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Another porpoise battering
At 16:00hrs on Thurs 3 April, we received a call from a member of the public reporting a stranded baby dolphin lying adjacent to the harbour in Whitehills near Banff. Arriving promptly on scene, however, we were not too surprised (usually anticipated when folk phone in a baby dolphin) to discover a harbour porpoise (Phocoena phocoena) floating lifelessly in the quay.
Ironically the CRRU's boat Orca II (which is usually moored in this harbour) was out being serviced for the forthcoming field season, but with dry suits and ropes to hand we managed quite easily to reach the carcass (whilst Bob Reid from the Scottish Agricultural College was contacted to collect the animal for necropsy).
On recovery of the porpoise, however, we were shocked to discover perhaps the worst example of interspecific aggression any of us had ever seen. Inspection of the animal's torso revealed multiple lacerations and puncture wounds all over the body; the inter-dental distances between the inflictions indicative of multiple attack strikes by one or more bottlenose dolphins. This young female had literally had the life beaten out of her, and these injuries were almost certainly the reason for her death. It was a very sad sight altogether. As Barbi White (CRRU) attending the scene put it, "this shy little creature, that has such a hard time just to survive in this harsh environment, has been through such an ordeal that makes me shudder to contemplate".
Photographs were taken of the porpoise and the carcass was placed into a bodybag, which Bob Reid arrived promptly to collect before we all went our separate ways...
There are several theories for this particular interspecific aggression towards harbour porpoises by bottlenose dolphins in the Moray Firth. The most consistent reasoning is explained in terms of infanticide in bottlenose dolphin populations. Infanticide has been well documented in this species, and can be explained in a similar manner to that seen in lion communities. When a conquering male lion wins his right to a new pride of females, he is keen to ensure that all of his female pride quickly bear his own offspring. Consequently, any young cubs belonging to the former male of the pride are killed by the new male. Removing the offspring of the lionesses from the equation ensures that these females come into oestrus far more quickly and are therefore receptive to the new king of the pride.
Female bottlenose dolphins produce a single calf every 3-4 years. An amorous male dolphin, who knows that a calf belonging to a female is not his own, is not going to wait around for this female for 3 years in order for her to bear his own calf. In a similar logic to the lion, if the male dolphin can remove this calf, the female may be receptive to him within a month and bear his own seed. This may explain why schools of bottlenoses are primarily made up of females and calves - there are many more advantageous to living together in a school than simply finding food and sharing maternal responsibilities! (Of course, this also explains why female dolphins are polygamous - by having multiple relationships with a variety of males, the males themselves cannot be sure whether they are the father or not, and in the confusion the new born calf is spared!)
So how does this relate to porpoises? Well, results from autopsies of deceased harbour porpoises known to have been killed by bottlenose dolphins have revealed that the animals targeted are of a similar size and weight to that of a bottlenose calf. The suggestion here is that the breeding success of adult male bottlenoses in a small population may be related to their skills in removing calves fathered by rival males, and that these skills may be acquired by actively practising them on the more numerous harbour porpoises, i.e. the skills required to separate a single porpoise from its school, pursue it and kill it would be similar to those needed to remove a bottlenose calf.
Whilst competition for food resources or habitat selection have also been suggested as alternative reasons for this behavioural phenomenon, the latter cannot be explained in terms of energetics, as the porpoises are pursued far outwith predicted territorial boundaries, the chase continuing until the animals have been killed. Out of 410 harbour porpoise necropsied by the Scottish Agricultural College since 1992, 137 (33.4%) have died to date as a result of attacks by bottlenose dolphins. This stranding represents yet another example of the hard brutality and evolutionary pressures of the marine world.
To learn more about the social ecology of the bottlenose dolphin and the harbour porpoise visit our fact file section HERE.Back to news