Animal Tree Rings Aid Biologists in Species Survival

We have a natural fascination with time - how landscapes have been carved over millennia, how our bodies grow and sag with age, how the stars traverse the sky each night. Scientists probe the layers beneath our feet to understand the secrets of our past. Geologists and palaeontologists sample ice, rock and fossils to reconstruct past climates and species and archaeologists pick through ancient "dustbins" (middens) in excavation sites to reimagine our historical dinner time.

Similarly, most living things produce records of their own existence in layered body tissues - often in the form of daily or yearly growth bands. The most familiar of these so-called biochronologies are tree rings, which form every year in response to seasonal cycles in temperature and rainfall.

Dendrochronology - the art of tree-ring counting - allows us to precisely date trees. Based on the rings in its trunk, a bristlecone pine in eastern California known as Methuselah is said to be the world's oldest living thing at 4,856 years old.

It's not just the number of rings, either - their width tells us whether the tree was thriving in a particular year, or suffering due to drought. Chemical compounds locked into the wood offer clues about atmospheric changes, including those produced by volcanic eruptions .

Let's not not stop at trees - your own tooth cement, nails and hair are forming chemical and visual records of your own life experience right now, storing traces of food, drink and drugs you have consumed. They can also produce "stress marks" during trauma or pregnancy, when a mother literally breaks her own body tissues to grow and nourish her baby.

Elsewhere in the natural world, some of the more surprising examples of biochronologies include whale earwax , narwhal tusks , bird feathers and the bony plates (scutes) on turtle shells .

Recent studies, for instance, have applied forensic analyses of whale earwax to explore their stress levels during historic whaling days. Narwhal tusks , meanwhile, have helped explain how declining Arctic sea ice has affected their diet and exposure to pollution.

The importance of otoliths

In my lab, we work with aquatic animals - from fish scales and ear bones to squid eyes and beaks. Like decoding a biological black box, we analyse chemical constituents in the growth layers to reconstruct a detailed picture of the individual's prior health, diet and movements.

Some biochronologies are more "fickle", forming layers at unpredictable rates, including the eye lenses of fish and turtle scutes. Others, such as bird feathers, are shorter lived due to periodic moulting. Yet they all share the important feature of serial growth, producing valuable archives that we can probe to build a picture of the animal's life.

Probably the best known biochronometer in the animal world - and my own personal obsession - is the fish otolith, or ear bone (Ancient Greek: oto is ear and líthos is stone). We humans have tiny ear stones (otoconia), whose primary function is to maintain balance, but fish otoliths are also crucial for hearing, as well as featuring specific properties that make them particularly valuable markers of biochronology.

Unlike "normal" bones, fish otoliths are composed of calcium carbonate crystals and are metabolically inert, meaning they never get broken down and rebuilt. Instead they keep growing - even during periods of starvation - producing daily and annual growth bands.

These beautiful crystalline structures are also highly resistant to degradation and vary in shape between species. This enables scientists to use a combination of "otolith atlases" and artificial intelligence to identify popular choices of fish from otoliths left behind in ancient human middens, as well as in the contemporary stomach contents or poop of predators such as seals, albatrosses and squid.

Otoliths have driven my research for almost two decades. I've been fascinated by animal migration and the ecological and evolutionary processes underpinning these long and dangerous journeys ever since taking a "movement ecology" class at the University of Edinburgh with the brilliant Professor Victoria Braithwaite in 2003.

I decided I wanted to track marine animals myself, and my lab now primarily uses otolith and eye lens chemistry to reconstruct fish habitat use and growth rates, and the temperatures they experienced through their lives. We are now also investigating how well these same structures track reproductive events, chronic stress and exposure to pollution.

And we are working with international teams to understand how hypoxia (low oxygen zones or "dead zones") affect fish growth and reproduction. Ultimately, this data allows us to connect stressful events in a fish's past to its lifetime health and survival, which is important for predicting a species' persistence.

For example, a recent study used otolith-derived metabolic rates of Atlantic bluefin tuna to show their vulnerability to future climate change. Meanwhile in California, we used otolith chemistry to understand the impact of dams on salmon migration and survival, revealing that - on many rivers - dams have made it impossible for salmon to escape into the mountains during summer, which is essential for enabling them to resist the increasingly severe droughts afflicting the region.

Conservation

Fisheries managers read the rings on millions of otoliths each year to track individual cohorts and look for warning signs of overfishing, but I would argue that biochronologies are still underused in this field. For example, fisheries managers could use otoliths to track the movements of juveniles too small to be tagged (those under 4cm long), since chemical markers make it possible to identify where they grew up. This would allow these managers to earmark productive or struggling "nursery habitats" for protection or improvement, respectively.

We consistently find that rivers and estuaries play a critical role in the survival and growth of valuable species such as salmon, sea bass and anchovies. Juvenile fish often have such high natural mortality rates - often only 1% survive to their first birthday - that even small improvements to their survival can result in large boosts in abundance and make wild fisheries more sustainable.

As such, let's keep up the momentum to clean and restore our rivers and beaches, and to embrace monitoring tools such as biochronologies to learn which actions produce the biggest benefits. Next time you think about banging the glass at an aquarium, just remember that the fish inside are listening - and recording you too.

Anna Sturrock receives funding from a UKRI Future Leaders Fellowship