Whale of the Month - Atlantic White-Sided Dolphin

Species fact file

Common names: Atlantic white-sided dolphin

Icelandic name: leiftur

Scientific name: Lagenorhynchus acutus

Family: Delphiniidae (dolphins)

Max length: 2.7m (males), 2.5m (females)

Distribution: Offshore waters of the cold temperate North Atlantic

Atlantic white-sided dolphins are possibly the most colourful species of cetacean, sporting impressive and striking patterns that make them very distinctive. Despite their unique colouration, white-sided dolphins can be easily confused with white-beaked dolphins while out at sea, as they share a very similar distribution.

An Atlantic white-sided dolphin shows off its unique, colourful patterns. Image from Wikipedia.

They are an oceanic species, and prefer deeper waters – so they are not often seen close to land. In addition to this, Atlantic white-sided dolphins are very widely ranging and can travel long distances in a short time. This makes this species particularly difficult to study, and their population size is not well known – especially in the eastern Atlantic. Currently, the best estimate for the population in the whole Atlantic ranges from 150,000 to 300,000.

Despite preferring deep, offshore regions, Atlantic white-sided dolphins are not deep divers. Their maximum recorded dive time is 4 minutes, but most last less than 1 minute. These dolphins are fairly generalist in their diet and will take a variety of prey, including herring, cod, mackerel, sand lance and squid. A study in the eastern Atlantic found that mid-water fishes and squids are probably the most important prey for white-sided dolphins here.

A social species, Atlantic white-sided dolphins live in groups. Group size seems to depend on location: in Icelandic waters, most consist of less than 10 members. These social groups don’t appear to be defined by relatedness, but what other factors could be involved in group formation are as yet unknown.

White-sided dolphins are often seen associating with other cetaceans, particularly other dolphin species as well as baleen whales such as fin or humpback whales. The dolphins may seek out baleen whales for energetic benefits, as they use the waves created by these larger species to ‘bow-ride’, making their travelling more efficient. Alternatively, these associations may give the dolphins an easy meal: they can scavenge on fish that are brought to the surface by feeding whales.

Atlantic white-sided dolphins are known to be very active at the surface and are commonly seen breaching, tail slapping, and bow-riding. These are likely to be social behaviours rather than part of foraging, as they are most often performed in large groups.

There is ongoing discussion about the genetic relationships between the Atlantic white-sided dolphin and other species. The genus Lagenorhychus contains six species: Atlantic white-sided dolphins, Pacific white-sided dolphins, white-beaked dolphins, dusky dolphins, Peale’s dolphins, and hourglass dolphins. But genetic studies consistently find that most of these species are more closely related to others outside the genus than they are to each other. A paper in 2019 suggested reassigning five of these six dolphins to other genera, a move which would see the Atlantic white-sided dolphin rebranded from Lagenorhynchus acutus to Leucopleurus acutus. Although it is yet to be officially adopted, change is on the horizon for the Atlantic white-sided dolphin and other members of the Lagenorhychus genus.

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Revision of the Lagenorhynchus genus from studies of genetics, biogeography, and acoustics