This wasn’t the first case where male grey seals were observed targeting mammals for food.
In the weeks preceding the cannibalism event, our small band of researchers had spent more than one breezy hike across the island discussing the new evidence that male grey seals were predating on harbor porpoises in the North Sea. It was exciting and thought-provoking because while initial observations of males handling porpoise carcasses could have just been opportunistic scavenging, new DNA evidence in wounds of stranded porpoises, and a recent report with direct video observations of a seal chasing own a live porpoise seemed to be changing our perceptions. Little did we know that in the following weeks, a paper would be submitted describing a male grey seal predating a harbor seal in Germany, and that we would witness a male cannibalizing a pup ourselves.
Answering both these questions is actually pretty tricky. From the >20 years of monitoring on the Isle of May and other UK colonies like North Rona, nothing like this has been observed before. But other places haven’t had as continuous of observations, so we don’t know yet whether these cases reflect a real increase in a previously rare behaviour, or that greater monitoring effort is revealing what a few aberrant animals have always been doing.
Since we don’t have a reliable baseline for the past, a first step to answering this might be to determine how common it is for males currently to be eating mammals through a diet study. Traditionally, diet can be examined by studying feces from live animals, or stomach contents of dead animals. In our observations, the male seemed to only eat the blubber and a bit of the skin of the pup, so looking for hard parts in stomachs and scats would not provide a good representation of diet in this case.
Instead, we might be able to employ modern molecular techniques such as stable isotope analysis. Stable isotopes are chemical signals in animal tissues like whiskers, feces or hair. By analyzing these tissues, we can track where in the food-web individuals are foraging. For example, as you forage higher up the food chain, the amount of Nitrogen isotope (δ15N) increases. So a seal eating small fish like sand lance would have a lower Nitrogen signal than seals eating bigger fish like pollock, and a seal eating other mammals would have an even higher level than either of those. By testing a number of colonies, we might be able to understand what percentages of populations appear to be eating ‘above’ their usual diet of fish.
From this information, we could then track over time if the proportion of fish/mammal eating seals is changing in certain locations, or remains the same.
Maybe it is more common in certain areas than we thought.
Maybe there are places where it isn’t seen at all now–but if it appears in the future we will know roughly when and where it started.
Maybe it is just one or two males that are doing this and the ‘increase’ is a coincidence.
Diet studies are a great tool for these types of questions, but due to the time and costs of collecting samples and analysis, we might still miss subtle shifts or changes. That is why keeping a vigilant eye out for and reporting rare behaviours and strandings is also essential. Can you think of a way to involve the community in such an effort?
For the sake of this thought experiment, let’s make the big assumption that these cases are in fact increasing in frequency; then the next question to consider would be “Are these observations early behavioural indications of shifts in the ecosystem?”
Polar bears have been observed cannibalizing young, eating geese, and chasing reindeer. Researchers have suggested that this shift from their ‘usual’ diet of seals could be the result of reductions of sea ice limiting their natural foraging behaviours. With changes in the ecosystem, some individuals might be opportunistically taking advantage of novel food sources. Is this what we are seeing with grey seals eating porpoise, harbor seals and cannibalizing? Or are the two unrelated?
To really test this hypothesis however, we need more than just a collection of a few observations across time and coincidence. Instead of being an afterthought, rigorous attention to and recording of rare or unusual behaviours is necessary for generating a timeline and baseline to which future changes can be compared. We also need to start thinking outside our species-specific boxes and take a broader, ecosystem-level approach to monitoring and understanding changes. For example, to test the hypothesis that competition or reduction of available prey resources in the North Sea is driving these behaviours; we need information on changes in: grey seal populations, the diets and distributions of other top predators, industrial activities, fish population dynamics, environmental variables, oceanographic processes….the list goes on. Obviously no single scientist can do it all; but through opening doors to interdisciplinary collaborations, we can start to work on some of these difficult, but pressing questions.