True’s beaked whales are rarely confidently identified at sea due to their similarities with other beaked whales. It is one of the least known species of its genus Mesoplodon.
Physical description and behaviour:
True’s beaked whales body colour is slate grey on the back with a lighter belly. Some individuals might have a few pale patches around the anal and genital area. There may also be a clear diagonal white blaze going from the beak across the blowhole to the melon, and to the eye and start of the mouth-line. Individuals can also have a dark eye patch.
The species is about 4 m to 5.5 m length and has a long spindle-shaped body. Its body shape is closer to Cuvier’s beaked whale body shape than Sowerby’s beaked whale’s. It has a slightly concave trailing edge on its tail and its flippers are relatively narrow and small. Its dorsal fin is located almost two-thirds down the back and tends to be slightly recurved or triangular. The species’ head is small, has a bulge forward of the blowhole and has a pronounced but short beak.
Similarly to other species of the same genus, True’s beaked whales have a single pair of teeth directed upward and forward at the very tip of their lower jaw. These teeth are only visible in adult males above the mouth-line as they are smaller and below the gum in females and juveniles.
Little is known about True’s beaked whale behaviour and diet. They are thought to feed predominantly on mesopelagic fish and occasionally on cephalopods, based on stomach content analysis following strandings (Lusher et al., 2015). The species may also have a generalist foraging behaviour, which is related to how much time and energy goes into dives (MacLeod et al., 2003, MacLeod, 2005). They likely stay in relatively smaller groups than other species of the same family Ziphiidae (MacLeod & D’Amico, 2006).
Distribution and abundance:
Little is known about the species distribution and range. However they appear to be widespread and occur in deep waters. They have been encountered not only in the temperate parts of the Atlantic Ocean but also in the South Atlantic and southern Indian and Pacific Oceans.
Since 1899 there has been 11 records of strandings in Europe and 10 of those occurred in western Ireland. There has also been recent records of strandings on the Mediterranean coast of Turkey in 2016 (Öztürk et al., 2016). Sightings have occured in the Azores, the Bay of Biscay and the Canaries. Their distribution could be influenced by a southern branch of the North Atlantic Current.
IUCN Red List of Threatened Species™ Assessment
Due to a lack of knowledge on the species, there is relatively little literature regarding the various threats impacting it. Similarly to other species of beaked whales, anthropogenic underwater noise is thought to be one of the dominant threats impacting True’s beaked whales.
Macro- and microplastic pollution could also be an issue based on the digestive tract of stranded individuals in Ireland (Lusher et al., 2015). It was however unclear as to whether the microplastic was directly ingested or if it was consumed by preys, after which it would end up higher up in the food chain.
|CMS Instruments||ASCOBANS, ACCOBAMS, Western African Aquatic Mammals, Pacific Islands Cetaceans|
|IUCN Status||Least concern|
No pictures for Mesoplodon mirus
|English||True's Beaked Whale|
|Scientific name||Mesoplodon mirus|
|Additional notes||Updates have been made in August 2021 as per European Whales, Dolphins and Porpoises by Peter G. H. Evans (2020) unless stated otherwise.|