• Leanne Rosser

When dolphins attack: writing my first paper

When I first moved to Japan I never imagined that I would be a part of the research opportunities I've had here. At best I thought I could possibly find some volunteer work that had some vague association with cetaceans but not actual research. And so to have published my first paper is a surreal yet brilliant bonus to my time in this beautiful country.


© Leanne Rosser, Mutsu Bay Dolphin Research

It all began during my time in Aomori working for Mutsu Bay Dolphin Research on one of our boat surveys. It was a lovely June morning and we were surveying the Wakinosawa coast searching for the Pacific white-sided dolphins that visit every spring, getting shots for photo ID and studying their behaviour. This particular day their behaviour was very…unusual. Splashing and quick surface-rushes are often an indication that a group is feeding, however today feeding was not their priority. As our boat slowly approached the commotion, our cameras zoomed-in for a closer look on the group, we saw something small being flipped in the air. It soon became apparent that the small unidentified creature was a baby Pacific white-sided dolphin, possibly around 2 months old, and this group were in the middle of an attack. We stayed with the group of approximately 10 dolphins, documenting this rarely observed behaviour, and witnessed calf-directed aggression continue from four attackers (1 male, 3 unknown sex). Individuals would ram the baby with their head or rostrum (beak), flip the baby into the air or work together to sandwich and squeeze the baby.

© Leanne Rosser, Mutsu Bay Dolphin Research

Often the dolphins would submerge the

calf in an attempt to drown it and the multiple rake marks found all over the calf's body showed that they also used their teeth in the attack, biting the calf and even drawing blood.


One particular dolphin made persistent attempts to stay with the calf, desperately trying to rejoin the calf whenever the pair were separated. This individual also appeared to support the infant at the surface, trying to rescue it from its attackers. When an animal assists another in distress it is referred to as epimeletic behaviour, and in this case suggested the individual to be the mother. Along with the calf-directed aggression, the attackers pursued the suspected mother displaying sexual behaviour and separating her from her calf throughout the observation.

Mother and calf - © Leanne Rosser, Mutsu Bay Dolphin Research

Something which is not usually observed during this kind of behaviour is a group switchover. After 54 minutes however, a second group arrived on the scene and the attack was taken over by 6 new aggressors (3 male,1 possible male, 2 unknown sex). This second group attacked with more vigour and ferocity than the first group who seemingly gave up. The second group often split into two subgroups, one that focused their efforts on attacking the baby and the other that guarded the presumed mother. The attack group switch could reveal second group dominance or alternatively cooperation, with the second group pitching in after the first used up their energy.


After 75 minutes of watching the event unfold, with a total of 23 different dolphins identified throughout the observation and the baby still desperately trying to flee, we had to leave the area as our survey time was up. The fate of the baby was never revealed…but the attack was brutal and persistent enough to assume an unfortunate end to the calf's life. After our encounter and back on dry land, we all felt a mix of emotions. Of course the observation was difficult to watch but as this is the first report of such behaviour in Pacific white-sided dolphins, the data collected is an invaluable addition to the knowledge and understanding of the species.

© Leanne Rosser, Mutsu Bay Dolphin Research

So why would this group of dolphins want to kill a baby?

Infanticide, the killing of young, is a rarely witnessed phenomenon at sea but not an uncommon occurrence for a variety of species. For dolphins and other mammals with long inter-birth intervals, it has been suggested that a male will kill an unrelated calf in order to end the mother's lactation period and bring her back into estrus so that he can mate with her. By killing an infant a male dolphin can increase his reproductive success. Yet other factors may also be attributed to such behaviour, like male sexual frustration as a result of a skewed male to female ratio or being less experienced than older males, or high testosterone levels coinciding with mating season. In our observation the mother-calf pair were slightly more vulnerable to male harassment due to their small group size and location. Dolphin mother-calf pairs usually travel together in the relative safety of nursery groups and small calves are usually seen further North in Funka Bay, Hokkaido. This is actually the first time we have seen such a young calf in five years of surveying the Wakinosawa area so being there may have left the baby exposed to danger. Apart from a possible ‘helper’ dolphin ramming an attacker at the beginning, the mother-calf pair in our observation were mainly left to fend for themselves.


I began work on this paper shortly after our encounter in July 2020 and it has taken until now (May 2022) for it to be published. The process began with myself and co-authors making sense of the photos and videos from the event and identifying individual dolphins, using photo-ID, and their attack tactics. The behavioural types were categorised (ram, flip, sandwich, submerge and bite) and attack 'bouts' were counted within and between groups. After this, statistical analysis was conducted to determine who were the dominant attackers and which of the two groups performed the most attack behaviours. The first draft was then

submitted to the scientific journal by the end of December 2020 and by February 2021 I had my first taste of scientific criticism. The process was quite tough. This paper had to go through three rejections and multiple alterations in accordance with the expertise of the reviewers. A manuscript must go through peer review where around three professionals from the field will examine your work, give feedback and decide if it is scientifically worthy enough to be published in a journal. Although getting rejected multiple times feels frustrating, the paper was hugely improved from these comments and it was a vital learning experience for me as a researcher. I’ll admit there were times when I felt very disheartened and that it would never be published but with the help of co-authors and science friends, I persevered. I really wanted to get this story out into the world and highlight our work at Mutsu Bay Dolphin Research, so I'm really happy it's finally published.


This June I will be back in Aomori to conduct more research with the team. The unpredictability of life at sea always excites me. Watching wildlife is a constant surprise and dolphins are especially remarkable to observe. Who knows what we will see this year!


To access our paper 'Calf-directed aggression as a possible infanticide attempt in Pacific white-sided dolphins' published in Aquatic Mammals click here.

Feel free to get in touch to discuss this work: leannerachaelrosser@gmail.com

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