Every scientist starts with a question.
For marine scientist Dr Jessica Rudd from the University of Exeter, questions about the movements and behaviours of marine life have taken her on a journey around the UK, from tracking Atlantic bluefin tuna in the waters of southwest England to investigating basking shark breaching [2], an elusive behaviour which may provide clues about reproduction. By attaching tiny electronic devices known as tags to marine animals, Jessica can understand how they move through a watery world that we rarely get to see.
Researchers call this use of tags to study wild animal behaviour “field biologging”. Think of the tags as smartwatches for wildlife. Instead of counting our steps, these devices measure stats such as depth, water temperature, light levels and even movement, allowing scientists to reconstruct where an animal has travelled and how it behaves. Every dive, turn and sudden burst of speed is another clue about life beneath the waves. It’s almost as if the research team can swim alongside the fish underwater!
One of the things she loves most about biologging, Jessica tells me over Teams from where she is currently based in Exeter, is that the animals rarely follow your research plan. You might begin with one scientific question, only for the data to reveal something completely unexpected…
A twist in the tale...
One tag told a story that no one expected. As part of her PhD, Jessica’s supervisor handed her an unexplored dataset from an Atlantic bluefin tuna [1] which had been tagged back in 2018, and asked her to take a closer look.
For this project, Atlantic bluefin tuna caught by recreational anglers in Falmouth had been fitted with “MiniPAT tags”, small electronic devices that use satellites to transmit data, which are attached near the dorsal fin of the animal. The tags can remain attached for up to two years before they are programmed to detach and float to the surface. In the first twist of fate in our story, one of the tags eventually washed up on a beach in France – before being discovered by a passing dog walker. It was posted back to the UK – carrying months of underwater information with it.
Initially, Jessica hoped the data from the tag might reveal new information about tuna migration routes in the Bay of Biscay.
At first, everything looked normal. The tuna carried out regular deep dives, moving up and down through the water column in a predictable pattern in search of prey. In the purple box in the graph below, the repeated peaks and dips in the pitch and roll mirror changes in depth, showing the smooth, controlled swimming behaviour expected from a healthy Atlantic bluefin tuna.
Then Jessica noticed that the data (or rather, the tuna it came from) started to behave strangely…
As you can see in the red box, at about 2am in the Bay of Biscay, the tuna suddenly shot upwards, rocketing towards the surface at 150% greater acceleration than usual, and pitching and rolling erratically in ways that a fish simply shouldn’t.
“Tuna don’t work like that,” Jessica confirmed.
The scientists realised that the data only made sense if the tag was no longer attached to a living tuna. The frantic outburst of movement suggested the tuna had been chased. The sudden warmth and complete darkness indicate that afterwards, it had been swallowed whole.
One final clue sealed the case. Just before the light disappeared, the tag detected a brief flash of light at around 2am. There was only one possible explanation for this: moonlight. For the sensor to detect it, the tag had to be at or just above the surface of the water. At exactly the same moment, the roll data showed the tuna had rotated almost 180º, suggesting it was held belly up, with the tag briefly exposed to the sky before disappearing into darkness.
By now, Jessica and the team were convinced: a predator had entered the story.
A predator enters the story.
Instead of recording a live tuna, the tag was now recording behaviour inside the stomach of whatever had eaten it – but what?
The first clue to the culprit was temperature. A reading of 35ºC suggested the mystery predator wasn’t a fish at all, but a warm-blooded marine mammal. The depth data added another piece to the puzzle: whatever had swallowed the tag was regularly surfacing to breathe before diving back down again. Tuna are a tasty treat for a large variety of aquatic life including sharks, but sharks are fish that don’t breathe air or maintain body temperatures anywhere near 35ºC.
Using diet and geographic distribution of marine mammals, the team had narrowed the suspects down to just three species of cetaceans found in the Bay of Biscay: the killer whale (orca), the false killer whale, and the pygmy killer whale.
The ongoing data from the tag showed that the predator made its deepest dives during the day – this didn’t match the pygmy whale, which typically dives deeper during the night. Next, the longest recorded dive lasted 11.1 minutes – shorter than the dives expected from false killer whales, which can remain underwater for up to 14.6 minutes.
By process of elimination, the team had their answer. The tag wasn’t recording the final moments of a tuna anymore, but the hidden hunting behaviour of one of the ocean’s most intelligent predators: the orca.
A dramatic recreation of events – by Kim Conroy, Tap Root Visuals.
The secret lives of orca.
By complete accident, the research team had ended up with inside access to the lives of orcas.
They noticed small periodic drops in internal temperature, suggesting that the orca was swallowing other cold-blooded prey, as cetaceans don’t tend to drink seawater. The researchers counted 77 separate feeding events. It seems that rather than one enormous meal, orca enjoy feeding regularly in smaller portions.
At night, orcas hung out fairly close to the surface, making dives of only around 20 metres. In the day, however, they regularly reached depths of up to 120 metres (that’s about the same as Egypt’s tallest pyramid!). This coincided with favoured feeding times, suggesting that whales may be ambushing tuna at the deepest points of their dive. The distinctive black and white colouring of an orca may even work as camouflage through a phenomenon known as countershading, making it harder for tuna to see them against the changing light of the ocean at deeper depths.
