Events in February 2014 and media coverage
On 17 February I assisted the Scottish Marine Animal Strandings Scheme (SMASS) with the collection of two dead harbour porpoises from St Cyrus beach in Aberdeenshire. The skin of both porpoises exhibited extensive tooth raking from bottlenose dolphins, and at this stage an attack by dolphins is presumed to have caused their deaths. These animals were transported to the Scottish Agricultural College in Aberdeen where they await post-mortem.
Two days later I happened to notice some commotion in the sea just outside my house, and was able to witness an aggressive dolphin-porpoise interaction firsthand. Several bottlenose dolphins were involved in a prolonged chase of a single porpoise, involving the porpoise being rammed and thrown violently into the air on multiple occasions. I was able to capture some distant images of this event from my window.
Because this event was likely to be of interest to the Scottish marine mammal community and was also relevant to the stranded animals I had collected two days earlier, I shared the images of the attack with SMASS and they were published on the SMASS Facebook site along with some images of the stranded animals.
Subsequently, various media took the images of the strandings and the attack from the SMASS Facebook site and merged them to create an over-sensationalised and incorrect news ‘story’ which bore little resemblance to the facts. Examples of this include articles in the Daily Mail (http://www.dailymail.co.uk/news/article-2572968/Pictured-Horrific-moment-bottlenose-dolphins-attack-kill-two-porpoises-FUN-cat-mouse-game.html) and the Huffington Post (http://www.huffingtonpost.co.uk/2014/03/05/dolphins-kill-porpoises-pictures_n_4903116.html). None of these sources discussed the events with me before using my images and publishing their stories.
Several elements of those media stories were incorrect:
- The two dead animals collected from St Cyrus and the bottlenose-porpoise interaction photographed from my house were separate events on different dates and at different locations (several miles apart). The media reported the various images as relating to a single event which was not the case.
- I did not observe two porpoises being killed by dolphins as reported in the media. Rather, I witnessed a single porpoise being attacked by several dolphins and the outcome of the attack remains unknown. However, a single dead porpoise exhibiting dolphin tooth rakes was washed up in the same area one week later (SMASS, pers. comm.).
- The media reported that the stranded porpoises had “horrific injuries to their heads, and deep, gaping holes around their eyes”, implying that the wounds were the result of the dolphin attack. However, those wounds were almost certainly unrelated to dolphins and were most likely the result of scavenging by birds following death.
- For the media to state that porpoises were attacked by dolphins “for fun” and to report dolphins to be “psychopathic killers” (Huffington Post) is not only sensationally anthropomorphic but is not supported by any credible evidence. Potential biological reasons for the attacks by dolphins on porpoises have been investigated in several peer-reviewed scientific papers (see below).
- If dolphins have an image as “giggling, smiley-faced cutie pies” (Huffington Post) then that is predominantly the result of media misrepresentation, and is equally as inappropriate as the negative portrayal in these articles. Biologists and naturalists who observe dolphins in the field understand that they are wild animals with a complex suite of behaviours which scientists are still at a very early stage of interpreting.
It was disappointing that the media were so eager to take a series of interesting observations and use them to create an incorrect and exaggerated story without first establishing the facts. Since over 60% of porpoises stranded in Scottish waters since the 1990s exhibit signs of attack by bottlenose dolphins (Ross & Wilson, 1996), it is debateable whether this story even represents ‘news’.
Why do bottlenose dolphins attack harbour porpoises?
Attacks by dolphins on porpoises have been investigated in several scientific papers. Bottlenose dolphins and harbour porpoises do not prey upon each other, and consequently a predatory basis for the interactions can be eliminated. However, several other mammal species engage in inter-specific killings where direct predation is not the primary driver, for example when eliminating potential competitors for food resources (e.g. wolves killing coyotes).
Ross & Wilson (1996) suggested several potential explanations for dolphin attacks on porpoises including competition for food, interspecific interference in feeding activities, perception by dolphins that porpoises may represent a threat to young or ill dolphins, object-orientated play, practice-fighting, sexual frustration and aberrant behaviour of a few individuals. Patterson et al. (1998) pointed out that aberrant (i.e. atypical for the species) behaviour by particular individuals was unlikely since similar attacks have been documented in other locations around the UK. Attacks have now been documented as far afield as California (Cotter et al., 2011), further reducing the likelihood of aberrant behaviour as a feasible explanation.
While Spitz et al. (2006) provided some evidence for dietary overlap between bottlenose dolphins and porpoises in the Bay of Biscay, Cotter et al. (2011) found that the species did not overlap greatly in diet in Pacific waters where several attacks have been documented. Competition for food resources is therefore unlikely to be a sole explanation.
The possibility of bottlenose dolphins attacking porpoises in order to develop the skills used during infanticidal behaviour has also been proposed (Patterson et al., 1998). Cotter et al. (2011) found that only male dolphins participated in observed attacks on porpoises in Californian waters, which may lay credence to this hypothesis.
Cotter et al. (2011) considered it likely that multiple factors may play a role in the expression of these aggressive interspecific interactions.
In addition to porpoises, attacks by bottlenose dolphins on at least four delphinid species have recently been documented in UK waters (Barnett et al., 2009). Similarly, apparently aggressive behaviour towards harbour porpoises by other dolphin species has been recorded, for example by Pacific white-sided dolphins (Baird, 1998) and white-beaked dolphins (Haelters & Everaarts, 2011). These observations suggest that the underlying reasons for the attacks may apply also to other small odontocete species and are not solely limited to bottlenose dolphins and harbour porpoises.
Trying to establish the underlying drivers for such behaviours is one aspect of what makes dolphins, and other mammals, such fascinating study subjects.
Baird, R.W. (1998). An interaction between Pacific white-sided dolphins and a neonatal harbor porpoise. Mammalia, 62: 129-134.
Barnett, J., Davison, N., Deaville, R., Monies, R., Loveridge, J., Tregenza, N. and Jepson, P.D. (2009). Postmortem evidence of interactions of bottlenose dolphins (Tursiops truncatus) with other dolphin species in south-west England. Veterinary Record, 165: 441-444.
Cotter, M.P., Maldini, D. and Jefferson, T.A. (2011). “Porpicide” in California: killing of harbor porpoises (Phocoena phocoena) by coastal bottlenose dolphins (Tursiops truncatus). Marine Mammal Science, 28: E1-E15.
Haelters, J. and Everaarts, E. (2011). Two cases of physical interaction between white-beaked dolphins (Lagenorhynchus albirostris) and juvenile harbour porpoises (Phocoena phocoena) in the southern North Sea. Aquatic Mammals, 37: 198-201.
Patterson, I.A.P., Reid, R.J., Wilson, B., Grellier, K., Ross, H.M. and Thompson, P.M. (1998). Evidence for infanticide in bottlenose dolphins: an explanation for violent interactions with harbour porpoises? Proceedings of The Royal Society of London B, 265: 1167-1170.
Ross, H.M. and Wilson, B. (1996). Violent interactions between bottlenose dolphins and harbour porpoises. Proceedings of The Royal Society of London B, 263: 283-286.
Spitz, J., Rousseau, Y. and Ridoux, V. (2006). Diet overlap between harbor porpoise and bottlenose dolphin: an argument in favor of interference competition for food? Estuarine, Coastal and Shelf Science, 70: 259-270.