3.3. Cetacean biology
All marine mammals – Cetaceans, Pinnipeds, Sirenians, the sea otter (Enhydra lutris) and polar bear (Ursus maritimus) – are considered to have evolved at various times from land dwelling ancestors. The Delphinidae constitute the largest and most diverse family of cetaceans, including the Killer whale (Orcinus orca). Currently there are three suborders of cetaceans: Archaeocetes – now completely extinct, Odontocetes – the toothed whales and Mysticetes – the baleen whales. It is commonly believed that both Odontocetes and Mysticetes share a common ancestor from among the Archaeocetes. Many recent discoveries of new species made through genetic research indicate that the number of living cetaceans is significantly higher than about 80 currently recognised.
Although cetaceans look more like fish than their closest living relatives, the Hippopotamus (Hippopotamus amphibius), their anatomy, physiology and embryology are certainly mammalian. The cetacean body is designed to provide the least hydrodynamic resistance whilst swimming. The dolphin skin, feels smooth and soft, yet its entire body is covered by micro-folds which direct the flow of water and avoid turbulence while swimming quickly. Subcutaneous fat layer helps avoid the dispersion of heat, makes the body more rounded and hydrodynamic, balances the weight of the animal making it easier to float. There are three types of fins to help propel and direct the animal. The pectoral fin (evolution of the front limbs) and dorsal fin direct and stabilise the animal whilst the tail (caudal fin) is used for propulsion. One of the most recognisable features of cetaceans is their horizontal tail. It is the most efficient system of travelling vertically through the water in order to breathe at the surface and feed underwater.
Living in water has led to some physiological changes within cetaceans. To be able to avoid pressure sickness, associated with movement through the water column, the lungs of cetaceans have become more efficient. They can exchange up to 80% of their lung volume with every breath, and have the ability to decrease lung volume with depth due to the flexibility of the ribcage, veins and trachea. To prolong the time underwater the pigments in the blood have become specialised allowing more efficient transfer of oxygen from the lungs into the blood. To reduce heat loss the peripheral circulation of the blood can also be changed. They also have the ability to slow the heart beat from 70-100 beats per minute on the surface to 30-40 beats per minute whilst diving.
The separation of the larynx and pharynx and the migration of the nares (nostrils) to the top of the head are some of the most important adaptations to life in the water. It means that only a small part of the animal needs to exit the water to breathe. Like a swimmer, cetaceans start to exhale before they reach the surface making the time they spend at the surface as effective for gas exchange as possible. The dolphins have one nare whereas the baleen whales have two distinct nares. Because of their undersea environment, whales and dolphins have to actively decide when to breathe. To avoid drowning during sleep marine mammals retain control of their blowhole. While sleeping, the dolphin shuts down half of its brain, along with the opposite eye. The other half of the brain stays awake at a low level of alertness. This attentive side is used to watch for predators, obstacles and other animals. After approximately two hours, the animal will reverse this process, resting the active side of the brain and awaking the rested half.
Our famous five senses, in cetaceans are reduced to four; in fact the smell is virtually absent while the sight and the touch are rather developed in almost all species. Dolphins have similar eyes to horses and other ungulates. The curvature of the lens can be changed to allow the animal to see clearly both in and out of the water. The eye is also sensitive to extremely low levels of light. To reduce friction in the water the external ear has been reduced to a small hole. All sounds are received through the lower jaw and directly transmitted to the inner ear. The inner ear is insulated from the rest of the head by fatty material, which allows the animal to discern the direction, tone and frequency of the sound waves. For dolphins, it has been necessary to adapt to an environment of limited visibility. This has led them to develop a sophisticated system of sound production and reception – echolocation. The system is based on the emission of sounds and the analysis of the echoes as the sound waves reflect off different objects in the environment. Baleen whales, on the other hand, do not have a biosonar, and produce intense but low-frequency sound signals, whose function is to keep individuals in contact with each other, even tens or hundreds of miles away. Sound travels through water nearly five times faster than in air, so it provides a better method of investigating the environment.
