At 10am on 27 June 2018, researchers studying dolphins in a Belize mangrove, witnessed a gruesome spectacle. First, a group of four bottlenose dolphins separated a baby manatee from his mother. Then, one of them attacked the calf, biting it over and over again, and ramming it out of the water. The youngster later died from its injuries.
Animals kill other animals all the time, but there is usually an explanation. Animals kill for food, for dominance, for mating rights or to avoid being killed. Sometimes, however, there is no obvious motive.
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Dolphins don’t eat manatee calves, nor are manatee calves a threat to the cetaceans. According to the scientists who witnessed the encounter, the aggressors here had nothing to gain. Not was this a one off.
Between 1999 and 2020, researchers documented ten baby manatee attacks by dolphins, of which at least two were fatal.
When animals kill excessively, it’s called surplus killing. Dolphins killing manatees is one example, but the term is typically used to describe the behaviour of predators which kill more prey than they can immediately eat.
Foxes, for instance, will sometimes kill a glut of chickens, leading to the practice’s alternative name of ‘henhouse syndrome.’ Here are five animals that practice surplus killing.
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Bottlenose dolphins
Dolphins don’t just kill baby manatees – they also kill porpoises. Harbour porpoises are smaller and lighter than bottlenose dolphins, so they pose no threat. They are also not eaten.
In recent years, researchers have documented a growing number of dolphin-on-porpoise attacks. In 2016, six porpoises were part of a larger group found stranded on the Baltic Sea coast of Schleswig-Holstein.
The two calves and four adults stood out because they bore the characteristic marks of a dolphin attack, including rib fractures, internal haemorrhages and specific teeth marks that match the mouths of dolphins. Similar attacks have also been documented in UK and American waters.
Orcas
Porpoises do not get off lightly. Orcas, otherwise known as killer whales, are not true whales but the world’s largest species of dolphin.
Over the last 60 years, one particular population, living off the north-west Pacific coast of North America, have chased down at least 78 porpoises, and killed 28 of them. Like the dolphins, they didn’t eat the porpoises, but here at least, there is a possible explanation.
The whales often targeted young porpoises that were similar in size to their preferred prey item, the adult Chinook salmon. Adult orcas engaged in the killing, but the behaviour was most common amongst juveniles. It’s possible, therefore, that the porpicide was a form of hunting practice.
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Grey wolves
Grey wolves are social animals that hunt and fell their prey in large, co-ordinated groups. Typically, the carnivores kill one animal at a time, such as an elk or a deer, and then feed on it for a couple of days.
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Waste not want not, they often consume more than 90 per cent of a carcass’s edible bits, including organs, meat and bones. Occasionally, however, they go rogue.
In winter 2016, one grey wolf pack is thought to have dispatched 19 elk in a one night. Nine individuals belonging to a group called the Rim Pack, killed two adult cows and 17 calves in Wyoming.
Surplus killings like this are rare but when they do occur, it’s often during the winter months when it may represent the wolves caching food for the hard times ahead.
Midges
Corethrella appendiculata is a little midge, with a life cycle comprising four stages: egg; larva; pupa and adult. Two of these stages are predatory.
In their adult form, Corethrella midges are specialised parasites that feed mainly on frog blood. In its larval form, C. appendiculata lives in water-filled cavities in trees, where it predates other aquatic invertebrates, such as mosquito larvae.
Sometimes, the larvae of C. appendiculata really go to town. A single midge larva can kill around 200 mosquito larvae by biting them on the thorax (the bit inbetween the head and the abdomen).
This is far more than they can eat and is thought to be an adaptation to their crowded environment. They may kill extra prey to reduce competition from neighbouring species.
Domestic cats
In the UK and many other countries, most domestic cats are well-fed by their owners. They don’t need to bring home dismembered bodies, yet the behaviour is so instinctive, very often they do.
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As part of a 2025 UK study called ‘What the Cat Dragged in’, 553 outdoor domestic cats were monitored over three and a half years. While most cats brought home a maximum of one prey item per month, a handful of serial killers regularly returned over 15 individuals.
Scaling the numbers up, this means that the UK’s pet cats kill around 140 million wild animals per year, most of which are small mammals. This presents a threat to local wildlife, so if you’re concerned, add a bell. Some studies suggest that cats which wear bells bring home less wildlife.
Top image: a breaching orca hunting a harbour porpoise. Credit: GeoStock/Getty Images