Whilst the tag could tell researchers what was happening inside the predator, it couldn’t confirm what was happening outside. That changed when the team compared their findings with 36 minutes of video footage using a CATS cam (a small camera attached to an orca with a suction-cup tag), from CIRCE (Conservation, Information and Research on Cetaceans). Snapshots from the footage (below) captured three orcas feeding on an Atlantic bluefin tuna, with one individual holding the fish while the others tore pieces of it away. Instead of competing over the meal, they worked together, taking turns to feed before leaving the tuna carcass on the seafloor.
Jessica explained that food sharing helps strengthen the social bonds within an orca pod, where cooperation is often essential for successfully hunting large prey. As orcas must regularly surface to breathe, working as a team allows them to tire out larger fast-swimming prey before sharing the reward: a process known as endurance-exhaustion [3] . The tagged bluefin tuna measured around 1.75 metres, which is no easy target and would certainly require collaboration!
Animals don’t read research proposals...
Just like that, what began as a simple project tracking Atlantic bluefin tuna became an unexpected window into the behaviour of one of the ocean’s most iconic predators. This work revealed not only how orcas hunt and share prey, but also how tightly their survival is linked to the animals they depend on. As climate change pushes Atlantic bluefin tuna further north into cooler waters4, understanding these prey-predator relationships will become increasingly important for predicting how marine ecosystems may change in the future.
For Jessica though, one of the biggest lessons wasn’t about orcas at all. It was about the nature of science itself.
“Animals don’t care what your research question is,” she laughed. “The amazing thing about tagging animals is that they lead the way. You can go into a project with one question in mind, but the animal is going to do whatever it wants which can ultimately lead you to different questions and discoveries”.
That’s what makes biologging so powerful. Every tagged animal carries back a story scientists never could have scripted. The same dataset can be revisited years later to answer completely new questions as technology improves. With smaller, less invasive tags now collecting increasingly detailed information and even real-time data, researchers can build a bigger picture of how animals migrate, hunt and respond to our changing oceans. When these behavioural patterns are combined with information on shipping, fishing activity or climate change, they become powerful tools for protecting marine wildlife.
It’s a beautiful example of how sometimes the biggest breakthroughs don’t happen because we find exactly what we were looking for, they happen because we’re curious enough to follow where the data leads. After all, mother nature has always been writing the story, we are simply learning how to read it.
How to find another way...
Scientists can’t be everywhere at once. Citizen science projects allow anyone to contribute observations that help researchers monitor wildlife, track environmental change and answer questions we haven’t even thought to ask yet. Whether you’re recording whales, photographing mammals or surveying rock pools, every observation helps build a clearer picture of how wildlife is changing.
Here are a few projects you could get started with:
Animal Tracker App (Max Planck Institute): Report the movements of animals who have already been tagged and follow their journeys- https://www.icarus.mpg.de/29143/animal-tracker-app
Shoresearch (Wildlife Trust): Explore your local coastline while recording the incredible wildlife found in the intertidal zone- https://www.wildlifetrusts.org/get-involved/citizen-science-projects/shoresearch#volunteer
OceanWatchers (ORCA): Become a marine mammal surveyor and learn how to spot and ID whales and dolphins and understand their behaviour- https://orca.org.uk/training/oceanwatchers
MammalWeb: Deploy camera traps to monitor wild mammals or act as a spotter where you identify species from the images- https://www.mammalweb.org/en/
Bugs Matter (Kent Wildlife Trust): Count squished bug numbers during car journeys to monitor the health of flying insect populations- https://www.kentwildlifetrust.org.uk/get-involved/our-projects/bugs-matter
Zooniverse: Choose from several projects based on your interests- https://www.zooniverse.org/projects?discipline=biology
Remember how the tag from the tuna eventually washed up on a beach in France? It was recovered by a member of the public before finding its way back to Jessica and the research team. If you ever come across any animal tags in the wild, getting in contact with the organisation that deployed it can really help scientists recover valuable data. You never know what incredible story that tiny device might hold.
Acknowledgements
A huge thank you to Dr Jessica Rudd for generously sharing her research journey with me, as well as the amazing resources used in this article. I’m excited to see what future projects and discoveries lie ahead!
References
Primary source
[1] Rudd, J.L. et al. (2024). “High-resolution biologging of an Atlantic bluefin tuna captured and eaten by a supposed orca”. Scientific Reports 14 (29352). https://doi.org/10.1038/s41598-024-80744-0
Other sources
[2] Rudd, J.L. et al. (2021). “High resolution biologging of breaching by the world’s second largest shark species”. Scientific Reports 11 (5236). https://doi.org/10.1038/s41598-021-84670-3
[3] Guinet, C. et al. (2007) ‘Killer whale predation on bluefin tuna: exploring the hypothesis of the endurance-exhaustion technique”. Marine Ecological Progress Series, 347, pp. 111-119. https://doi.org/10.3354/meps07035
[4] Erauskin-Extramiana, M. et al. (2019). “Large-scale distribution of tuna species in a warming ocean”. Global Change Biology 25, 6, pp. 2043-2069. https://doi.org/10.1111/gcb.14630
