Aging vs. life span

[shmeeter]

Do animals with longer life spans actually age more slowly at the genetic or cellular level, or do they have some kind of mechanism for compensating for aging that is better than that of other animals?

 

My hunch is the former, but I really don't know. I was brushing my dog, and thinking about his grey hairs at the age of 13, and one thought led to another, and here I am at science forums with my first post.

 

Thanks in advance!

Ian

[TonyMcC]

There seem to be links between animal size and lifespan and also heartbeat rate and lifespan. It has been suggested that for all species the number of heartbeats in a lifetime is about the same. Here is one attempt to quantify. I will say that the subject and conclusions are debatable.

http://www.math-physics-tutor.com/web_documents/bader2.pdf

Edited February 13, 2011 by TonyMcC

[ewmon]

I think it's important to look at how populations of different species survive (or not). One insightful method involves normalizing lifespan and survival of various species by graphing percent of maximum lifespan versus percent of population survival, as shown below. Thus all curves begin at 100% survival at % 0% lifespan, and end at 100% lifespan and 0% survival I quote the text directly from the source:

 

 

When discussing aging it is important to distinguish two points on survival curves. Mean lifespan (average lifespan) corresponds to the age at which the horizontal line for 50% survival intersects the survival curve. Maximum lifespan corresponds to the age at which the survival curves touch the age-axis (0% survival) — and this represents the age at which the oldest known member of the species has died. (In animal studies, maximum lifespan is typically taken to be the mean lifespan of the most long-lived 10%.) Curve A as shown is a pure exponential decay curve. Curve B corresponds to the survival of small animals, such as mice or squirrels in a natural environment. Human survival was still close to curve B in ancient Rome when average lifespan was 22 years, but by the mid-1800s the typical North American lived to be 40 — more like curve C. Today, people in the most developed countries have an average lifespan of about 80 — resembling curve D. Reduction of infant mortality has accounted for most of the increased longevity, but since the 1960s mortality rates among those over 80 years has been decreasing by about 1.5% per year. Maximum lifespan for humans, however, has remained about 115-120 all through known history.

 

Curing specific diseases such as heart disease or cancer can do no more than further "square" the survival curve (toward curve E), with no effect on maximum lifespan. Curing cancer would add about 2 years to human life, whereas eliminating heart disease would add 3 or 4 years. Mean lifespan varies with susceptibility to disease, accident & homicide/suicide, whereas maximum lifespan is determined by "rate of aging". In aging research, maximum lifespan is regarded as a proxy for aging. Chemicals, calorie restriction with adequate nutrition, or other interventions which increase maximum lifespan are said to have slowed the aging process.

I have seen the A curve represent survival rates for animals such as amoebas, which die at a fairly steady rate throughout their lives. The main point I want to make is that dogs would have a lower average lifespan (and probably a lower maximum maximum lifespan) if they lived in the wild (I would guess somewhere around the B or C curves). However, having been domesticated, they enjoy better healthcare throughout their lives (lower infant mortality, fewer deaths by disease, protection from violence, and better healthcare for their "elderly"), and thus, their population enjoys an extended survival closer to the D curve (modern people).

Edited February 13, 2011 by ewmon

[Blahah]

This is a rather well publicised idea which stems from people's general perception of a correlation between body size and lifespan in endothermic ('warm blooded') animal species, and some research papers which compared metabolic rate with mass for some species. The entire concept is called the allometric scaling laws of biology.

 

I'll try to tie the various ideas together in historical context...

 

The first person to study it was Max Rubner in 1883. He compared the body mass and metabolic rates of 7 breeds of dog ranging from ~3kg to ~30kg, and found that plotting the data on logarithmic scales produced a straight line relationship (with gradient 2/3). He later worked this into a theory after doing more experiments, called the 'Rate of living' theory.

 

It was widely thought that the reason that the relationship worked out to approx [math] metabolic\,rate \approx mass^{2/3}[/math] was because this same relationship holds for surface area to volume of a cube: [math]surface\,area = volume^{2/3}[/math] and the relationship is similar for surface area to volume of animals. This gave heat conservation as a reason for larger animals to live 'slower'. Metabolism produces heat, and as volume increases animals retain more heat and therefore larger animals must have a slower metabolic rate in order to avoid overheating. Biological systems in general start to operate less efficiently over about 37°C, a limitation caused by the denaturation of proteins.

