Observation: Reports of grey seals predating on mammals have involved only one male at a time, but these behaviours have been reported at multiple locations.
Although males do travel between sites, the fact that these behaviours have been spotted in widely different parts of the North Sea within a similar time frame tends to rule out the idea of an individual “rogue” male as the culprit for all cases of cannibalism/infanticide/porpoise predation. So how are similar behaviors occurring in all these locations?
One possible hypothesis is that this new foraging strategy is spreading between and within populations via cultural transmission or social learning. For example, bottlenose dolphins learned to forage using sponges from the seafloor as tools, and a recent study has shown that the cultural learning of foraging specialties, passed from mother to offspring, has directly shaped the evolution of killer whales (while some killer whale groups specialize on fish, others prefer mammals as prey). In both these examples, it is hypothesized that in the past, a few behaviourally flexible individuals began to utilize new habitats, ecological niches, or tools for accessing resources, and then group culture passed that strategy along through the generations.
It seems that in cases of cultural transmission, the social structure and behavioural ecology of the animals is one key to facilitating the spread of behaviours. Killer whales form stable, matrilineal groups (aka: Grandma is running the show!) and bottlenose dolphins have complex social networks between relatives and often feed cooperatively. Grey seals are not typically considered “social” foragers and after 18 days of nursing pups are left to figure out how to be a ‘seal’ on their own. However, adult seals are in close proximity during the breeding season and many individuals return to the same place each year—suggesting their neighbors might be fairly stable over time. We know that some behaviours used during breeding such as the male “Body Slap” or “Yodel” are unique to specific breeding colonies, and are assumed to be maintained in the population through some form of learning. We also know that grey seals can learn vocalization patterns.
Although cultural transmission does occur in marine mammal and other wildlife populations, we should also consider the possibility that the recent cases of grey seals predating on mammals simply reflect single individuals independently switching food resources and that the behaviour is not spreading. However, since our understanding of seal behaviour (discussed above) suggests that it is possible that predation on marine mammals and cannibalism could spread in a population of seals, it will be interesting to gather evidence to test these two hypotheses.
How would we do this? As mentioned before, the first and most important step is ensuring we report rare behaviours. Then, identifying the initial, focal individuals performing rare behaviours is essential to determining if it is spreading. In the case of lob-tailing for humpback whales, a behaviour that involves a whale throwing its tail up into the air and slamming it back down on the water, the very first use this technique was observed in 1980. With consistent follow-up monitoring and reporting, researchers were able to actually track the behaviour as it spread from individual to individual over time!
Comprehensive photo-ID databases are essential to this type of monitoring and for determining if and how behaviours spread. Grey seals can be identified as individuals by the patterns on their pelage (fur) and females are currently monitored using this method throughout the UK. So to monitor cannibalism and marine mammal predation, we might consider incorporating males into these existing photo-ID projects. Males are sometimes considered trickier to match due to darker colouration, less distinct ‘blotchy-ness’ and a habit of rolling in mud and sand, but as our research has shown, it might be difficult but not impossible!
Once males have been identified, we can then track the locations of predation events, observations of their interactions with other males and build long-term databases that again give us a baseline to compare future observations against. Answering these questions however requires long-term funding and consistency of effort. With modern technology of drones, smart phones and apps, can you think of any cost-effective ways to provide this information to scientists?
While many of these questions seem to be ‘blue skies’ research (aka science for the sake of learning), this information will be vital for managers making policy decisions and for conservation as well! The observations of cannibalism, and the findings of subsequent necropsies, have provided information to managers and policy makers trying to make decisions on shipping regulations and how to best protect declining populations of harbor seals. Our colleagues are tackling some of these pressing conservation issues; and knowing if such behaviours are becoming more or less frequent, how or if they are spreading, and what is driving the males’ behaviours and decisions will all be essential to broader ecosystem conservation in the future.
*Map locations approximated from these reports:
- Boyle, D. P. (2009). Grey seal breeding census: Wildlife trust of south and west Wales (CCW Regional Report, CCW/WW/10/04).
- Bouveroux, et al. (2014). Direct evidence of gray seal (Halichoerus grypus) predation and scavenging on harbor porpoises (Phocoena phocoena). Marine Mammal Science, 30, 1542-1548. http://dx.doi.org/10.1111/mms. 12111
- Haelters, et al. (2012). The grey seal (Halichoerus grypus) as a predator of harbour porpoise (Phocoena phocoena). Aquatic Mammals, 38(4), 343-353. http://dx.doi.org/10.1578/AM.38.4.2012.343
- Jauniaux et al. 2014. Bite injuries of grey seals (Halichoerus grypus) on harbour porpoises (Phocoena phocoena). PLOS ONE, 9(12), e108993. http://dx.doi.org/10.1371/journal.pone.0108993
- Leopold et al. (2015a). Exposing the grey seal as a major predator of harbour porpoises. Proceedings of the Royal Society B, 282(1798), 20142429. http://dx.doi.org/10.1098/rspb.2014.2429
- Leopold et al, (2015b). Porpoises: From predators to prey. Journal of Sea Research, 97, 14-23. http://dx.doi.org/10.1016/j.seares.2014.12.005
- Stingell et al. (2015). Predation of harbour porpoises (Phocoena phocoena) by grey seals (Halichoerus grypus) in Wales. Aquatic Mammals, 41, 188-191. http://dx.doi.org/10.1578/AM.41.2.2015.188
- van Bleijswijk, J. D. L., Begeman, L., Witte, H. J., IJsseldijk, L. L., Brasseur, S. M. J. M., Gröne, A., & Leopold, M. F. (2014). Detection of grey seal Halichoerus grypus DNA in attack wounds on stranded harbour porpoise Phocoena phocoena. Marine Ecology Progress Series, 513, 277-281. http://dx.doi.org/10.3354/meps11004
- van Neer, A., Jensen L. F., & Siebert, U. (2015). Grey seal (Halichoerus grypus) predation on harbour seals (Phoca vitulina) on the island of Helgoland, Germany. Journal of Sea Research, 97, 1-4. http://dx.doi.org/10.1016/j.seares.2014.11.006