Aquatic mammals & their adaptations

AQUATIC MAMMALS & THEIR ADAPTATIONS

IntroductionMammals are primarily terrestrial animals .Some of them have wonderfully adapted for a secondary aquatic life.Based on the relation to water and the degree of aquatic adaptations aquatic mammals are grouped into 1.Completely aquatic mammals 2.semi-aquatic or amphibious mammals

Semi aquatic animals morphological modificationsSleek & slender body Dense &soft fur (hairy coat)Elongated snoutSmall ear lobesFlat tailWebbed hands & feetSub cutaneous fat

Semi aquatic mammalsGroupMonotremata Marsupialia Rodentia Artiodactyla Pinnipidia ExamplesOrnithorhynchusChironectesBeavers , Musk ratsHippopotamusWalruses , otters , seals , sea lions

Monotremata : egg laying mammals Ornithorhynchus

Poison gland

Marsupialia : Chironectes

Rodentia :long incisors that grow continuously Beavers

Musk rats

Artiodactyla : hooved mammals Hippopotamus

Pigmy Hippopotamus

Pinnipidia : carnivorous aquatic mammals Walruses

Otters

Seals

Sea lion

Sea lionSeal

Completely aquatic mammalsGroup Cetacea

Sirenia ExamplesWhales , dolphins , porpoisesDugongs , manatees

Cetacea : whale like mammals Baleen whale

Blue whale

Sperm whale

Killer whale

Dolphins

Porpoises

Sirenia : aquatic herbivorous mammalsDugongs (sea cows)

Manatees

All secondarily aquatic mammals are believed to have evolved from terrestrial ancestors.

A great disadvantage of all aquatic mammals is retention of the lung breathing habit

Aquatic adaptationsModification of body contourIntegumentary modificationsLocomotor adaptationsSkeletal modificationsModifications in the digestive systemRespiratory modificationsModifications of sense organsReproductive modifications

Modification of body contourMost aquatic mammals have a tapering & streamlined body-----only least resistance in swimmingElongation of skullReduction of zygomatic archesDevelopment of cylindrical chestReduction of projecting structures------ hind limbs , pinnae , scrotum etc..

Integumentary modificationsThe modifications mainly loss of hair & skin glands & the formation of blubber (fatty layer of adipose tissue , beneath the epidermis ---thick coat )Blubber serve as a heat insulator and there by prevents the loss of body heat & maintains a constant body temperature.Reduce specific gravity of the body in the waterAct as shock absorber to absorb the hydraulic pressure during deep divingSkin surface has become smooth by the loss of hairHair represented by a few bristles around the mouth (exception in white whales) present in whales & sea cows

Locomotor adaptationsElongation of the bodyReduction or loss of hind limbsFormation of dorsal fin , tail flukes & flippers or swimming paddlesHyperdactyly (formation of extra fingers) & hyperphalangy (formation of extra phalanges)

Modification , reduction and loss of hind limbsIn cetaceans & sirenians hind limb altogether absent , since tail , fore limb & dorsal fin are mainly involved in locomotion.Ornithorhynchus and chironectes ,fore limbs & hind limbs have webbed digits.In pinnipidians hind limbs are enclosed by skin , forming oar like structures

Formation of dorsal fin & tail flukesTail of aquatic mammals , in general is horizontally flattened , forming a sort of caudal finIn cetaceans , the flattened tail is provide with lateral expansions of the skin , called tail flukes .The movement of tail & tail flukes help to forward & vertical movement of an organism.Dorsal fin & dorsal fluke, which act as a rudder or keel in swimming

Formation of flippers In semi aquatic mammals fingers are webbed . In completely aquatic form fore limbs are modified into swimming paddles or flippers.A flippers consist of an internal support of fused bones , covered by skin Flippers act as balancers & stabilizers in swimming

Hyperdactyly & HyperphalangyFlippers are characterized by hyperdactyly and hyperphalangy (development of extra fingers & extra phalanges)This gives them sufficient strength and support and increases their surface area

Skeletal modificationsEndoskeleton is lighter by the presence of oil inside bonesDorso ventrally flattened cranial regionElongated facial region for catching preyVertebrae are simple with well developed neural spines and transverse process --- for the attachment of musclesCervical vertebrae are fused to form a small solid bone------- neck region reduced

Ribs are movable & dorsally attached this enhance the lung capacity and also increases the thoracic cavity backward for flotation

Modifications in the digestive systemIn fully aquatic jaws are used only for catching the prey & not for masticationMastication is practically absent in oral cavitySalivary glands are very much reducedStomach of mostly aquatic mammals has chambered & specialized for crushing and digesting the food

Teeth are simple and dentition is homodont and monophyodontIn toothed Whales teeth are quite numerous (more than 250 )In baleen whales teeth are present only embryo in the adult they will be replaced by numerous baleen plates or whalebones these are arranged on upper jaw in to transverse rows , baleen plates act as strainers for filtering planktonic food

Respiratory modificationsRespiratory system of fully aquatic mammals is highly modified for a life in water.External nostrils are valvular & located far back on the top of the headDorsal position of the nostril enables the animal to inhale atmospheric air ,without raising headNostrils are valvular , they can be closed during under water stayEpiglottis has become tubular, elongated

Blow hole

Storage of large quantity of air for underwater stayModification of lungsHigh concentration of myoglobin in musclesStorage large volume of oxygen in rete mirabileLungs are large , highly elastic & non lobular with high storage capacityDiaphragm is oblique & this makes the thoracic cavity sufficiently large for maximum expansion of lungs.30 minute survive in under water

Modification of sense organsEyes ::::::::::1 )relatively smaller in size 2 ) cornea is flat & lens is reduced 3 ) nictitating membrane , lacrimal gland ,lacrimal ducts are absent 4 ) during under water stay eyes are protected by a fatty secretion from Harderian gland

Ear :::::::: 1) external auditory opening is small 2) auditory canal is filled with secretion 3) Acoustic system is reduced

Reproductive modificationsTestes are intra abdominal in position & scrotal sacs are absentThe period of gestation is considerably long (more than one year) so that young ones will be much grow and nearly one-third of the mothers size when born

Development of young ones Can be divided in 3 categoryNeoformations : development of tail flukes dorsal fin , baleen plates, , harderian gland etc..Degenerations : loss of ancestral structures , loss of hair , hind limbs , finger nails , pinnae , cutaneous glands , lacrimal glands , nictitating membrane , scrotum etc

Modifications : streamlined body , dorsal valvular nostril blubber rete mirabile highly oblique diaphragm flippers simple bilobed liver non lobular lungs tubular & elongated epiglottis chambered stomach elongation of skull hyperdactyly ,hyperphalangy reduced sternum shortening fusion of cervical vertebrae etc

Thank you by Anand