In this article we will discuss about Mammals: 1. Introduction to Mammals 2. Locomotion in Mammals 3. Sense Organs 4. Hearing and Balance 5. Origin.
Contents:
- Introduction to Mammals
- Locomotion in Mammals
- Sense Organs in Mammals
- Hearing and Balance in Mammals
- Origin of Mammals
1. Introduction to Mammals:
Mammals occupy the highest position in the ladder of evolution. The structural diversities amongst the different groups of mammals are profound. They vary in size from that a field mouse barely 2.5 cm in length to that of a whale attaining a length of more than 30 metres.
A shrew, Sorex minutus (order Insectivora) is the smallest living mammal weighing about 3 grammes in comparison to whale, the Balaenoptera which is the largest mammalian form of about 122 tonnes in weight. Elephants are largest among the land mammals. The giraffe is the longest for its elongated neck.
Mammals have colonised all environments in course of their evolution. Adaptive radiation at its zenith is encountered amongst the mammals.
2. Locomotion in Mammals:
The mammals are basically quadruped animals. The legs are like the ‘towers of the bridge’ and the backbone is the ‘arched cantilever system’ supported by the ‘towers’. This whole system carries the animal and helps to secure food, shelter and other biological needs.
Mammals exhibit extensive adaptive radiation for locomotion. The skeletal framework becomes greatly modified in relation to diverse modes of locomotion.
Plantigrade (Ambulatory)—the central type of locomotion:
The ancestral mammals were plantigrade, i.e., the feet (soles) and toes touched the ground during locomotion. This type of locomotion is observed in human beings. The other mammals which serve as examples of this type of locomotion are: opossums, bears, raccoon, shrews, mice, etc.
This is the central type of locomotion from which other types of locomotion have radiated in mammals. The mammals under this category walk on the entire foot and are typically five-toed. The metatarsals and metacarpals are not fused and are longer than the phalanges.
The wrist and ankle bones permit movement in various planes. In larger mammals (exemplified by bears) the locomotion is ambulatory while in smaller forms (e.g., shrews, opossums, etc.) the locomotion tends toward the cursorial types. Human beings practice an ambulatory bipedal plantigrade type of locomotion.
Cursorial (Running) type of locomotion:
Surface-oriented larger mammals depending on speed for catching prey or survival show cursorial locomotion. Larger mammals including carnivores, horses, zebras, deer, pronghorn, antelopes, cattle, bison, giraffe show this type of locomotion. Cursorial type of locomotion reaches its peak in ungulates living on the plains. Cursorial mammals have an elongated body and neck.
The elongated neck is used to shift the centre of gravity forward when the animal attains momentum during locomotion. Odocoileus virginianus (white-tailed deer) stretches its neck far forward when it moves at its greatest speed.
The limbs become lengthened with the tendency towards fusion or loss of metacarpal and metatarsal bones into the cannon bones. The joint surfaces become tongue-and-groove types restricting the movement of the limbs in a single plane parallel to the long axis of the body.
Depending on the degree of contact with the ground, cursorial locomotion is divided into two types:
A. Digitigrade type:
Only the toes touching the ground, i.e., walking on toes. Most of the carnivores, cats, dogs, etc., are digitigrade animals. It is estimated that Cheetah (Acinonyx jubatus) attains a maximum speed of 60-65 mph (100 km/hr). It gallops in a ‘measuring-worm fashion’.
B. Unguligrade type:
Only the tips of toes touch the ground, i.e., walking on tips of toes.
Examples:
Zebra (Equus), deer (Odocoileus), pronghorn (Antilocapra americana), moose (Alces americana), African antelopes etc.
Jumping type of locomotion:
The jumping type of locomotion is closely similar to that of cursorial type. Some mammals always move by jumping and use both saltatorial and ricochetal jumping depending on speed. Dipodomys (Kangaroo rats) and Zapus (Jumping mice) use both methods of jumping.
