‘What? You're 40? You don't look a day over 30!’ This is sweet music to some of our ears! We all know that every animal is born, grows, matures and ages in a predictable way. After all, ageing is part of the life cycle of every living thing on Earth. Yet, scientists still do not have a clear picture as to why some people do not look their age, while others age much faster than the majority. Is age just a number or is it much more than that?

To answer this question, veterinary epidemiologist Stephanie Venn-Watson from Epitracker, Inc., CA, USA, along with a team of researchers from institutions in California and Arizona, USA, turned to studying ageing in captive bottlenose dolphins. In addition to being extremely lovable, dolphins are a great study organism because they are long lived and show similar afflictions to humans when they age, such as loss of memory, inflammation in the joints, anaemia (or low blood iron levels) and type II diabetes. Working with the US Navy Marine Mammal Program – an organization training and caring for aquatic mammals – the research team accessed blood samples collected from dolphins from 1994 to 2018. The group was particularly interested in how blood composition changes over time and whether that correlated with ageing rates in 44 dolphins.

The scientists found that haemoglobin, which is an iron-containing protein found in red blood cells that carries oxygen to tissues, declined throughout the lifespan of the dolphins, leading to low blood iron levels and anaemia in old age. In fact, the group concluded that faster ageing dolphins, meaning those that showed more markers of ageing than others, had larger declines in blood haemoglobin than those dolphins that aged slower over a period of 30 years. Anaemia is also a characteristic of ageing in humans, often referred to as ‘anaemia of old age’. The work conducted by Venn-Watson and her colleagues suggests that not only are dolphins a great model to study human ageing but also that the ageing rate itself may depend on declines in blood iron levels over time.

In addition, the researchers found that platelets – small cells in the blood that help form clots – along with lymphocytes, which are white blood cells involved in the body's immune response, can also be used as markers of ageing rate in bottlenose dolphins. Both platelet and lymphocyte levels declined in fast ageing animals, while in those with slower ageing rates, the decline over 30 years was much slower. Interestingly, the authors did not find any differences in ageing rates between male and female dolphins, even though the females are known to live longer in the wild. The authors attributed this to the lifestyle of the animals: here, they were kept in captivity and were well cared for, while in the wild, the males are more exposed to predation because of their vast territory, and are more aggressive than the females and, therefore, more prone to injury by other males, making them more at risk of dying.

In their study, Venn-Watson and her colleagues found that bottlenose dolphins, much like humans, age at different rates, despite being held in identical conditions. Those animals with faster ageing rates also tended to be more susceptible to age-related conditions, such as anaemia and a compromised immune system. These findings may dare scientists to look at the genetic code behind these ageing systems, to try to slow down ageing in humans or, at the very least, improve the quality of life in the ageing population. It may be the case that in the not too distant future, age will, indeed, be just a number.

E. D.
N. J.
A 25-y longitudinal dolphin cohort supports that long-lived individuals in same environment exhibit variation in aging rates
Proc. Natl. Acad. Sci. USA