What evidence for non-ageing species?
pjhynes at ozemail.com.au
Fri Sep 22 17:16:02 EST 1995
On Mar 16, 1995 steve at chambers.ak.planet.co.nz (Steve Chambers) wrote :
>In an article in Scientific American on the very old, it reported
>that after 80 there was no relationship between old age and high
>death rates in humans.
As I recall this topic has reared its head several times. I have two
queries which appear to be loosely related to Steve's - and others'-
1/ The alleged independence of death rates from chronological age
in the very old.
*** Is this not merely an artefact of the Gaussian Curve ?
When t>2*sigma, dN/dt --> 0 as a bit of algebra or simple inspection
of the curve will show.
So long as a group of individuals is far from its specie's mean age
of death, the death RATE looks approx constant.
In human males the mean age appears to be around 70.7 yr with an
SD of about 8.5 yr. (The death rate curve is approx normal.)
So IF you make it past say 85, your chances of dying APPEAR to
2/ An oft used index of death rates is the "Age Specific Mortality,
See : Carey et al, "Slowing of Mortality Rates at Older Ages in
Large Medfly Cohorts"
Science Vol 458, 457-461, 16\10\92
and Curtsinger et al, "Demography of Genotypes : Failure of
the Limited Lifespan Paradigm in D. melanogaster",
Science 258, 461-464, 16\10\92
These guys have real good data - over 10^6 flies fed and pampered
till they all kicked off from old age. Trouble is, even if you
start with a million, at the end you have just a handful left.
And that leads to LARGE fluctuations in the stats for the tail
enders. (Small sample noise.)
Bad for Qx which is supposed - in the above papers - to show
departures from the Gompertz Law in a large, ageing population.
So, who thinks these papers really do show that? who has read'em,
anyway? If Qx does not validate non-Gompertzian trends, where
does that leave the assertion of a slowdown in death rates
among the really old?
Or am I missing it altogether?
Bye all ... PJH.
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