A. Saltatorial type:
When four feet are used in jumping. Rabbits are the best examples. Rabbits, hares and jumping mice possess longer hind limbs. The hind limbs are more muscular than the forelimbs. The forelimbs are usually used for digging or manipulation. The neck becomes short.
B. Ricochetal type:
When only the hind limbs are used in jumping. Kangaroos are the best examples.
Amphibious, aquatic and marine locomotion:
Adaptations for living and swimming in water are all secondary. The mammals under this category have evolved from previous terrestrial mammals.
Depending on the degree of modifications they are divided into the following types:
A. Amphibious type:
The mammals of this type include the beaver (Castor), musk rat (Ondatra), nutria (Myocaster), otter (Lutra), mink (Mustela) and many others. An increase in thickness and quality of hair is usually encountered in these mammals.
The tail becomes modified for aquatic locomotion. It is dorsoventrally flattened in beaver and laterally flattened in musk-rat and nutria. The surface area of the feet has been increased by webbing or addition of stiff hairs.
B. Aquatic type:
The mammals under this category spend most of the time in water and usually come to land for reproduction. The typical examples are seals and hippopotamus. The forelimbs and the hind limbs become highly modified into paddle or fin for swimming.
C. Marine type:
These mammals never come to land. The typical examples are whales. They underwent adaptations to live in water and never leave it. The body is ovoid with short and rigid neck. The skin is hairless except for vibrissae.
Presence of subcutaneous fat (blubber) is a physiological adaptation. The tail becomes modified into a horizontal fluke which serves as the propelling organ. Besides, there are many other morphological as well as physiological adaptations for marine life.
Fossorial type:
There are many mammals who spend their entire life in the underground. They become specially adapted for this mode of life. The pocket gophers and moles are the typical representatives. Most of them are small in size.
The digging apparatus becomes highly evolved in this group of mammals. There are some semi-fossorial type [e.g., badger (Taxidea)] which spends much of its time above the ground. In fossorial mammals the profile of the head is triangular and flat (e.g., Spalax). Besides modifications of skull the post-cranial portion of the skeleton becomes also modified.
The forelimbs together with the pectoral girdle are modified for digging efficiency in different ways (Fig. 10.120). The forelimb may be provided with sesamoid and hetero-tropic bones. In Scalopus the palmar regions are furnished with stiff hairs.
In much semi-fossorial type (Taxidea) the claws become greatly elongated. These claws grow at a quicker rate to make up for the wear and tear for digging. In pocket gopher (Thomomys bot-tae) three centre claws grow 0.23 mm/day or over 0.84 cm/year.
Graviportal (Recti-grade or Sub-unguligrade) type:
This type of locomotion is best illustrated by an elephant which means movement on pillars. The limbs are pillar-like and the articulating ends are flattened. Each limb has five digits arranged in a circle around its edge and an elastic tissue pad is present under the foot.
Arboreal type:
Many mammals, specially living in forest areas, have become modified to live on trees. This mode of living is named as arboreal. Arboreal mammals are able to climb the trees and use their branches as the highways.
Modifications for holding onto tree branches are observed in these mammals. Tree squirrels and sloths have well-developed claws. Some arboreal mammals possess prehensile tail. The tarsier develops adhesive discs on the front toes.
Sloths spend most of their time hanging upside down the trees and lead a sedentary life. The skeletal system becomes greatly modified. The neck is short with the unusual number of cervical vertebrae (i.e., seven in number) in Choleopus tridactylus (two-toed sloth), but in Bradypus tridactylus (three-toed sloth) there are nine cervical vertebrae.
Strong shoulder girdle, well-formed clavicle, increase in the number of ribs are some of the important arboreal adaptations.
The squirrels use the trees for climbing and jumping rather than hanging. In typical forms the body is elongated, the hind limbs have well-developed musculature, well- developed and sharp claws and well-formed sense organs.
