SPECIES: Physeter macrocephalus (Linnaeus, 1758)
Capodoglio, sperm whale, cachalote, cachalot, Pottwal
The sperm whale is the largest of the Odontoceti (toothed whales), presenting a high level of sexual dimorphism, with females being 2/3 of the male length and 1/3 of the male body mass.
In the Mediterranean Sea, males have an average length of 14-15meters, and females 12meters, with a weight of 40-50 tons. Newborns are about 3,5- 4,5 meters long.
Characteristic of the species is the profile of the enormous head, which constitutes 1/3 of the entire body and in which there is the spermaceti organ, a unique anatomic structure used for echolocation (bearing in the environment, foraging and feeding, communication with other individuals), which in other odontocete cetaceans (toothed whales) is represented by the melon. The colour is uniform dark grey, but only along the outside of the upper and lower jaw, the skin is often white. The lower jaw is shorter so it doesn’t reach the front. There are about 7 – 30 teeth for each side of the jaw. The dorsal fin is small and round in shape, the pectoral fins are relatively short and wide. The tail is big with a straight-lined back edge and a central depression.
The breather, or ‘blow hole’, is located on the head, shifted to the left and the breath, or the ‘blow’ is low, messy and directed obliquely forward with an inclination of about 45°.
Foraging entails long deep dives up to 1000 meters and lasting about 45 minutes. Before diving the sperm whale surfaces for 10-12 minutes, then it arches his back, and right before deep diving, it pulls out the big tail.
The sperm whale holds a few world records in nature: it is the biggest known toothed animal; it has the biggest brain in all living creatures on the planet (the brain of an adult sperm whale weighs about 7kg and there are past records of 9kg); it is the deepest diving marine mammal (up to a depth of 2200 meters) holding its breath for longer than 2 hours; it is the strongest biological sound source (sperm whale clicks have frequencies higher than 230 dB at one micropascal of pressure, at one meter distance).
The social structure of the sperm whale is built around a social group; it’s composed of females and offspring, individuals of both sexes; recent genetic studies have shown that these groups are formed by one or more permanent matrilineal units. Females seem to remain in their birth groups for life, whereas the males, once they reach “adulthood” (between 12 and 21 years of age), leave their mothers to form groups of “bachelors” (bachelor groups). As they get older, sperm whales tend to isolate themselves, and the big sexually mature male adults, are often solitary (“bulls”) and join the reproductive groups only during mating season.
The species is regularly observed both in the continental slope (150-200m deep) and in the open sea. In the Mediterranean Sea, group size is decreasing and the big groups above 30 individuals described in the 1950’s, are now just a memory.
This species is cosmopolitan in Italy, with the exception of the Adriatic Sea, where it is present only in the southern most part of the basin, in which it finds the right conditions to satisfy its biological needs.
The sperm whale feeds mainly on bathypelagic and mesopelagic cephalopods, mainly small to medium sized squids (1 meter, 1-3 kg) and demersal species of fish.
The status of the Mediterranean sub-population of sperm whale is more delicate than the neighbouring Atlantic one, in fact, in 2012, it has been listed as ‘endangered’ in the IUCN Red List, due to the thinning of the population and the decreasing number of adult individuals.
Main threats include ship-collisions and driftnet bycatch, the latter being especially heavy up until the end of 21st century (still occasionally occurring). Plastic ingestion, anthropogenic noise and chemical contaminants are other factors influencing the conservation status of the sperm whale in the Mediterranean Sea.
The presence of the sperm whale in the Pontino and Campano Archipelagos has been monitored since 1991 by Ischia Dolphin Project. Since 2004, thanks to the development of an acoustic detection system provided by IFAW ((International Fund for Animal Welfare), the number of sightings has increased remarkably.
Residency of the sperm whale and its habitat use have been studied regularly through photo-identification data collected in a 16-year- period (2003-2018).
In total, 88 individuals have been photo-identified on the basis of natural marks on the tail. The increase of the number of individuals on the photo-identification catalogue (recruiting rate) caries randomly in years, suggesting that the waters off Ischia Island represent only a portion of the population’s home range, that the area is used by different groups, and that the number of individuals to add to the catalogue is still wide.
Data confirms the presence of all types of known social groups: young immature males, groups of females and calves, and solitary males, supporting the hypothesis that this area is relevant for all ages of this species. Thanks to data gathered on this species by our Organisation, the Pontino and Campano Archipelagos have been recognised as Important Marine Mammal Area (IMMA) by the IUCN’s Marine Mammal Protected Areas Task Force.
Finally, it is important that a stable association in time among young males has been documented in this area.This type of association is little known and in the past it had been impossible to give evidence to such association. This association, documented by us through the years, suggests that the social structure of groups of non reproductive males can mirror that of females, in terms of complexity and long term relations among individuals.
Monitoring the social structure and the population status of the sperm whale in the area is of the utmost importance for the conservation of the species in the Mediterranean Sea.
This blog was written by our project partners at the Ischia Whale and Dolphin Project.
For more information about getting involved with whales an dolphins of the Mediterranean please visit their website: www.oceanomaredelphis.org