Category: Cetaceans

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Sperm whales make direction decisions democratically

Researchers have found that sperm whales decide on which way to turn democratically. While these whales only socialize with members of their own clan. You might think that this might restrict them, but generally clans have 20,000 females, making these groups huge (one of the distinguishing marks of the clan is by vocal dialects – which may end up as symbolic markers of the clan identity).

When travelling in a group they can take over 1 hour to decide which way to turn. These groups can be extremely large, and yet decide things together. Decisions like direction and speed of travel and also when to feed. While it is not clear how they reach consensus or how they “speak” to each other, it is clear that decisions are made together.

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Killer whales trapped in drift ice off the coast of Hokkiaido in Japan have got free

This pod of Orca have been trapped, with only this small opening to breathe from. Unlike other marine mammals, orca generally stay close to the surface, and breathe regularly, which can make this a death trap.

While Orca have been timed, holding their breathe for 15 minutes, they usually take shallow dives of up to a minute, or deeper dives lasting 3-4 minutes. While Orca can hit speeds of almost 30 miles per hour, this is usually only over short distances, and they could not sustain this for long under water – especially without being able to take a breathe.

Thankfully, in the last few hours, these animals have managed to get free.

Common Dolphin

Common Dolphin

  • The most abundant cetacean in the world, with around 6 million (it should be noted that there are 1350 humans in the world, for every individual common dolphin).

Despite this fact and its name, the common dolphin is not thought of as the model dolphin (that honour goes to the bottlenose dolphin due to its popular appearances in aquaria and the media). It did, however, feature heavily in Ancient Greek and Roman art and culture, most notably in a mural painted by the Greek Minoan civilization.

It is currently the only member of the genus Delphinus. The common dolphin belongs to the subfamily Delphininae, making this dolphin closely related to the three different species of bottlenose dolphins, humpback dolphins, striped dolphins, spinner dolphins, clymene dolphin, spotted dolphins, fraser’s dolphin and the tucuxi and guiana dolphin.[5] The common dolphin was originally categorized into two different species (now thought to be ecotypes), the short-beaked common dolphin and the long-beaked common dolphin. However, recent evidence has shown that generally long-beaked dolphins of this species have originated from the short-beaked population, and therefore there is no close links between different long-beaked dolphins in any part of the world.

Currently, the common dolphin is divided into four subspecies:

  • D. d. delphis, the nominate subspecies
  • D. d. bairdii, the Eastern North Pacific long-beaked common dolphin
  • D. d. ponticus, the Black Sea common dolphin
  • D. d. tropicalis, the Indo-Pacific common dolphin

A number of fossils were erroneously placed in the same genus, but this has since been corrected. 

Common dolphins can live in aggregations of hundreds or even thousands of dolphins,though are often seen in groups numbering several hundred individuals (with subgroups consisting of 20-30 individuals). Occasionally, different groups will come together to form mega-pods which can consist of over 10,000 dolphins – quite a site to witness. Genetic studies in the Northeast Atlantic suggest that common dolphin pods generally do not consist of close kin, but rather of members that are not closely related. Unlike many delphinids, common dolphins do not live in a matriarchal society. That being said, closely related individuals are usually found in similar geographical locations fairly consistently, providing evidence that this species displays site fidelity (at least in the North-eastern Atlantic). Male common dolphins display greater site fidelity in relation to their kin than females.

Common dolphin pod structure often consists of nursery pods (which includes females and calves), bachelor pods (consisting of all males) and mixed groups of males and females, including sub-adults and calves. Genetic evidence seems to indicate that common dolphins live in fission-fusion societies, where dolphins form pods that are not necessarily stable and do not necessarily consist of related individuals. It is not known if common dolphins form lifelong bonds with other individuals  like the long-term male alliances seen in bottlenose dolphins.

There is some evidence that common dolphins use signature whistles, similar to that of the bottlenose dolphin. These whistles are believed to serve as an acoustic label the dolphin equivalent of a name.  It takes approximately 1 year for a calf to learn its signature whistle after which it remains stable for the rest of a dolphin’s life.

In South Africa, as many as 29 common dolphin signature whistle types were detected. However, it was difficult to determine if each dolphin had its own signature whistle due to the vast number of dolphins present (over 1,000) and anthropogenic background noise. Additionally, considering the vast number of dolphins present and  taking into account their feeding and diving behaviour, it appears that common dolphin signature whistles are also used for group cohesion. Another hypothesis for the function of signature whistles, is that they serve as a beacon for lost individuals.

