Aging: Topics for Discussion(About which I am not advocating any particular theory, except I doubt "wear-and-tear" explanations.)
(There are some very good Wikipedia pages on this subject, whose facts fit what I have read in scientific books.)
A) The ~ three-fold difference between life expectancies of Great Danes (etc.) as compared with the smallest breeds of dogs. All their symptoms of aging are the same as in humans (arthritis, weakened bones, cataracts, deafness, hair turning grey or white), except these symptoms occur after 6 or 7 years in large dogs, but after 15-20 years in very small dogs. Can we make sense of these differences in terms of...
(II) Telomeres and telomerase?
(III) Programmed self-destruction (favored by evolution, kin selection, reduced-competition from grandparents), altruism, etc.
(IV) Consider very big breeds of dogs as if using a Volkswagen motor to propel a Winnebago.
(V) Please suggest other possibilities.
C) Women's life expectancy is 8-10 years longer than men. (I wonder if this male-female difference occurs in animals; and which species?) Also, human males have slightly higher death rates, even as babies, and fetuses.
D) Based on my own experience going to college reunions, people aged 50 differ widely in apparent age, from some who look 35 and others who look 65. (and every apparent age in between, spread out in a bell-shaped curve). And those getting the most "wear-and-tear", exercise, running, world travel, boating, mountain climbing, mostly seemed the youngest.
E) Some species of animals have very long life-expectancy: Sea Anemones, some clams, Hydra, (sponges?), Some species of turtles, especially the American Box Turtle. Two common features are that all these continue to grow and to reproduce during their entire lives.
F) A few species of animals (salmon fish) seem to age very, very rapidly (in a day or two) after spawning eggs and sperm (and after swimming up-stream). Octopus undergo something similar, at whatever age they breed (and without the swimming upstream part; our political process might not withstand the social effects if thousands of octopuses migrated up rivers and streams!).
"Annual" species of plants self-destruct after flowering, their first year. Some other plants flower and degenerate after two years. Other species ("perennials") keep on flowering, year after year. This may or might not be related to aging in animals.
G) One-celled organisms don't senesce. Likewise the primordial germ cells from which sperm and oocytes develop. (But a wikipedia article mentions aging in yeast.)
H) Some cancer cells in tissue culture, "HeLa cells" are the best example, continue to grow without limit. There are also mouse cell lines that go on forever ("L cells"), despite the 2 year life span of mice! Also the myeloma cells used in the monoclonal antibody technique. These are all (?) either derived from cancers, or from non-cancerous cells that were converted into cancer cells in culture.
I) Nematode and fly researchers have selectively bred genetically longer-living strains of their organism (up to ~ three-fold longer life-spans, or somewhat more). Why not? What might be the limiting factor?
J) Supposedly, in some very high mountain areas (parts of the Himalayas, the Andes, and the Caucasus), human life spans are extremely long (&120 years, and up). (Although surveys in these areas find almost nobody claiming to be 80 or 90 years old, as if they jumped from 90 to 110+!) These people would have been subject to more than the average wear and tear, but seem to age more slowly.
Maybe harsh conditions select for the most hardy (& only they survive?) Or was there selection among their ancestors?
Anyway, we should ask what correlations occur between infant mortality rates and maximum ages reached by survivors.
(What lessons could we learn from such correlations?)
K) Very extreme starvation diets cause mice and rats to age more slowly, and survive about 150% as long as normal. (But please note that this starvation is so extreme that the animals never reach full size and can't reproduce).
L) In regard to measuring survival after blood loss, or at freezing temperatures, to quantitate stamina, perhaps we should consider these as measures of homeostatic mechanisms. Then, if we visualize homeostasis as analogous to a small bead sliding down the inside walls of a cone-shaped indentation, what are some alternative ways to think about aging?
M) Way back, in the days before telomerase and NF-kappaB, a very popular theory of aging was the "Error Catastrophe Hypothesis", of which the key idea was a vicious cycle of accumulation of mutations in genes for proteins that participate in replicating DNA, and transcribing RNA, and translating mRNA to make proteins, with the result of producing higher and higher mutation rates.
This was tested by comparing mutation rates in viruses infecting cells from young versus aged animals. Guess what was the result? Can you explain the logical basis of this experimental approach?