On a cool crisp December morning in 2014, on our study colony of the Isle of May, Amy (now Dr Amy Bishop) spotted, and recorded a rare and startling behaviour – a clear case of cannibalism. Amy has a detailed description of this and other instances of cannibalism about to be published in the Journal Aquatic Mammals. The story has also been picked up by New Scientist.
From a distance of about 50m, Amy observed an adult male grey seal dragging a weaned seal pup, grasped in his jaws, across the rugged terrain of the Isle of May colony. The pup was alive while being carried, and was calling. The male hauled the pup into a freshwater pool. He then proceeded to force the pup under the water, still gripping the pup by the scruff of the neck in his jaws. The male then anchored the pup’s lower body to the ground with his fore flippers, and wrenched his jaws upwards, using his powerful neck muscles to tear the skin and blubber of the pup. Amazingly, the male then proceeded to tear strips of blubber and skin off using his teeth and swallow chunks of the pup. Amy had witnessed, and fortunately video recorded, a rare, but complete instance of cannibalism in a wild grey seal! Over the course of the next 7 days, a team of researchers from Durham University and the Sea Mammal Research Unit (SMRU) carefully watched the area where this male spent most of his time on the colony; they witnessed the male perform a similar pattern of behaviours 5 more times and carcasses found around the colony suggest a total of 11 pups were killed in this manner.
This is a rare but fascinating, if somewhat horrific, behaviour because of other recent observations of grey seals eating other marine mammal species, including harbour seals and porpoises around the UK coast. Cannibalism by grey seals has been formally reported in the early 1990s in Canada, but Amy’s observation was the first complete observation of a grey seal eating another grey seal in the North Sea!
Grey seals are long lived animals: Females can reach ages greater than 30, males over 20 years. Each autumn, grey seals come ashore at remote island sites, where females give birth to single pups that were sired the previous year. Males provide no parental care and compete to mate with females who become fertile 14-16 days after giving birth. Males and females tend to be site faithful – they come back to the same spots on the same island over multiple years to breed. Amy’s observations are particularly intriguing, because the best way for a male to gain the most mating opportunities with females is to stay ashore on the island for as long as possible. Normally, males attempt to do this by building up a thick layer of fat (blubber) prior to the breeding season while feeding out at sea. A male’s blubber reserves fuels his activity while on the island, where there is no access to a grey seal’s normal marine diet. But, Amy’s observation suggests that this cannibal male may have found another way to extend his stay on the colony – by eating pups!
But, if the male is in the same location on the colony that he held in the previous year, then surely he would be eating his own pups? Not a good strategy! This is where having the complete record of the cannibalism observation, and not just watching a male eat a carcass, comes in handy: Amy observed this male moving over 40m away from his usual location (the pool), bypassing numerous pups, in order to select victims that were distant from his ‘territory’ on the colony. Could it be that this male is deliberately targeting pups outside of his territory that are unlikely to be his own offspring? Researchers at Durham are keen to find out!
Gaining more understanding of this unusual behaviour will be challenging. Long term studies of grey seal behaviour by Dr Sean Twiss (Durham University) and Dr Paddy Pomeroy (SMRU) spanning over 30 years turned up no observations of cannibalism – so it may be that that this is not only a rare behaviour, but also a relatively new behaviour. An intriguing question is whether other males might learn to adopt this strategy too.
Only continued long term behavioural studies using skilled observers to identify individual seals will help to unravel these mysteries. Throughout the duration of this blog series, we have spent some time discussing the merits, and importance of doing “non-sexy” science; of sitting quietly and observing animals performing their natural behaviours. In fact, our research group has built our entire program on these methods, and our publication list stands as a testament to the broad topics that can be addressed using individual-based behavioural observations. However, in addition to these organized studies, every once in a while, something unexpected happens. A rare behaviour, a strange sighting, a missing piece of a puzzle. We will be the first to say that these sorts of sightings can always, in a way, be attributed to luck—being in the right place at the right time. However, even with our research team being in the right place at the right time, it requires an attentive disposition to stay focused for over 8 hours of observations, an observant mind to notice something strange is happening, and naturalist instincts to turn the camera on, and let nature take its course.
We would like to add that if you have any observations of rare grey seal behaviours: cannibalism, predation on harbor seals, predation on porpoises, or just something you think was ‘weird’—we’d love to hear from you!! Pictures and videos are best, but even just a description is helpful, and you will be given full credit in any further use of your sighting.