Daily consumption of food is proportional to body weight and can vary from 3-14%, higher in smaller odontocetes and lower in larger species. As a member of the Odontoceti suborder, dolphins have teeth, whereas baleen whales have baleen plates. Toothed whales use their teeth to grasp their prey, they generally have little tooth differentiation (all the teeth are of the conical shape) and some species may have up to 300 teeth. In baleen whales a filtering system has been developed using baleen plates, consisting of keratin, in place of teeth. Some baleen whales actively filter the water by bringing the water into the mouth then using the tongue to force the water out through the baleen plates. Others passively filter by allowing the water to pass through with their mouth open.
Like all true mammals, cetaceans give birth to live young and females possess mammary glands. The gestation period in cetaceans lasts from 10 up to 14 months in some species. Since the newborns first have to develop the insulating blubber layer, the mating season has peaks in spring and autumn so that giving birth tends to occur when water temperatures are highest. When nursing, the mother by contracting special muscles actively wrings out the milk into the mouth of the calf. The milk of a cetacean is much thicker than that of terrestrial animals, it has just 40-55% of water and is richer in fats (30-40%) and proteins (20-75%). An example of the calorific value of this milk is that newborn fin whales can grow up to 100 kg per day. Calves nurse for several months and then the transition from milk to the prey takes place. Nevertheless, the close relationship between mother and calf will continue for a few years as the mother, through parental care, teaches calf all that is indispensable for its survival as independent individual.
3.3.1 Once million years ago marine mammals were land animals
It is true that animals that lived on land have moved to the sea like people move from one country to another. There is a saying what exists on land exits in the sea. So how did these animals get to be in the sea. That first transition of land animals going into the ocean coincides with the largest mass extinction event of all time, larger even than the one that killed off the dinosaurs .
Fossils have revealed the shapes and sizes and other details of mammals such as their skulls and teeth that used to live on land but now live in water . The first and foremost reason is different types of environmental changes that occurred over the years. Another reason is that some of these mammals that lived on land liked to be near the water’s edge so adapted to the niches of the water . In addition to there seemed to be a shortage of food which pushed many animals to look elsewhere for nourishment. Many animals changed the way they ate after becoming water animals so they could survive there. These changes were not easy and many problems occurred on the way in breathing, reproduction and many more . Also deferent parks over there body changes to attacked to the water too.
Today lots of marine animals we know started out as land based animals for example a walrus ancestor looks like the modern otter, whales come from an ancient deer-like creature .
Scientist cannot fully explain all the changes that happened to the animals and are there going to be any more changes in the future to come.
3.3.2 Adaptations of cetaceans for life in the sea
Cetaceans have evolved throughout time in order to be able to adapt to their living habitat. The changes can be categorised in three main groups namely physical, behavioural and structural adaptations. Cetaceans have evolved in order to overcome problems caused by swimming and locomotion, diving, thermoregulation, orientation in the ocean and threats coming from predators.
As far as physical adaptations are concerned, their surface area is on average 23% less than other mammals and their smooth body surface which is achieved by having no hind limbs but instead they have caudal flukes allows them to swim faster and easier. In combination with their cylindrical shape heat loss is minimized. Furthermore, blubber deposition follows a specific pattern in order to allow for a smoother body surface. Cetaceans have a very well developed sense of hearing that can detect sounds from extremely large distances away an ability of supreme importance as far as mating and detection of potential predators is concerned.
As far as behavioural adaptations are concerned, they propel themselves through the water in such a way so that propulsion is lift-based rather than drag-based with the so called caudal oscillation that increases thrust, efficiency, power and speed. One of the most important behavioural adaptations of marine mammals is their ability to swim and dive.
As far as structural adaptations are concerned, their organisms have adapted to survive oxygen conservation and dealing with extremely high pressures at depth. This is achieved by bradycardia, peripheral vasoconstriction and increased oxygen storage. They have the capacity to store extremely high amounts of oxygen in their blood and muscles. Additionally, they have adapted to collapse their thoracic cavity, lungs and alveolar sacs. Moreover, they have adapted a system known as a counter-current heat exchanged which is used to keep the blood warm by keeping it near the centre of the body.
Cetaceans evolved dramatically.