Common dolphins sometimes associate with other dolphin species, such as pilot whales (note, not actually whales). In the Gulf of Corinth, common dolphins frequently display mixed species association, especially with striped and Rissos’ dolphins. Over one third of all dolphin sightings in the gulf consisted of mixed species associations that partially consisted of common dolphins. In mixed species associations, the ratio of striped to common dolphins ranged from 6-11:1. When Rissos’ dolphins were present (there would usually be only one or two individuals), it appeared that much of their scars were the result of interactions between striped and spinner dolphins. In much of the interactions, the Rissos’ dolphins would chase and herd the common dolphins toward the boat, while the common dolphins would try and swim under the Rissos’ dolphin. When groups of common and striped dolphins would charge at each other, the Rissos’ dolphin would chase the striped dolphins. Sometimes these interactions appeared to be playful, and at other times aggressive. Synchronized swimming and surfacing was commonly observed. These interactions take place in the deepest part of the Gulf, furthest from shore and usually consist of a total of 60 dolphins from all three species.

There have been 15 cases of common dolphin and striped dolphin hybrids being recorded. Genetic and observational evidence has demonstrated that the hybrids are fertile and are capable of not only reproducing with other hybrids, but are capable of reproducing with each of the parent species. Striped dolphins have been known to mate with other dolphins, as the Clymene dolphin is the result of hybrid speciation between striped and spinner dolphins. However, this is unlikely to happen with common dolphins, as their population in the Gulf of Corinth is too low. Common dolphins and bottlenose dolphins have been known to interbreed in captivity. There is one confirmed case of a hybrid between a bottlenose and common dolphin in Southern Spain, an important feeding ground for both species. The mother was a female bottlenose dolphin (dubbed as Billie) who has spent 10 years within a common dolphin pod. Billie was observed assisting common calves reach the surface at three different intervals and would babysit the calves after the mother went through labour. They have also been observed bow riding on baleen whales, and they also bow ride on boats. They are fast swimmers and breaching behaviour and aerial acrobatics are common with this species. They are also known to display altruistic behaviours to support injured members.

The short-beaked common dolphin is pregnant for 10 to 11 months. The new-born calf has a length of 70 to 100 centimetres (2.3 to 3.3 ft) and weighs about 10 kilograms. For the Black Sea population, weaning occurs at between five and six months, but occurs later (up to about 19 months) in other areas. Typical interbirth interval ranges from one year for the Black Sea population to three years for eastern Pacific Ocean populations. Age of sexual maturity also varies by location, but can range between two and seven years for females and three and 12 years for males. No evidence exists of any major reproductive differences between the two species. In captivity, the long-beaked common dolphin has hybridized with the common bottlenose dolphin . One of the hybrids has been bred back to a bottlenose dolphin, demonstrating such hybrids are fertile.

Find our news section below this video of a megapod of common dolphins

Yangtze finless porpoise

Also known as Narrow ridged finless dolphin. this is a river dolphin, found only in the Yangtze river of China. Its cousin the Baiji has gone extinct but this species is currently doing better- though not by much, there is only thought to be around 1000 remaining. Unfortunately, this species faces many of the same threats so may be on the same road, but a few decades behind. We must fight for this not to be the case.

The Yangtze River has a large amount of human activity – causing population declines due to pollution, illegal fishing, vessel traffic, and the creation of dams (which splits up the population into smaller groups. Both the Chinese government and various charities are working to halt its slip towards extinction. Bycatch is also a significant, with a variety of banned fishing tools such as gillnets still in use.

They are highly opportunistic feeders, which likely explains significant differences in their diet at different times of the year.

There are 5 reserves that have been created, and activities in these areas are heavily controlled which will hopefully give at least small areas from which the porpoise can survive – time will tell.

Narrow-ridged (or East Asian) finless porpoise

Narrow ridged (east Asian) finless Porpoise

This species is native to the East China Sea, Yellow Sea, and the seas around Japan. The Yangtze finless was originally thought to be a subspecies of this porpoise, though further study showed its differences to be greater than that (hence it now being recognized as a separate species). The east asian finless porpoise is a subspecies of the narrow ridged finless porpoise (the above picture is one of these).

