Grampus griseus

Grampus griseus

Risso’s dolphin (Grampus griseus (G. Cuvier, 1812)) is a cetacean belonging to the Delphinidae family.

Systematics –
from a systematic point of view it belongs to:
Eukaryota domain,
Kingdom Animalia,
Subkingdom Eumetazoa,
Superphylum Deuterostomia,
Phylum Chordata,
Subphylum Vertebrata,
Infraphylum Gnathostomata,
Superclass Tetrapoda,
Mammalia class,
Subclass Theria,
Infraclass Eutheria,
Superorder Laurasiatheria,
Order Cetacea,
Suborder Odontoceti,
Superfamily Delphinoidea,
family Delphinidae,
Subfamily Globicephalinae,
Genus Grampus,
G. griseus species.
The term is basionym:
– Delphinus griseus G.Cuvier, 1812.
The terms are synonyms:
– Delphinus aires G.Cuvier, 1812;
– Delphinus griseus G.Cuvier, 1812;
– Delphinus griseus Gray, 1828;
– Delphinus orca Temminck & Schlegel, 1841;
– Delphinus risso Risso, 1826;
– Delphinus rissoanus Desmarest, 1822;
– Delphinus rissoi Gervais, 1859;
– Globicephalus rissii Hamilton, 1837;
– Gramphidelphis griseus Kellogg, 1940;
– Grampidelphis exilis Iredale & Troughton, 1933;
– Grampidelphis griseus Iredale & Troughton, 1933;
– Grampidelphis kuzira Iredale & Troughton, 1933;
– Grampus cuvieri Gray, 1846;
– Grampus griseus subsp. richardsoni Gray, 1850;
– Grampus griseus subsp. sakamata (Gray, 1846);
– Grampus griseus subsp. sowerbianus (Fischer, 1881);
– Grampus griseus subsp. stearnsi (Dall, 1873);
– Grampus richardsoni Gray, 1850;
– Grampus rissoanus (Desmarest, 1822);
– Grampus sakamata Gray, 1846;
– Grampus sakata Tomilin, 1957;
– Grampus sowerbianus Fischer, 1881;
– Grampus stearnsii Dall, 1873;
– Phocaena griseus F.Cuvier, 1836;
– Phocaena rissonus F.Cuvier, 1836.

Geographic Distribution and Habitat –
Grampus griseus is a circumglobal pelagic cetacean that lives in deep seas even if it often approaches the coast.
It lives worldwide in temperate and tropical waters, in the Indian, Pacific and Atlantic oceans, also in the Persian Gulf, the Mediterranean Sea and the Red Sea, but not in the Black Sea (one stranding was recorded in the Marmara Sea in 2012). Their range extends north to the Gulf of Alaska and southern Greenland and south to Tierra del Fuego.
Since 2017, Risso’s dolphins have begun to appear off the Norwegian coast, in Bleik’s Canyon, Andenes. Repeated regular sightings imply an enlargement of their natural range. Possible explanations are changing climate and currents, as well as northward expansion of prey species or competition with other species in their natural range, such as pilot whales.
In the Mediterranean Sea it is quite common, especially in the Ligurian Sea, in the Tuscan Archipelago and north of Sicily. It is present in abundance in the Gulf of Taranto.
Their preferred habitat is just off the continental shelf on steep banks, with water depths ranging from 400 to 1,000 m and water temperatures of at least 10°C and preferably 15-20°C. Dives to a depth of 600 m in pursuit of prey are recorded.

Description –
Grampus griseus is a cetacean that can reach a length of 4 m and a weight of 500-600 kg.
It has a head without a rostrum and a rounded but not globular forehead, which has a characteristic V-shaped furrow in the middle, with the apex facing downwards.
The upper jaw protrudes slightly.
The dorsal fin is located about halfway down the body, very high, pointed and sickle-shaped. The pectoral fins are long and pointed. It also has very few teeth because it feeds on slimy prey such as squid.
The livery is unmistakable; newborns are of a uniform very light gray colour, but as they grow they first become brownish and then the slate gray of the adult. Over the years, the body is covered with numerous and extensive light scratches, which end up making it take on an almost white color, especially in the front part.
It is believed that these scratches are an effect of social interactions, but the possible adaptive function of this very particular depigmentation remains a mystery. Some scratches are thought to result from squid bites. In some specimens a thin and barely hinted saddle cloth is visible. On the ventral side there is an anchor-shaped whitish spot, similar in shape and position to that of pilot whales.

