Bottlenose Dolphins: The Ultimate Sea Bully? (A Guest Post)

By Kayla Fuller

Imagine this situation: you’ve brought your favorite lunch to work. Everyone is jealous of your food, continuously eyeing it up. A few coworkers, who have brought in disappointing lunches in comparison, approach and demand that you hand it over. After you refuse, they beat you until your body lies lifeless and they take your lunch anyway.

Woah, woah, woah… that took a dramatic turn!

Photo of a harbour porpoise, taken by AVampireTear (Wikimedia Commons)

But for harbour porpoises in the northeastern Atlantic, this fight for food has become a reality, and bottlenose dolphins are the suspected culprit. In 1996, Harry M. Ross (SAC Veterinary Services, U.K.) and Ben Wilson (University of Aberdeen, U.K.) documented fractured rib cages, damaged internal organs and joint dislocations of deceased harbour porpoises in the northeastern Atlantic. Why would bottlenose dolphins be causing such damage? Who could ever associate such a cute and cuddly creature with a horrific crime like this?

Photo of a bottlenose dolphin, taken by NASA (Wikimedia Commons)

Researchers Jérôme Spitz, Yann Rousseau, and Vincent Ridoux with the Center for Research on Marine Mammals: Institute for Coastal and Environmental Research at the University of La Rochelle in France become the judge and jury in this trial. Jérôme, Yann, and Vincent obtained 29 harbour porpoises and 25 bottlenose dolphins that had been beached and died in the Bay of Biscay (between Spain, France, and England). At the time of the study, more harbour porpoises were being found dead in the bay than in previous years. They hypothesized that bottlenose dolphins and harbour porpoises may have had similar enough diets to cause competition and violence between the two species.

Photo of a harbour porpoise that received injuries thought to be from a
bottlenose dolphin before death (circled), from Ross and Wilson (1996)

The researchers’ goal was to analyze stomach contents to directly see what each mammal was eating at the time of their death. To do this, Jérôme, Yann, and Vincent removed the stomachs from the harbour porpoise bodies and weighed them with all contents included. After weighing stomach casings separately, they calculated total weight inside of the animals’ stomachs. Then, they washed stomach contents through a filter to separate out larger matter. Now, if you have a weak stomach, this probably wouldn’t be the job for you. Jérôme, Yann, and Vincent separated food items within the stomachs into identifiable categories. It could sometimes be difficult to recognize whole animals in a stomach due to breakdown, so methods like pairing dismantled eyes or counting fish bones was necessary to identify them! This same process was repeated for bottlenose dolphin carcasses. From there, the scientists compared specimens for prey presence, abundance, mass, and size to see if there was overlap between diets of the harbour porpoises and bottlenose dolphins.

So what did they find? More food mass, a greater number of species, and a more diverse size range of prey was found in the stomachs of bottlenose dolphins in comparison to harbour porpoises. Although bottlenose dolphins have a habitat that includes more deep-ocean areas while harbor porpoises inhabit coastal surroundings, certain prey species were eaten by both. Since bottlenose dolphins are bigger and hunt in larger groups, they would logically be more dominant in a face-off over a common prey item. Why are they fighting more over the same foods? This shift could be a result of humans harvesting species from the ocean that are diet items for bottlenose dolphins. It could also be a result of warming ocean temperatures that could be changing the dwelling places of available food for bottlenose dolphins. This would explain why more habour porpoises are being attacked by these marine tyrants moving into shallower waters.

Poor porpoises, all they want to do is eat their lunch in peace. Who knows, maybe in the next few million years, we’ll see highly evolved harbour porpoises covered in spikes to ward off the dolphins. That’ll teach those bullies!


References:

Ross, H., & Wilson, B. (1996). Violent Interactions between Bottlenose Dolphins and Harbour Porpoises Proceedings of the Royal Society B: Biological Sciences, 263 (1368), 283-286 DOI: 10.1098/rspb.1996.0043

Spitz, J., Rousseau, Y., & Ridoux, V. (2006). Diet overlap between harbour porpoise and bottlenose dolphin: An argument in favour of interference competition for food? Estuarine, Coastal and Shelf Science, 70 (1-2), 259-270 DOI: 10.1016/j.ecss.2006.04.020