Brachiation:
This is a specialised type of arboreal locomotion which means swinging from branch to branch by using the forelimbs only. The forelimbs become greatly lengthened. In gibbons the forelimbs may touch the ground. The stereoscopic vision is very good which helps a gibbon to have a 12-metre-jump from one branch to another with precision.
Volant, gliding and glissant:
Volant and glissant are interchangeable names for gliding type of locomotion. The flying squirrels (Rodentia), flying phalangers (Mursupialia) and the flying lemur (Dermoptera) have developed extra sustaining surfaces formed by the flap of skin (Patagium).
The patagium may extend from the fore to hind limbs on both sides. In colugo (Cynocephalus) the patagium connects the head, forelimbs, hind limbs and tail. Gliding has evolved at different times in different groups of Mammalia.
True flight (flying) exists in bats only. The ‘winged hands’ are the lifting surfaces required for true flight. The hand and arm have modified into a wing. Greatly elongated radius and digits ll-IV of the hand support the patagium. The hind limbs can rotate 180° when it remains suspended upside down from a branch of tree.
Methods of Terrestrial locomotion:
The method, most frequently practiced by the great majority of land mammals, is four-footed or quadrupedal locomotion.
The mechanics of locomotion can be translated in the following way:
Walk:
In walk, the animal raises the two diagonally opposite feet, for example the anterior left and posterior right, it advances them while the other diagonal pair support and propel the body. The animal then replaces on the ground the feet it has forwarded and raises the other two. This diagonal gait is exhibited by the members of the cat family and dog family.
Pacing:
In pacing the legs of the same side are moved simultaneously. Thus when the two feet on the right side are on the ground the other two on the left side are raised, and just when the latter are put down the others are raised. Giraffe, brown bear and camel exhibit such pacing.
Gallop:
It is nothing but succession of leaps. The animal throws itself by means of the hind legs, extends its body in the air and lands on the forefeet.
Example:
Stoat.
Leaping:
In certain mammals a very peculiar progression by leaps is observed in which the tail plays an important role. Australian kangaroos progress in this fashion. When it walks slowly, it supports its body partly on its forelegs and partly on its tail and then raises the hind-legs together, then in a second movement, it is supported on its hind-feet, while its forefeet and tail are raised.
When the animal accelerates its gait, it employs its hind-legs and during the leap, the tail acts as counterbalance and is held horizontal.
Bipedal:
Man alone is bipedal and walks with the help of the two hind limbs. Chimpanzees and gorillas are partially bipedal. They cannot stand upright and while walking they generally touch the ground with their digits of the forelimbs.
Methods of Arboreal locomotion:
Many mammals spend their entire lives on trees, descending to the ground rarely or accidentally. This is true for certain monkeys and apes as well as for many rodents and marsupials. The techniques of arboreal locomotion are most probably derived directly from the techniques of walking.
Climbing:
In mounting a tree trunk alternate grasping with the hand and foot of one or the other diagonal is practiced by some monkeys, and three toed ant-eater, Tamandua. Some animals climb by alternate movement of the limbs of one side and then of the other. Others climb by successive holds first by the forelimbs and then by the hind ones.
Possession of a prehensile tail facilitates arboreal locomotion. In leaping from one branch to another, spider monkeys use hands, arms and tail. Some flying squirrels possess scales beneath the tail and the scales act as anti-skids.
Brachiation:
The special mode of progression of gibbons and spider monkeys has been termed Brachiation. It corresponds in a way to the bipedal walk but here the front limbs are used. The mechanism of the two-handed locomotion is simple.
First one of the hands grasps a branch and draws the body forward. Then the body oscillates on that pivot made by the hand and the other hand extends to hold another branch. If the animal wants to go from one branch to another little far off branch a short glide occurs between the holds.
Aquatic Locomotion:
Swimming:
Aquatic mammals swim in water with the aid of modified forelimbs or hind limbs and with the undulation of tail. Many terrestrial animals can swim.
