Bear Consumption and the Appearance of Programmed Aging

I seem to recall salmon held up as an argument for programmed aging in general - as opposed to aging as wear and tear - due to their very fast decline into senescence following spawning. This circumstance is very specific to salmon, however, having much more to do with evolutionary adaptation and the appetites of bears than any broader form of programmed aging:

Pacific salmon are noted for not feeding during their breeding period, relying instead on stored energy reserves and for their rapid senescence - the physiological deterioration associated with aging - once breeding is over. It is, thus, more beneficial for bears to consume fish with fewer signs of senescence because these fish have more energy reserves. However, these “fresh” fish are also more vigorous and harder to catch and so are more effectively caught in smaller, shallower streams.

Carlson and colleagues studied populations of salmon and brown bears in six creeks in southwest Alaska to determine whether the rate of senescence in salmon was driven primarily by the rate of predation by bears or by the tendency of the bears to prey on salmon with less evidence of senescence. They measured the reproductive lifespan of each fish as the number of days between stream entry and death and recorded the mode of death for each fish. They found that the selectivity of the bears for salmon of various senescent conditions was the prime factor determining the rate of senescence in the salmon.

In populations where bears killed old, decrepit salmon, the salmon senesced more slowly relative to populations where bears killed young, “fresh” salmon. This result contrasts with the established expectation that senescence evolves because of the number of individuals killed by predators, rather than their physical characteristics.

Here's the original paper via PLoS One:

Predation by Bears Drives Senescence in Natural Populations of Salmon

We found that the degree of condition dependence in extrinsic mortality was very important in driving senescence: populations where bears selectively killed fish showing advanced senescence were those that senesced least rapidly.

Lazy bears mean longer lived salmon - an extreme example of the way in which evolution favors successful mechanisms for early reproductive success over the longevity of those mechanisms. Evolutionary circumstances are wildly difference for different species, however, and we should expect to see wildly different mechanisms, rates and circumstances of aging - and so we do. Few variations out there in the animal kingdom will be directly relevant to the quest to intervene successfully in human aging, but until you dig in you won't know for sure.

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