Why do some species live so much longer than others?

[Machine_Elf]

The concept/phenomenon of negligible senescence may have popped up at one of the upthread links, but just in case it hasn’t, I figured I’d post it here.

[pool]

That’s crazy DSeid, I had no idea that pygmies had such a short natural lifespan, and apparently they don’t respond to HGH with greater height right, because they’re receptors are inactive or they don’t have as many?

[drewder]

@Toledo_Jim

I’m a bit surprised. If you look at life span vs size, with a couple of exceptions, there is a strong trend that larger is related to longer lived. Most vertebrates get about 2 billion heartbeats. Elephants and tortises have slow pulse rates, and live a looong time. Mice and small birds have frantically beating hearts and seldom see a second year. We don’t know why some of the parrot family go so long, but they almost certainly don’t last that long in the wild.

The other interesting exception is dogs, the smaller ones tending to outlast the bigger ones. Sadly, the most annoying dogs outlive the mellow companions.

And, of course, us. My second billionth heartbeat was about 20 years ago, and I could easily have another billion to go.

I was going to mention the heartbeat thing too. This is why old people tend to be overweight because they wisely rationed their heartbeats in their youth.

[DSeid]

pool,

The little I can quickly find points to fewer GH receptors, or at least less expression of the gene for the GH receptor. 8-fold less.

adult Pygmies show normal baseline and post-stimulation serum growth hormone (GH) levels but low values of baseline serum GH-binding protein (GHBP) and insulin-like growth factor-I (IGF-I). This discrepancy suggests a defective response to GH occurring in adulthood since Pygmies lack both the pubertal serum IGF-I surge and the growth spurt. However, sequencing of the key genes of the GH-IGF-I axis failed to identify Pygmy specific variants or haplotypes. … the GH receptor (GHR) gene expression is 8-fold reduced in adult Pygmies in comparison with sympatric adult Bantu, and that this reduction is not associated with sequence variants of the GHR gene.

Note that it belies a simple explanation of GH or IGF-1 alone being key. Dogs, low IGF-1 associated with small size and longer lifespan. Human Laron Dwafism low IGF-1 also, and GH-receptor deficient, with very long lifespans. Pygmies low IGF-1, GH receptor deficient, small stature, and fast maturation with early death.

When they figure it out is going to be very confusing I think.

[Chronos]

Quoth pool:

That’s crazy DSeid, I had no idea that pygmies had such a short natural lifespan,…

DSeid’s cite didn’t say anything about pygmy lifespan, just their life expectancy, and a life expectancy that low is almost certainly due mostly to environmental effects (I’m guessing mostly a lack of access to pediatric medicine). In fact, it could even be that their earlier sexual maturity is a cause, by causing more complications in childbirth.

Life expectancy is how long, on average, members of a population live. Life span is the age at which members of a population typically die of old age. If you have a lifespan of 80 years, but a 75% infant mortality rate, then your life expectancy will be only 20 (or less, if other things also kill your people before reaching 80). This does not mean that anyone is actually 20 years old when they die; most likely, if you reach 20, you’ll probably have about 60 more years ahead of you.

[DSeid]

Actually the article uses the word “lifespan” not the phrase “life expectancy” … still they may be confused so I tried to look for more. Got this article and figure 2a is the one that matters most.

The age-specific survivorship curves for the various Pygmy populations are all pretty comparable and all very different that nearby non-Pygmy populations.

Yes, a huge drop off in the first several years of life compared to non-Pygmy native hunter-gatherer populations. Also a much more consistently linear decrease from there. Of the relatively few Pygmy group members who make it to 40 less than half will survive to 50. Of other native hunter-gather groups of the relatively large number who make it to 40 roughly 80 to 90% will still be alive at 50, and without a steep drop-off until sometime after 60.

Or put in how the article puts it

life expectancy at adulthood (defined here as age 15 years) is, respectively, 29.5, 27.3, 22.5, 20, and 32.5 years in Batak (3), Aeta (3), Aka (1), Mbuti (22), and Efe (3), against 41.5, 45.8, and 46.6 years in !Kung (19), Ache (18), and Turkana (20), respectively. Shortened lifespans also reduce the number of pygmy women who reach the end of the reproductive period.

Still that does not prove that the cause of relatively early adult death is early old age … still hard to declare anything for certain about lifespan. Which is chicken and which is egg?