cell division would markedly increase cancer rates.
Genetic defects:
Here we will see why exceptional longevity may be ultimately harmful to an organism. The following section will explain why.
The DNA molecule is made up of 'base pairs' or "nucleotides". The sequence of these nucleotides is unique between individuals, with the result that everyone is different in appearance, behavior, and everyone has a slightly different genome to others, generating genetic diversity. (Even 'clones' don't have exactly the same DNA as their genetic donor- their mitochondrial DNA differs.)
If you are thinking that identical twins are exactly as the name implies because they came from the same fertilised egg, you are half right, and that leads directly into the realm of DNA replication errors and random mutations. It is true that, at first, identical twins are exactly identical, even down to their mitochondrial DNA. However, every time DNA replicates itself in preparation for mitosis, there are substitutions that occur between the base pairs.
The aforementioned nucleotides come in four different types (disregarding Uracil in RNA). These are: Adenine, Guanine, Cytosine and Thymine. Adenine pairs with Thymine and Guanine with Cytosine. Sometimes, when DNA is replicating, a nucleotide may be switched with another and even transformed into another. This happens at a very low frequency, as few as once per billion base pairs replicated (there are three billion base pairs in the human genome), however these substitutions can mount up over successive cell divisions, leading to differences in phenotype (the appearance of an individual, a result of the interaction of their DNA and the environment they live in). Therefore identical twins are only truly identical just after fertilisation.
The process of obtaining mutations in the DNA sequence can be sped up by exposure to chemicals, drugs, radiation, etc. These can cause base pair deletions, additions, substitutions, or can affect whole genes or even whole chromosomes (these types of mutations will not be discussed here). These changes in the genes can cause the genes to malfunction, or not produce enough of a certain protein, or produce too much continuously; the effects can be very wide-ranging. When all these mutations build to dangerous levels, the cell will undergo apoptosis, or suicide. This death is to prevent the cell from becoming cancerous or producing certain substances which could endanger the entire organism.
If a dragon lived for thousands of years or even many hundreds of years, then it would gradually accumulate many mutations in its DNA (although, like birds, probably would have a high DNA repair rate [Barja, 1998]). As mentioned earlier, it would need the enzyme telomerase in order to survive this long, and this unlimited cell division in addition to the accumulated genetic mutations would be extremely dangerous to the animal's health.
Research at the University of Michigan has even discovered that certain cells possessing the following types of surface proteins: CD44 and ESA, as well as lacking the protein CD24, commonly gave rise to tumours even when only a small amount were present (Ananthaswamy, 2003). Some stem cells (cells which differentiate into many varied cell types) also contain these proteins (however it should be noted that many other cell types developed into tumours at a much lower frequency) (Ananthaswamy, 2003).
Conclusion:
From the main points of the essay, an estimate of the typical dragon's lifespan has been proposed: 45 years (give or take a bit). This of course is not static, as it differs between individuals depending on their species, lifestyle habits and genetic makeup. Some wingless, sedentary, possibly ectothermic dragons may live as long as the great tortoises of the world, that is, up to two centuries, while small Faerie dragons would probably be the shortest-lived of the dragons, perhaps living as long as a mouse (an average of three years, though five may be a more likely lifespan) (Achenbach, 2003). Dragons that have less stress in their lives, ie. are fighting fewer knights in shining armour, ingesting less pointy jewellery with their virgin maidens, would perhaps enjoy a considerably longer life than their village-plundering brethren.
An extremely long-lived dragon would be at risk of contracting cancer, or would die if its genes were so mutated they were non-functional. If telomerase wasn't present in all cells (and there is a very remote chance it would be) then each division would shorten the telomeres. It could possibly be argued that the telomeres in dragons may be excessively long to begin with, although this still leaves the problem of DNA mutations occurring, as well as the chromosomes themselves taking up too much space in the cell. Obviously telomere length, which has been honed through evolutionary processes, is already well-suited to each organism.
Although the dragon lifespan proposed is far shorter than usual thinking dictates, it doesn't make the creature seem any less noble or magnificent. Also, it is just a theory and you are perfectly entitled to believe dragons can live for millennia; in the end only your own opinion should matter and it is up to you.
1*This estimate is assuming mammalian/ avian style metabolism, rather than reptilian metabolism. Reptiles do live longer than comparably-sized mammals, however the dragon's lifestyle and energy needs doesn't fit with the reptilian model.
2*This is not to say that cloned animals have shorter lifespans than naturally conceived animals, which is the theme of an ongoing debate. Dolly the sheep was cloned from a mammary cell of a six-year-old ewe, and she lived for a further six years before dying (Knight, 2003). The average lifespan of her breed was twelve years, so her telomeres were twelve years old at the time of her death.
