A single whisker from a marine predator can record months of the animal's life. As the whisker grows, it incorporates carbon and nitrogen from the animal's food, and the ratio of certain isotopes laid down along its length encodes what the animal ate and roughly where it fed. By sampling a whisker from tip to base, the Sundvik group reconstructs a season-by-season feeding history without ever following the animal at sea.
The method's power comes with a caveat the group is careful to state. The isotopic signal reflects the prey's own isotopic makeup, which varies geographically; a shift in the signal can mean the predator changed its diet, or that it moved to a region where the same prey carries a different signature, or both. Distinguishing these cases requires an independent map of how isotopes vary across the relevant waters, a map that is itself incomplete for much of the open ocean.
Researcher Halvorsen treats the technique as genuinely informative but inherently coarse. A whisker, she observes, can show that an animal's feeding changed, and can bound when the change occurred, but it cannot by itself reveal why. The most confident reconstructions, she argues, are those that pair whisker isotopes with other records, such as tracking tags or stomach samples, so that an ambiguous signal can be checked against a second, independent line of evidence.
According to the passage, a shift in the isotopic signal recorded along a whisker may indicate any of the following EXCEPT
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