Why do we have to visibly age as we get older? It might sound like a silly question – the terms “aging” and “growing old” are virtually synonymous – but for many organisms on the planet, this question actually does not need to be asked. Earlier this week, a paper was published in the journal Nature that explores the aging process in a wide range of organisms, including other mammals, vertebrates, invertebrates, plants, and a green alga – 46 species altogether. While the human aging process is characterized by an increase in mortality, and a decrease in fertility, the group of researchers found that this trend is simply is not the case for many other species. For example, some species have the same mortality and fertility throughout their entire lives, and others even have an increase in fertility near the end of their lives. These differing patterns raise many questions.
For decades, evolutionary biologists have argued that the “aging process” – declining fertility and increasing mortality with age – is inevitable due to evolution. Basically, the theory is that there’s no selective pressure after successful reproduction, and so (over countless generations) this leads to organisms having increased mortality after passing their reproductive age. But because the recent paper in Nature clearly shows that this aging process is not what’s seen in other organisms, we should no longer look at aging as an unavoidable fate, but instead we should investigate why we age differently. Could we “improve” our aging process by better understanding it in these other organisms?
Several animals, including hermit crabs (Pagurus longicarpus), red abalone (Haliotis rufescens), red-legged frogs (Rana aurora), and hydra (Hydra magnipapillata), were found to all have about the same mortality and fertility throughout their lives. On the other hand, some corals (red gorgonians [Paramuricea clavata]), birds (alpine swifts [Apus melba]), crocodiles (freshwater crocodiles [Crocodylus johnsoni]), and different plant species (viburnum [Viburnum furcatum], white mangroves [Avicennia marina], borderea [Borderea pyrenaica], yellow cryptantha [Cryptantha flava], hypericum [Hypericum cumulicola], agaves [Agave marmorata]) were found to actually have an increase in fertility as they grow older. Some organisms even had a decrease in mortality as they gained years (including netleaf oaks [Quercus rugosa], desert tortoises [Gopherus agassizii], white mangroves [Avicennia marina]). That all said, several species were also found to have an aging process fairly similar to ours, including some birds (southern fulmars [Fulmarus glacialoides]) and other mammals (chimpanzees [Pan troglodytes], killer whales [Orcinus orca], roe deer [Capreolus capreolus]). Clearly the aging process is far from universal for all of the inhabitants of this planet.
In addition to sharing some fascinating findings, this paper also serves the purpose of reminding us why it’s so important to study species other than humans. By better understanding how other organisms get old without “aging” like we do (they’re still fertile and have relatively low mortality), we may be able to improve our own aging process. In other words, we may be able to age less as we grow old.
For further reading:
- Owen R. Jones et al.’s article “Diversity of ageing across the tree of life” in Nature
- Virginia Hughes’ article “Why Do We Age? A 46-Species Comparison” in National Geographic
- Ria Misra’s article “There is a class of animals that never grow old”
- Teisha J. Rowland’s book Biology Bytes: Digestible Essays on Animals Both Commonplace and Bizarre
Discussion
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