Growing to 2.27m at most, and 72kg, though most are smaller.

They generally stay in water 50m deep or less, and mostly stay close to shore (though not always – it has been spotted over 100km from land).

The biggest threat to them is habitat degradation and pollution. The population has fallen by 50% in the last 3 generations.

They have been kept in captivity in Japan, Indonesia and China. A total of 94 are thought to have been held. Unfortunately these do not live long, and of all of the pregnancies that have occurred in captivity, only 2 resulted in a love birth and this did not survive for long.

Indo-Pacific finless porpoise

Indo-Pacific Finless dolphin

 They are found in most of the Indian Ocean, as well as the tropical and subtropical Pacific from Indonesia north to the Taiwan Strait. Overlapping with this species in the Taiwan Strait and replacing it northwards is the East Asian finless porpoise.

They are one of the species protected by the Sundarbans national park, which lies between India and Bangladesh.

Growing to 2.3m at most, and with a weight of up to 72kg (most individuals are far, far, smaller). They eat a wide range of foods from fish and crustations to vegetation such as leaves and rice, and eggs that have been deposited on these leaves.

Harbour Porpoise

Harbour Porpoise

This is one of the smallest cetaceans, which spends the majority of its time in estuaries and harbours – though it will sometimes venture up rivers, and have been found 100km from the coast.

They get their name from an anglicized version of the french word Porpois. This in turn comes from a medieval Latin amalgamation of two word – these are porcus (pig) and Piscus (fish). I think that these names are rather unfair, but still.

Its range is the North Atlantic, North Pacific and the Black sea. The populations are not continuous and as such there is a series of sub-populations that are geographically limited.

They are thought to have a global population of around 700,000, the Baltic sea is the only one which is seriously endangered, and that is the Baltic sea population which is thought to be around 500. Others could become endangered if current trends continue such as the Black sea population which is currently estimated to number around 12,000.

They have to spend a great deal of time looking for food, as they need to eat 7-8% of their bodyweight each day.

Generally, they live 8-13 years although individuals have been recorded living to over 20.

Dalls Porpoise

Dall's Porpoise

Lying in its own genus of Phocoenoides, the dalls porpoise is found to the west of North America and the East of Asia. The largest porpoise species they can grow to 2.3m and 220kg. Living in fluid groups of 2-10, Males compete for females, and many will guard a single female during the breeding season, in order to be sure that any offspring is theirs.

Females give birth around once every 3 years, with pregnancy lasting 11-12 months. They often ride boats bow-ride, and even will do the same on bigger whales.

They are abundant, in their range, and have a population of around 1 million in the wild.

There is a range of different sub-populations, but given the size of the overall population it should be secure in the long-term. Of course the problem is that this could change fast.

Burmeister Porpoise

Bermeister Porpoise

First described by Hermann Burmister in 1865, they are thought to have a population that numbers at least in the 10,000s. Found from Peru in the Pacific to Brazil in the Atlantic, while usually found in shallow coastal waters, they have been located 1000m down.

Most photos of this species are of a dead individual. It is black which explains its other name – the black porpoise, though it is thought that they turn black after death and are dark grey beforehand. The underside is lighter, though this is common as it helps with camouflage against the light surface of the water. Usually 1.5m in length and with a weight of 50-75kg, they can be mistaken for the Chilean dolphin, which is a similar size. Their dorsal fin is more triangular and points back rather than up so should not be confused.

They are shy, usually moving away from boats at a high speed. Usually seen in pairs or alone. However, they have been seen in larger groups, with a group spotted off the coast of Chile numbering 50.

Eating Hake, Maceral and Anchovies, which are all species impacted greatly by el nino. Many of the marine mammals in the area starved or struggled until the fish returned.

Narwhal

Narwhal

Generally, the source of any so called unicorn horns, they have a tooth that can extend more than 3m from their mouths and some have 2. Most recent populations suggest around 170,000, Induit from Canada and Greenland have hunted them for both meat and ivory.

 

They migrate in winter, and much of their food comes from Arctic cod and Greenland Halibut and live around 50 years. Young narwhals are predated by both Polar bears at breathing holes and Greenland sharks.

They are essentially an odd looking species of dolphin. There are around 123,000 individuals, living in 12 different groups in areas around the arctic.

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