In this world, it is inevitable that all living organisms age and die – sadly, there is no way of escaping death. But not all organisms follow the same pattern of weakening and deterioration towards old age and death, counterintuitive as it may seem. I stumbled upon the idea of ‘negligible senescence’ when doing research on certain tree species, and it took me down a deep rabbit hole on how some species (both plant and animal) can actually decrease their odds of death as they age, or show minimal signs of ageing at all. To start at the beginning, let’s look at what the term ‘negligible senescence’ means.
Getting older is an inevitable part of life, and often animals are thought to be nearing closer to death as they age. But is it possible to show no signs of ageing, or to even have a better chance of survival the older you get?
What is Negligible Senescence?
The term ‘senescence’ refers to the process of deterioration with age. When organisms age, they usually show signs of this ageing: like reduced ability to reproduce, getting weaker, and overall they are more likely to die as they get older. But in some species, scientists have not seen these signs of ageing, even as the organisms grow older.
Negligible senescence occurs when mortality risk is stable or decreases with age. In other words, these organisms don’t become more likely to die as they get older. In fact, their risk of dying stays the same or even reduces as they age. Some species even seem to get healthier as they age (at least for some part of their life). This is called ‘negative senescence’. Certain animal and tree species have fascinated scientists because they seem to defy the typical ageing process seen in most species.
Ageless Tortoises
According to Rita da Silva, a Danish biologist, some species of turtles and tortoises have found ways to slow down or even completely switch off senescence. In her study, she finds, that their pattern of ageing does not resemble that of humans and out of 52 species, 75 % of them show extremely slow deterioration as they age.
At Londolozi, we often see the Leopard Tortoise and Speke’s Hinge-back Tortoise. Although the negligible senescence of these two particular species has not been directly studied, much time has gone into researching other species in their family. With both of these species living up to 100 years and 50 years respectively, there is not much in their physical appearance (despite their size and number of ridges on their shell) that easily gives away their age.
So how does this work?
Tortoises, and other animals such as certain fish, sharks, birds and naked mole rats, maintain their youthful characteristics and health for most of their lives. While this concept is not fully understood, there are a few key factors that help explain it in simpler terms:
- DNA Repair: Negligibly senescent animals have highly effective DNA repair mechanisms. DNA carries the genetic instructions for an organism, and over time, it can accumulate damage from various factors like radiation, toxins, and metabolic processes. These animals are exceptionally good at repairing and maintaining their DNA, which helps them avoid the typical age-related cellular damage.
- Efficient Metabolism: These animals often have highly efficient metabolic processes. Metabolism involves converting food into energy and managing waste products. Efficient metabolism helps reduce the production of harmful byproducts and oxidative stress, which can contribute to ageing in other species.
- Protective Mechanisms: Some of these animals have evolved unique protective mechanisms against age-related diseases. They may have enhanced immune systems, antioxidants, or other biological defences that keep them healthy as they age.
- Reproductive Strategies: In some cases, negligible senescence may be linked to the animal’s reproductive strategy. Species with low reproductive rates and longer lifespans may have evolved to invest more resources in staying healthy for extended periods to increase their chances of reproducing successfully.
Time-Defying Trees
This phenomenon has not only been studied in animals but also in other organisms in nature. Trees are the longest-living organisms on earth. Many trees live hundreds, even thousands of years, sending down new roots and sprouting new branches as their bodies grow. There are many reasons why trees live so long:
One of the reasons for their long life is that rather than declining in health and productivity with age, some trees actually get healthier and more productive. Another reason for a long life is that large trees have multiple stems or trunks, so even if one trunk dies there are others still alive.
The Leadwood Forest provides some of the oldest trees in Londolozi. Radiocarbon dating done in South Africa has established that a leadwood tree can live up to 1070 ± 40 years. A tree can remain standing for many years after it has died.
Trees are incredible in their ability to store energy internally in their roots and trunks. This energy storage enables the tree to recover quickly from fires or other damage because it has the resources available to make new branches, leaves and roots. Trees also defend themselves against disease with chemicals they produce called phenols.
Phenols act as antiseptics and antibacterial agents that deter pests and parasites. These phenols are stored in special cells just under the bark, and as trees age, they have greater concentrations of phenols, thus increasing their protection.
Intriguingly, as we continue to unravel the mysteries of negligible senescence in trees, these ancient giants serve as a living testament to the resilience and timelessness of the natural world.
On the Other Hand
It must be noted that with any scientific research, there are always alternative theories. Recent work done by Canwei Xia and Anders Pape Møller shows that some wild animals that are not expected to live very long, age slower than we thought. This discovery helps explain why some wild animals don’t seem to get old quickly.
While crocodiles can live up to around 70 years of age, they are another species of animal that don’t seem to show signs of ageing with time, despite increasing in size. The authors suggest that age-defying animals might just be a trick of nature, and they still age like other animals, however much, it might not seem so – particularly with reptiles.
They also suggest that because many of these animals live so long, researchers haven’t yet been able to observe their whole lifespan. They argue that animals might only appear to not be ageing, but in actual fact, the “ability to escape from senescence is possibly only a beautiful illusion in animals”.
I love discovering topics like this that are at the forefront of scientific research. There are so many unanswered questions, and still a lot of uncertainty around this topic. Many studies argued both for and against this idea, with some believing that we have not been monitoring certain species for long enough throughout their lifetime to claim them as being negligibly senescent. However, like with any theory, there needs to be a starting point to trigger more research in years to come. This kind of exploration is highly beneficial to us as humans as we often attempt to obtain insights from other creatures to better understand our own biology.
Robyn Morrison
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