9 Main Physiological Adaptations of Cetaceans | Aquatic Mammals

The following points highlight the nine main physiological adaptation of cetaceans. The physiological adaptations are: 1. Thermoregulation 2. Feeding Habits 3. Osmoregulation 4. Lung Ventilation and Deep Diving 5. Bradycardia 6. Retea Mirabile 7. Reproduction 8. Gestation and Parturition 9. Growth and Size.

Physiological Adaptation # 1. Thermoregulation:

Aquatic mammals have a well-developed thermoregulatory mechanism. In cetaceans, the skin is naked by the loss of all hair except a few sensory bristles around the snout in some species.

For compensation a sub-dermal fat layer, called blubber, acts as an insulating blanket. Retia mirabilia helps in conserving body heat and helps in transferring heat from arteries of flipper and flukes to other areas. Thus cooling effect of sea water is reduced.

Physiological Adaptation # 2. Feeding Habits:

Animals lose energy in different ways but gain it by only one source-food. The feeding mechanism is different among two groups of cetaceans. Most of the toothed whales con­sume squids, cuttle fishes, octopei and fish.

Porpoises and dolphins have similar peg-like teeth, situated on the jaws. The masticatory function of the teeth is reduced due to elonga­tion of the jaw bone and they probably serve to hold the prey. The killer, false killer and pygmy killer whales consume a great variety of cetaceans, seals, penguins, turtles and many other marine creatures.

The surfaces of the roof of the mouth and tongue become rough­ened to prevent the escape of food. Speed and agility of some dolphins are used in the chase and subsequent swallow of small fish.

Slower moving species such as the beluga or porpoi­ses, which have little or no beak, adopt other techniques suited in their habitat. The beluga (Delphinapterus leucas) and Irrawaddy dol­phin (Orcella brevirostris), have protruded lips by which they can dislodge bottom-living molluscs and forage using suction to draw invertebrates into the mouth.

The whale bone whales (Mysticeti) consist of just 11 species, all relatively large in size. They have no teeth in the adult stage but remain only in the foetus. In adult stage, they possess a row of closely packed, triangular baleen plates hanging from each side of the roof of the mouth. These plates are made of keratin and have hairy fringe on their inner edge.

With the exception of the grey whale, the usual food of whale bone whales is a variety of zooplankton known as krill. Blue whales, hump back whale and fin whales take shoaling fish in some areas. In all these cases the baleen plates act as strainer.

The stomach of cetaceans is 3 or 4 compartmented. The fore-stomach is a muscular crop which contains stones and is lined by squamous epithelium and without glands. The second part of the main stomach part has a folded mucosal layer and gastric glands. The third part is a pyloric stomach with few glands that open to the duodenum. The intestine is many times longer of the body.

Physiological Adaptation # 3. Osmoregulation:

Cetaceans are hypo-osmotic to their sur­rounding environment so that body fluids tend to lose water by osmosis and conserve salts by diffusion. In cetaceans the water loss cannot be compensated by drinking sea water.

Mainly water is conserved in the body through con­centration of the urine. Females, which have water loss through the milk, are compensated by the concentration of the milk. The milk of these mammals has 10 times more concentra­tion of fat as compared with cow’s milk.

Physiological Adaptation # 4. Lung Ventilation and Deep Diving:

Whales can ventilate the lungs more com­pletely than terrestrial mammals and possibly up to 90% of the inspired air is used at each breath. The volume of air contained in the lungs of a large whale has been estimated to be 2000 lbs.

But the whales which dive in deep water cannot store much O₂ and have smallest lung capacity proportionate to their body size because lungs collapse completely in about 100 m depth due to water pressure.

For this reason the deep- diving cetaceans store O₂ in the muscles and in the blood. The muscle of whales contains large quantities of myohaemoglobin, a protein that attracts oxygen from the blood, this helps in the long submergence of the animal by storing O₂.

Physiological Adaptation # 5. Bradycardia:

Bradycardia means the slowing of heart beat rate and occurs during submergence. The rate is usually reduced to ¹/₁₀ or ¹/₁₅ in normal condition and also decreases in deep diving. It is an oxygen saving mechanism.

Physiological Adaptation # 6. Retea Mirabile:

The blood vascular system of cetaceans contains networks of blood-vessels throughout the body, especially in the thoracic region, called retea mirabile. The anastomosing blood-vessels help for storage of extra oxygen which are used during submergence but Young (1981) confirms that the retea mirabile is connected with the accommodation of the animal to varying hydrostatic pressures and temperatures.

Physiological Adaptation # 7. Reproduction:

The reproduction shows various modifica­tions among cetaceans:

i) The testes are abdominal, do not descend into the scrotal sacs.

ii) The penis is long and is curled when not in erection.

iii) There are a pair of teats in the inguinal region.

iv) The uterus is bicornuate but a single young is carried in the uterus for a long time. On rare occasions twins develop within the uterus. Multiple foetuses are quite rare and all die before birth.

Physiological Adaptation # 8. Gestation and Parturition:

i) Gestation period varies from 10 to 16 months according to species. Gesta­tion period of blue whale is 12 months while the small beluga and narwhal are 14-15 months.

ii) Fertilization and birth both take place in water.

iii) The females give birth to a single pre­cocious young at a time.

Physiological Adaptation # 9. Growth and Size:

i) Growth of young cetaceans is very rapid.

ii) The blue whale young gains about 90 kg per day.

iii) The single blue whale calf is about 7 meter long and weighs about 2.5 tonnes at birth.