3.3.3 Oxygen mistery: How cetaceans are able to hold their breath for so long?
Cetacean is any member of an entirely aquatic group of mammals commonly known as whales, dolphins, and porpoises. The cetacean family shares a number of physiological traits with one another including having blowholes, breathing oxygen, maintaining constant awareness of their breathing and being able to dispose of additional salts that are taken in by their body when they consume food. To assist with long dives cetaceans have developed special lungs that help them inhale additional oxygen and transfer it to enlarged blood vessels where it can be used by the body. According to some researchers whales are also able to use up to 90% of the oxygen they inhale as compared to humans that are only able to use around 15% of the oxygen they inhale. Whales are also extremely efficient swimmers and use minimum effort to travel and hunt for food in order to conserve precious oxygen. Their red blood cells also carry more oxygen. And when diving, cetaceans' blood travels only to the parts of the body that need oxygen: the heart, the brain and the swimming muscles. Digestion and any other processes have to wait. Additionally, these animals have a higher tolerance for carbon dioxide (CO2). Their brains do not trigger a breathing response until the levels of CO2 are much higher than what humans can tolerate. These mechanisms, part of the marine mammal diving response, are adaptations to living in an aquatic environment and help during the process of sleeping. Cetaceans reduce the number of breaths they take during rest periods, a dolphin might average 8 to 12 breaths a minute when fairly active only to have their breathing rate drop to 3 to 7 per minute while resting.
3.3.4 How do cetaceans breathe?
Despite the fact that cetaceans are mammals they don't breathe like us. Instead they have a hole called a "blow hole" on the top of their heads. Sometimes, when a cetacean breath air out of its blow hole, it shows up as a spray or mist called a "spout" that can been seen many miles away blow holes are surrounded by muscles that keep the hole closed when the cetacean is under water and open when the animal is at the surface and needs to breath.
Cetaceans use their blow holes to take oxygen from the water that they live in.
Blowholes are located at the top of the cetacean's head and act as a passageway to the trachea where passes through the air passage ans fills the lungs in order to make it easier for the cetacean to breath . Also its easier for them to breath with minimal effort especially during times of rest when they can be seen logging around near or at the surface of the water.
The hot air from the cetacean its been mixed with the cold air of the ocean and it creates water droplets which gives the impression that the cetacean blows water instead of air
Since they can only breathe at the surface, whales have had to develop conscious or voluntary breathing. That means they choose when to take a breath. This is important because whales can't breathe underwater. So they surface every few minutes to blow out a mixture of water and air and take in a breath of fresh air.
The sperm cetaceans respiratory system is among the most efficient in the world it can hold its breath for 80 to 90 minutes at a stretch. Some cetaceans can swim without a breath for as long as two hours. A whale's "nostrils" are called blowholes and are on the top of its head .
3.3.5 How do cetaceans sleep without drowning?
As humans understand it, sleep tends to involve a level of unconsciousness that would seem dangerous to aquatic mammals.But how do cetaceans really manage to sleep without drowning? To begin with, observations of bottlenose dolphins in aquariums and zoos, and of whales and dolphins in the wild, show two basic methods of sleeping: they either rest quietly in the water, vertically or horizontally, or sleep while swimming slowly next to another animal. When marine mammals sleep and swim at once, they are in a state similar to napping.Young whales and dolphins actually rest, eat and sleep while their mother swims, towing them along in her slipstream which is a placement called echelon swimming. At these times, the mother will also sleep on the move. In fact, she cannot stop swimming for the first several weeks of a newborn's life. If she does for any length of time, the calf will begin to sink due to the fact that it is not born with enough body fat or blubber to float easily. While sleeping, the bottlenose dolphin shuts down only half of its brain, along with the opposite eye. The other half of the brain stays awake at a low level of alertness. This attentive side is used to watch for predators, obstacles and other animals. It also signals when to rise to the surface for a fresh breath of air. After approximately two hours, the animal will reverse this process, resting the active side of the brain and awaking the rested half. This pattern is often called cat-napping. It's impossible to know what sleeping with just half your brain at a time would feel like, and dolphins can't exactly explain it to us. On the other hand, humans, of course, can breathe while the conscious mind is asleep and our subconscious mechanisms have control of this involuntary system. But equipped with a voluntary respiratory system, whales and dolphins must keep part of the brain alert to trigger each breath in order to survive.
