Huge Fish or Sea MonstersWhales, dolphins and porpoises are marine mammals collectively known as cetaceans and are supremely adapted to their oceanic life. They all belong to the order of cetacea from the Latin word cetus (‘whale’ or ‘large sea animal’) and the Greek word ketos (‘sea monster’).
There are about 85 currently recognized species, which are divided into two suborders; the Odontoceti or toothed whales, and the Mysticeti or baleen whales.
Toothed whalesThe larger group, the odontocetes, consists of about 72 species. Depending on the species, gender and age, they carry between 2 to over 250 conical or spatula-shaped teeth in the upper and/or lower jaw. Among some beaked whale species the teeth erupt externally from the lower jaw only among males. They all have one blowhole on the top of their heads and males are bigger than females.
Baleen whalesThe suborder Mysticeti or baleen whales consists of about 13 species. Instead of teeth they carry two parallel rows of baleen plates hanging from the upper jaw to sieve out small zooplankton and fish. They are believed to have no echolocation system to find prey, but some, such as the humpback whales are known to communicate acoustically over long distances. They all have two blowholes and females are bigger than males.
MorphologyThe cetacean body is fusiformed or spindle-shaped, the forelimbs are modified into flippers while the hind limbs are either reduced to a tiny pelvis bone hidden within the body or not existent at all. To look at the tail is the best way to distinguish a cetacean from a fish: the flukes of a whale are horizontal and moving up and down when swimming, whereas a fish’s tail is vertical and moves from side to side. Under the hairless skin lies a layer of fat called blubber to store energy and insulate the body.
External features such as size, body form and colouration pattern, and dorsal fin shape, size and position vary greatly and might even differ between geographical populations. Body lengths and weights vary greatly. They range from about 1.5 m and 30 kg among small cetaceans (i.e. commerson’s dolphin and vaquita) to 30 m and 180 tons among the largest animal ever known to have lived on earth, the blue whale.
DistributionAlthough cetaceans occur worldwide in all oceans and river dolphins even live in distinct freshwater systems, their spatial and temporal distribution varies greatly. Some species are bound to specific temperature zones (tropical, temperate or polar waters) and to coastal or pelagic waters. While toothed whales are generally more stationary, some baleen whale species undertake extensive migrations moving between their feeding grounds in colder and breeding grounds in warmer waters.
DietJust as their appearance, size, and geographical distribution varies greatly, the diet of cetaceans is highly diverse and variable. Most large baleen whales feed on huge shoals of fish or different types of zooplankton, toothed whales hunt a variety of prey types such as fish, squid, crustaceans or even penguins and marine mammals like seals and other cetaceans.
Toothed whales use echolocation to search for and hunt individual prey items. As they generally have complex and strong social structures they also produce high frequency sounds to communicate acoustically with other members of the group. Some species are known to have developed different dialects.
Baleen whales create low frequency, which can travel over thousands of kilometres, to either communicate and possibly to navigate and to read the complex underwater landscape.
ReproductionAgain, reproduction differs greatly among cetacean species as some might have to travel to specific mating grounds, and others have to meet males from other social groups. After mating the usual gestation period lasts about 10 to 12 months when the foetus is born tail first at a rapid pace to allow the newborn to surface fast to take its first breath. Lactation periods range anywhere from four to six months (minke whales) to over a year. Female sperm whales even nurse their calves for at least two years and sometimes even much longer.
ThreatsThere are many threats for cetaceans worldwide today through human activities, which either kill animals directly or indirectly. The extent and seriousness of the various threats vary depending on the specie’s needs, population dynamics, size, diet, biology and behaviour.
While hunting for food consumption and bait and life-captures directly take animals out from a population or social group, the effects of indirect threats are not as easily visible. Habitat degradation, noise and chemical pollution, overfishing, by-catch, decline of food resources, entanglements, ingestion of debris, and boat collisions might have serious consequences not only for the individual animal but also on a distinct population.