Aortic Arches of Vertebrates:
Aortic arches are paired blood vessels that emerge from the ventricle of the heart which are basically similar in number and disposition in different vertebrates during the embryonic stages.
Embryonic arterial arches:
During the embryonic stages six pairs of arterial arches develop in most gnathostomes and are named according to the name of the visceral clefts.
The aortic arches are designated by Roman numerals (I—VI, Fig. 10.145A). The first aortic arch (I) is called mandibular aortic arch that proceeds upwards on either side of the pharynx and turn backwards as lateral longitudinal tubes, called radices aortae or lateral aortae which both join mesially to form the common dorsal aorta.
The second aortic arch becomes hyoid arch. The third (III), fourth (IV), fifth (V) and sixth (VI) are called branchial arches. Table 58 relates the modification of embryonic arterial arches in adult in different vertebrates.
3. Sense Organs in Mammals:
They have the same general plan of structure as encountered in birds and reptiles.
Smell:
Organ of Jacobson is well- developed in lower groups of mammals. The olfactory mucosa has become elaborate in higher mammals because of the convolutions of the ethmoturbinal bones. The nasal chamber has lost its sensory functions in the toothed whales where the olfactory nerves are almost vestigial.
Sight:
The structure of the eye resembles that of other vertebrates. The pecten of birds and reptiles is absent. The sclera is composed of condensed fibrous tissue. Most of the mammals are provided with three eyelids in each eye. The upper and lower eyelids are opaque and are provided with hair.
The third eyelid is transparent and hairless. In higher forms of the third eyelid is vestigial. Its vestige can be seen as a pink-fold in the inner canthus of each eye. The eyeball and the eyelids are kept moistened by the secretion of a lachrymal, a harderian and a series of meibomian glands for each eye. Following are the accounts of eye and its associated structures in mammals.
The eyes are ill-developed and almost functionless in burrowing insectivores, moles and marsupial, Notoryctes. The eyes of whales are small. In platanista it is vestigial. The eyes of the whales are variously modified. In them the cornea is flat and the lens is round. The sclera is thick and the eyelids are provided with specialised lid muscles for protection against pressure.
Cartilage is absent in the sclera. A tapetum lucidum is present in many nocturnal forms. Tapetum lucidum is a layer of light reflecting crystals located on the choroid coat adjacent to the retina. Hoofed mammals possess a tape-turn fibrosum. In it the portion of the choroid coat’ is made up of a tendinous type of connective tissue which glistens in a manner similar to a fresh tendon.
Carnivores, seals and lower primates possess another type of tapetum, called tapetum cellulosum. It is composed of several layers of cells filled with small crystals of unknown organic material. The pupil is round in most forms. A vertical slit is characteristic of the cat family. The vertical slit in them becomes round at night.
The slit is transversely arranged in many ungulates and whales. The portion of the iris bordering the pupil is modified to form an irregular, pigmented and fringe-like umbraculum in gazelle and camel. This is a device to protect the eye from excessive glare.
The retina of the eye bears rods and cones. But in lower orders like Edentata, Chiroptera and certain shrews the cones are absent. It has been shown that rods detect the differences in the intensity of light and hence the nocturnal animals usually have rods only. Rods contain a purple pigment, called rhodopsin, which is destroyed by light but is instantly manufactured by vitamin A.
The cones of the eye are sensitive to bright light and to various colours. Recently three more pigments have been detected in the human eyes. They are red-sensitive etythrolabe, green-sensitive chlorolabe and blue- sensitive cyanolabe. The capacity for colour vision, however, is restricted amongst the primates only.
4. Hearing and Balance in Mammals:
Amongst the vertebrates the ear is best developed in mammals. The accessory parts and the membranous labyrinth have become very complex in mammals. All mammals excepting monotremata, cetacea and sirenia possess large external pinna.
This is supported by cartilage and of various sizes and shapes in different mammals. It helps in collecting sound waves and it is turned in different directions by the mammals excepting man. Opening of the external auditory passage lies at the base of the pinna.
