New research suggests dolphin infanticide in Japan

Our friend Leanne Rosser from Japan just made a disturbing yet fascinating discovery: she observed a potential dolphin infanticide event (the killing of an infant) in the population of Pacific white-sided dolphins she studies in Mutsu Bay, Japan. In this post, she tells us about this event she witnessed, as well as the reasons behind infanticide in dolphin societies. Finally, she explains that infanticide is not completely uncommon in the animal world.

dolphin infanticide
The research team witnessed the brutal event in Mutsu Bay, Japan — Credit: Leanne Rosser

Let’s go to Japan

Dolphins have a surprisingly friendly reputation. As a researcher of these weird and wonderful creatures, I sometimes wonder how they earned this amiable image, especially after a rare observation in Northern Japan. Our team at Mutsu Bay Dolphin Research studies the ecology and behavior of Pacific white-sided dolphins on their annual migration into the bay every spring. Our boat-based surveys have been collecting data for our photo-ID project since 2016. We hadn’t previously encountered a young calf until June 2020, when one was spotted under attack.

A dramatic event…

A great commotion of splashing in the distance grabbed our attention. As the boat edged nearer, we found a group of around ten dolphins, four of which (one male, three of unknown sex) engaged in multiple aggressive behaviors directed at the calf. An attacker would ram using their head or flip the baby into the air. Working together, they frequently sandwiched the calf. In an attempt to drown the baby, attackers would also push down and submerge it. Rake and bite marks were visible on various parts of the calf’s body. The attackers even drew blood.  Other species of dolphins have used these strategies when attacking calves or harbor porpoises.

The calf caught in the middle of the attack – Credit: Leanne Rosser

However, what came after 54 minutes of consistent aggression is not usually considered part of this attack. Previously unseen individuals arrived at the scene, six of which (three males, one possible male, and two of unknown sex) took over the attack with increased vigor. The first group made no more attempts to harass the baby. Why would the first group devote so much time and effort to the attack only to give up? Analysis conducted for this study showed that the second group attacked the calf significantly more in a shorter period of time. This could, along with the fleeing of the first group, indicate their dominance. Or could it have been an act of cooperation? The second group pitching in after the first had drained their energy?

Mom to the rescue

Despite the group switch, one individual remained present throughout the entirety of the event. No matter how often the attackers separated the pair, this adult made persistent efforts to stay with the calf. As well as trying to support the baby at the surface, the dolphin also chased after the attackers. “Epimeletic behavior” is when an animal helps another in distress, such as mothers protecting their calves. This relentless struggle helped us identify this individual as the probable mother. The second group of dolphins was divided into two subgroups; one surrounded the mother while the other attacked the baby.

The fate of the baby

For 75 grueling minutes, attackers struck 115 times. Throughout the observation, we identified 23 dolphins either in or around the attack group. Although we were not able to directly witness the calf’s death, the tenacity of this kind of attack most likely resulted in an unfortunate end. The internal injuries that are likely to be sustained from the flipping and ramming behaviors are usually fatal.

The calf was most likely the victim of a dolphin infanticide — Credit: Leanne Rosser

Why would dolphins want to kill a baby?

This is the first record of possible infanticide in Pacific white-sided dolphins, but it is not uncommon within the repertoire of dolphin behavior. Infanticide, the killing of an infant, is a widespread phenomenon throughout the animal kingdom, with various factors influencing the behavior. Dolphins only give birth every 2-4 years, up to 4.5 years in Pacific white-sided dolphins. Calves remain highly dependent on their mothers for long periods.

Sexual selection

Previous research has suggested that the sexual selection hypothesis is a possible reason behind infanticide in dolphins. This is when a male can increase his reproductive success by killing an unrelated calf. The baby’s death will end the mother’s lactation period, making her ready for mating again. Half of the attackers were identified as male or possibly male, and sexual behavior was directed at the presumed mother throughout the observation. This could support the sexual selection hypothesis.

Males showing dominance

However, sexual behavior and aggression could also reflect high testosterone levels during a mating period. Male sexual frustration may also have been a factor, especially if there are more males than females in the population. Male dolphins can also become sexually ‘excited’ when displaying dominance. Furthermore, the mother-calf pair were more vulnerable in Mutsu Bay. Female dolphins tend to travel together in nursery groups with their calves. Around Japan, calves this young are more frequently spotted further north in Funka Bay, Hokkaido. This kind of exposure away from the relative safety of a nursery group may have given the males an opportunity to attack.

An encounter of this nature is rare, and the motives behind an attack on a baby can be hard to fathom. Yet it grants us further insight into the complexities of dolphin life and the pressures of survival in the animal kingdom.


Rosser, L. R. et al. Calf-Directed Aggression as a Possible Infanticide Attempt in Pacific White-Sided Dolphins (Lagenorhynchus obliquidens). Aquatic Mammals 48, 273-286 (2022)

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Leanne is a dolphin researcher for Mutsu Bay Dolphin Research in Japan. Her research interests include social behavior and all things Pacific white-sided dolphin. After more than five years in Japan, she is soon to start studying for her Marine Biology MSc at Bangor University in Wales.

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