3.3.6 Explore the senses of cetaceans!
Cetacean is the collective name for all marine mammals like whales dolphins and porpoises. It's a big group with about 90 different species. Their vision is limited to 13.7 m or so but species like river dolphins for example are almost blind. That may not be a problem considering that the depths of the ocean are quite dark and their vision is of no use.The sense of hearing they have is very important since water is an excellent conductor of sound. Whales and dolphins use echolocation for hunting, navigating and communicating with a series of sounds they generally produce which are termed "songs". Normally sounds from blue whales and fin whales are too low for humans to hear so we speed them. In addition they are believed to have the extra sense called geomagnetism that enables them to detect the magnetic field of the earth. Also the sense of taste is more important to them than smell because their nostrils or blowholes need to be kept shut when they are diving. But it seems that they have lost four taste modalities sour, sweet, umami and most of the ability to sense bitter tastes. Except the sense of salt tastes may be because of their function in NA+ reabsorption. The cetaceans have a very sensitive skin and they use their flippers and flukes to stroke and pet or as a way to greet one another. Cetaceans did not develop all the same abilities because they depend on the regions of the world they live in, the kind of food they need to hunt to survive and their social behavior and environment.
3.3.7 Echolocation - the ability to build the picture of the environmentusing sound
Echolocation, also called bio sonar, is the biological sonar used by several kinds of animals. Echo-locating animals emit calls out to the environment and listen to the echoes of those calls that return from various objects near them. They use these echoes to locate and identify the objects. Echolocation is used for navigation and for hunting in various environments.Equivalent to sonar or radar, echolocation is the production of sound used for communication. Echolocation is the use of ultra-high frequency sounds for navigation and locating prey. Bats and marine mammals are able to use sound to "see". It is the returning echoes that give the animal an "image" of some parts of its environment. In addition the echoes must be loud enough to return to the animal and short enough so that the echo of the sender returns back to the animal or human before the next one is sent out. Echolocation is used by mammals like dolphins, whales and bats. Humans have also learned this ability to interact with there environment when they are blind. The term was created by Donald Griffin, who was the first to conclusively demonstrate its existence in bats.In some bats the sounds are made and sent out by their noses but, most send different sounds through their mouth. The sounds bounce off objects and prey in their surroundings like insects or branches, and are picked up by the bat's sensitive ears. Although bats have good eyesight, they depend on the echolocation system to navigate as well as capture their food. Bats give off pulses at very high frequencies that are not audible to humans, at an impressive rate of 200 pulses per second. They can avoid obstacles no wider than a piece of thread, identify the size of objects, and capture flying insects using this method. There are about 800 different species of bats that use different patterns of frequencies. This allows scientist the ability to identify various types of bats in dark places, such as caves.
3.3.8 Toothed and baleen: What do cetaceans eat?
Cetaceans is a widely distributed and diverse clade of aquatic mammals that today consists of whales , dolphins , and porpoises and they are carnivorous and finned . The name is derived from the Latin word cetus "whale", itself from the Greek word “κῆτος” which means huge fish . Their size range is from 1 to 30 meters and they weight from 50 kilograms to 173 tons , and they are known as the largest animal to exist . Some of the cetaceans life span comes all the way up to 70 years . Most of the cetaceans species live in the sea , but there are some others that live in rivers . As it turns out , the cetaceans diet doesn’t consist mainly out of large marine animals but most of them in particular Baleen whales feed on the smalles animals of the ocean , which are krills and plankton by getting large amounts of water into their mouth and then squeezing it through the baleen , and leaving their food caught on the balleen , and then they wipe their baleen clean with their tongue before swallowing their food , Plankton are the diverse collection of organisms that live in large bodies of water which are unable to swim against a current , and krills are small crustaceans which are often attributed to species of fish and are found in all the world’s oceans . But there are some exceptitions , like the sperm whales that feed on animals like giant squids . More than that some orca whales feed on salmon and some other whale species feed on different marine mammals like sea otters and sea lions that can weight up to 40 kilograms .