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Using the DNA of dolphins to know their age

One of the most fundamental pieces of information about individuals (including dolphins) is their age. In biology studies, scientists need to know how old an animal is to understand many aspects of its life history and, for example, to estimate the viability of a population.

But estimating an animal’s age is no easy feat when its date of birth is unknown. Because there is no universal indicator for the aging process, it is especially difficult to age long-lived animals. In this post, we explain the results of a new study where researchers used Indo-pacific bottlenose dolphins’ DNA to calculate their age.

Knowing the age of different dolphins is important to study their population’s health. Speckling patterns on their skin can provide a rough estimate – Credit: Dr Simon J. Allen

How can you age cetaceans?

Whales and dolphins are a particular challenge, as they are very mobile and difficult to capture. A proven method of age estimation is to extract a tooth and analyze the growth layers of its dentine. This way, scientists can determine an individual’s age quite accurately. However, this method is highly invasive and difficult to use with live animals. For some species, such as Indo-Pacific bottlenose dolphins, the speckling pattern of their skin can indicate age. However, individuals differ in speckling intensity. It is also often difficult to see enough of the individual’s body, and the estimate is rough at best.

Because of these difficulties, most studies rely on long-term data collection, where animals have been regularly observed since their birth, and thus their age is known.

Shark Bay dolphins

Now that we can age the dolphins, we can understand more about their lives and biology – Credit: Dr Simon J. Allen

One of these long-term studies takes place in Shark Bay, Western Australia. There, the Shark Bay Dolphin Project, a population of Indo-Pacific bottlenose dolphins (Tursiops aduncus), has been studied for more than 30 years, using photo-identification and biopsy sampling. Because of this, we know the age of many of the animals, and we have small skin samples from them. This knowledge has now opened doors to new approaches to age estimation. Because we have DNA from several animals with known ages, we used an approach that estimates the age of a dolphin using epigenetics.

What is epigenetics?

Epigenetics studies how the environment and an individual’s behaviors can affect how genes work. Epigenetic changes can basically alter gene expression in the body and turn genes “on” and “off.” As animals age, their epigenetic patterns change throughout their life, also referred to as the ‘epigenetic clock.’

How does the epigenetic clock work?

Aging changes the DNA methylation levels (small additions to the DNA) in the body. Newborn dolphin calves have the highest level of DNA methylation, and old individuals have the lowest. Using DNA from skin samples, we measured DNA methylation levels in 165 dolphins with known ages. We then calibrated an epigenetic clock which we used to estimate the age of other dolphins of the same species based on their methylation levels. Our clock had an average accuracy of 2.1 years, which is really good!

We can now age dolphins simply by looking at their DNA! – Credit: Dr Katharina J. Peters

What is it useful for?

Dolphins can live for more than 30 years, making it very difficult to gather a large sample size of animals with known ages. Using the epigenetic clock, we can now estimate the age of individual dolphins based on their DNA, which means we only need a small skin sample of the dolphin, and we are good to go! This makes our clock extremely useful for scientists that study Indo-Pacific bottlenose dolphins but do not have access to long-term data.

What are the next steps?

We now know the age of many dolphins in the Shark Bay population. With this knowledge, we can ask further questions regarding their life histories, their timing of reproductive success, and differences in aging patterns between males and females. As so often in science, an answer leads to many more questions! This is an exciting time for us, and we cannot wait to get deeper into this work!

Authors: Katharina J Peters and Livia Gerber

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