The ageing lottery

hydra don't age or undergo senescence
Image by Stephen Friedt via Wiki Commons

Not all animals age like we do. In fact some animals don’t age at all. A recent paper compared patterns of fertility and mortality across 46 different species ranging from humans right down to alga. To be precise, the study looked at humans, primates, mammals, birds, reptiles, amphibians, fish, insects, molluscs, a cnidarian, a crustacean, plants, and an alga. In their words, humans are ‘extreme outliers’ when it comes to how we age. We have an early spike in fertility from about 13-40 years, then a plateau period where we quietly go about living our lives and raising our babies, then we have a massive increase in rate of mortality from around 60 years of age. This increase in mortality is called senescence.

The star of the ageing world is the Hydra (Hydra magnipapillata), a tiny tentacled fresh water animal that can live up to 1400 years. Hydra have negligible sensescnece – they maintain fertility and mortality throughout their considerable life span.

Other mammals show a similar increase in senescence at the end of their lifespan to humans – it just isn’t as dramatic as it is for us. Animals lower down the evolutionary chain, amphibians and reptiles, tend to show a much flatter mortality curve. Some of them, like the desert tortoise (Gopherus agassizii) and the red legged frog (Rana aurora) hit the ageing jackpot and actually become more fertile and less likely to die the older they get. This also goes for the white mangrove plant (Avicennia marina) and the oarweed alga (Laminaria digitata). This is called negative senescence and is the holy grail for ageing researchers.

Invertebrates and plants showed really variable fertility and mortality curves which shows that the way we age isn’t linked to how complex we are in the evolutionary sense.

Another phenomenon that occurs all the way up and down the evolutionary chain is the ability to live for a reasonable time once you are no longer fertile and able to reproduce. Humans do this, but so do Bali mynah birds (Leucopsar rothschildi), worms (Caenorhabditis elegans) and the microscopic plankton-esque rotifers (Macrotrachela sp.).

What does all this mean? If we could learn why hydra and the desert tortoise and the red legged frog seem to be able to avoid senescence, we might have a better understanding of how to delay ageing in humans.

The Scientific Paper:
Jones et al. Diversity of ageing across the tree of life. Nature. 2014

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