Biology –
Grampus griseus has a gestation which takes about 13-14 months, with intervals of 2.4 years.
Calving reaches seasonal peaks in winter in the eastern Pacific and summer and fall in the western Pacific.
Females sexually mature at age 8-10 and males at age 10-12. The oldest specimen reached 39.6 years.
The chick, just born, is about 1.5 m long; the weight of the newborn is not known.
Furthermore, a little known detail of the reproduction is that, it seems, that in the Mediterranean they mate in autumn.

Ecological role –
Grampus griseus is the only species of the genus Grampus Gray, 1828.
These cetaceans feed almost exclusively on neritic and oceanic squid, mainly nocturnal. Predation doesn’t seem significant. Mass strandings are rare. Analyzes of the stomach contents of specimens stranded in Scotland have shown that the most important species preyed upon in Scottish waters is the frilled octopus (Eledone cirrhosa).
One population occurs off Santa Catalina Island where they are sympatric with whales of the genus Globicephala macrorhynchus and both species feed on the squid population. While these species have not been seen interacting with each other, they do take advantage of commercial squid fishing that takes place at night.
They also travel with other cetaceans. They ride the bow waves of gray whales, as well as the waves of the ocean.
These dolphins have a stratified social organization. They typically travel in groups of between 10 and 51, but can sometimes form larger aggregations reaching up to a few thousand individuals. There are smaller, stable subgroups within larger groups. These groups tend to be similar in age or gender. This dolphin tends to experience loyalty to its own groups. Long-term bonds are seen to correlate with adult males. Younger individuals experience less fidelity and may leave and join groups. Mothers show high fidelity to a group of mother and young but it is unclear whether or not these females stay together after their young leave or remain in their natal systems.
Like many dolphin species, this cetacean also uses echolocation to target cephalopods and fish that feed below.
This cetacean can reach depths of more than 600 m using the air from the lungs and making several turns to descend rapidly, almost vertically, and increase the time spent looking for food. This allows the species to exploit a deep, dispersed layer of prey items such as squid, those that shelter during the day when they become more vulnerable to predation.
Furthermore, although it is capable of remarkable agility (it can reach 25 km/h), this dolphin usually has slow and relaxed movements. Unlike the common dolphin and the bottlenose dolphin, boats don’t seem to attract this cetacean, but it’s not difficult to approach it.
Like other dolphins and marine animals, there have been records of these dolphins being caught in purse seines and gillnets around the world. Many of these accidents have resulted in deaths. Small whaling operations have also been responsible for some of these deaths. Pollution has also affected many specimens that have ingested plastic. Samples from these animals show contamination within their tissues.
Concerning their conservation status, the populations of the Northern, Baltic and Mediterranean Seas are listed in Appendix II of the Convention on the Conservation of Migratory Species of Wildlife (CMS), as they have an unfavorable conservation status or would benefit significantly from international cooperation organized by targeted agreements.
In addition, this dolphin is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, Northeast Atlantic, Irish and Northern Seas (ASCOBANS), the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean and Contiguous Atlantic Region (ACCOBAMS), the Memorandum of Understanding for the Conservation of Cetaceans and their Habitats in the Pacific Islands Region (Pacific Cetacean MoU), and the Memorandum of Understanding on the Conservation of the Manatee and Small cetaceans of West Africa and Macaronesia (MoU on aquatic mammals of West Africa).
Risso’s dolphins are protected in the United States under the Marine Mammal Protection Act of 1992. Currently, Japan, Indonesia, the Solomon Islands and the Lesser Antilles still hunt this dolphin.
The threats for this cetacean are represented, in addition to hunting in some countries, by noise pollution mainly due to military activity (sonar), deep seismographs and accidental fishing. Furthermore, man competes by stealing prey, mainly squid. However, taking into account the vastness of the range and the consistency of the population, the IUCN Red List has attributed the status “LC” (minimum risk) to this species.

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Facilidad Global de Información sobre Biodiversidad.
– Gordon Corbet, Denys Ovenden, 2012. Guide to mammals of Europe. Franco Muzzio Publisher.
– John Woodward, Kim Dennis-Bryan, 2018. The Great Encyclopedia of Animals. Gribaudo Publisher.

Photo source:
– https://inaturalist-open-data.s3.amazonaws.com/photos/236847430/original.jpg
– https://www.artstation.com/artwork/Kb5RR