 

Max Kleiber also worked on this idea in the 1930s. He measured metabolic rate but did so for a range of species, and he produced a mathematical formulation of the relationship between metabolic rate and body weight:

[math]q_0 \approx M^{\frac{3}{4}}[/math]

where [math]q_0[/math] is metabolic rate and [math]M[/math] is mass.

This formulation is very famous and is found in many biology textbooks. It's often referred to as Kleiber's Law. It shows a similar relationship to that described by Rubner, but with a power of 3/4 instead of 2/3. This confused people for a while - if the relationship is not governed by the temperature constraints, then what causes it?

 

In 1997, Brown and Enquist published a paper in which they suggested that the efficiency of branching networks might be the reason for all the allometric scaling laws. For example, the circulatory sytem of mammals is a branching network.

 

Another theory, suggested by Hung-hai, Ulf and Rajindar (1993), relates size with lifespan. They suggested that free radicals released by mitochondria during respiration cause cellular damage and create ageing. This would suggest that animals which respire faster would die faster because the rate of free radical production is increased. Since animals essentially respire as fast as they can given other constraints (such as the heat constraints, and the limitations of their circulatory system) and these restrictions increase with size, smaller animals might indeed tend to have much shorter lifespans than longer animals. Faster metabolism would have to be supported by faster heartrate to provide oxygen and fuel for respiration, whilst slower metabolism so there MAY be a general similarity in total hearbeats per lifetime between endothermic mammals.

 

There have been some studies which specifically correlate heart rate and life expectancy, such as Levine's 1997 paper. This just goes to support the idea that living fast often means dying young.

 

There's a good book about the whole topic of allometric scaling in animals, by the famous animal physiologist Knut Schmidt-Nielson, called Scaling, why is animal size so important? And if you have access to journals, this paper by West, Woodruff and Brown (2002) ties together all the various scaling laws across all sizes of life.

 

[shmeeter]

But none of this explains why something that has a similar mass as us, like a deer or something, has a much shorter life expectancy. I mean, even in captivity, with health care, etc., one couldn't expect a deer to live longer than say, what, 20 years? Do humans have some way to counter some of the effects of aging, like free radicals? Could it be due to something like genetic complexity or diversity? (I read once somewhere that chromosomes get their ends lopped off during replication or something, and that this could be related to aging. Maybe the chromosome ends are null genes, existing only as a buffer against that?)

[Blahah]

But none of this explains why something that has a similar mass as us, like a deer or something, has a much shorter life expectancy. I mean, even in captivity, with health care, etc., one couldn't expect a deer to live longer than say, what, 20 years? Do humans have some way to counter some of the effects of aging, like free radicals? Could it be due to something like genetic complexity or diversity? (I read once somewhere that chromosomes get their ends lopped off during replication or something, and that this could be related to aging. Maybe the chromosome ends are null genes, existing only as a buffer against that?)

 

Remember that it's just a trend, there are many exceptions and the reasons are not necessarily obvious at all. Also, veterinary healthcare is nowhere near as advanced as human medicine. So an animal in captivity is not in similar circumstances of care to a human in the developed world. If you include humans in the scale it should be humans before modernisation - i.e. before 1700. The average lifespan at birth of a human in the Roman empire was 28 years. See this Wikipedia article for some other figures. The lifespan of the red deer is about 20 years, yes. There are slight variations between different animals, the point of the exercise is that endothermic animals tend towards the trend.

 

It is unlikely that genetic diversity is a major factor in human survival, I would suggest that intelligence and social living are the main reasons why humans might live longer. But certainly different species have different abilities to counter the effects of aging. Consider that species which care for their young have a stronger evolutionary pressure to survive longer to ensure their offspring survive. Human babies are completely useless until the age of about four, and not really equipped to survive on their own until much older. Deer fawns on the other hand are fairly competent after just one year. Ectothermic (cold-blooded) animals may survive longer because when then allow their body temperature to fall, as sea turtles and giant tortoises frequently do, free radicals (and all other molecules) are less active.