The external passage leads up to the tympanic membrane. The walls of the passage may be membranous or cartilaginous or osseous in part. The passage is formed of a series of incomplete rings in Tachyglossus. The middle ear or tympanic cavity is enclosed by periotic and tympanic bones.
The cavity communicates with the pharynx by the Eustachian tube. The inner wall of the tympanic cavity bears fenestra ovalis or fenestra rotunda.
A chain of auditory ossicles—the malleus, incus and stapes— run between the tympanic membrane and rotunda. These are the smallest bones in the body and show variation in form.
The stapes is with a foramen and the perforation is made by a minute artery as in rabbit. Stapes is rod- shaped in manis. The membranous labyrinth of the internal ear is with a specially developed cochlea. The cochlea is spirally coiled and is absent in monotremata.
5. Origin of Mammals:
History of the animal kingdom has been from the beginning a continuous process of evolution and highest in this evolutionary series stand mammals unique amongst the animals. It is an accepted idea that mammals originated from some groups of vertebrates that lie lower in the scale of the ladder of evolution.
These lower vertebrates are constituted by the Fishes, Amphibians, Reptiles and Birds. Of these the Fishes and the Birds can be eliminated since the former is too low and the latter is too specialised in their organisations.
Ancestry through Amphibia:
Amphibian ancestry of mammals through Hypotheria, a stage intermediate between amphibia and mammals, was advocated by T. H. Huxley (1880).
The points on which Huxley based his arguments were:
(a) Presence of two occipital condyles in both amphibia and mammals
(b) Presence of left aortic arch in mammals. As the left aortic arch is weak in reptiles they cannot hold the line of ancestry of mammals.
It is true that there are two occipital condyles in amphibia and mammals. But the source of the condyles is different. In amphibia the occipital condyles are derivatives of exoccipitals whereas in mammals the sources are the basioccipitals. So Huxley’s theory of the origin of mammals through Hypotheria is not justified.
Ancestry through Reptiles:
Paleontological evidences establish the fact that mammals arose from reptiles. Fossils of Synapsida that have been discovered in the carboniferous strata indicate many characters leading to mammalian line. The advanced forms of Synapsida showed a tendency towards reduction of skull bones and teeth differentiation.
From the Synapsida arose the Therapsids during late Permian and upper Triassic.
The Therapsids approached mammalian organisation by having:
(a) Strong and enlarged lateral temporal fossa,
(b) Secondary palate,
(c) Enlarged dentary and teeth differentiation,
(d) Double occipital condyles and
(e) Reduced quadrate and quadrato jugal.
The Therapsids diverged into Dicynodont and Theriodontia and during mid-Triassic from Theriodointia arose Cynodontia and Ictidosauria. It is believed that one of them or both gave rise to primitive mammals.
Different views regarding the origin of mammals:
Monophyletic origin:
Barghusen (1968), Crompton and Jenkins (1968), Crompton (1969) supported the monophyletic origin of mammals. According to them mammals have evolved from cynodonts, the last group of therapsids appeared in the late Permian.
By the end of Triassic non-mammalian cynodonts included some mammal-like groups and had given rise to mammals themselves. Recently Pough et al., (1996) presented the idea that the three groups of living mammals—the monotremes, marsupials and eutherians are all derived from the ‘holothere’ lineage.
Polyphyletic origin:
Colbert (1955):
It is quite possible that the mammals may have had a polyphyletic origin — that several groups of mammals-like reptiles contributed to the ancestry of the early mammals and in the Jurassic times the threshold had been crossed from reptiles to mammals. This view is almost supported by Olson (1959), Carter (1967), Simpson (1971), Bellairs and Attridge (1975), Griffiths (1978) and Young (1981).
Romer and Parsons (1986):
The mammals are descended from reptiles, but the fossil record shows that the reptilian line leading to them, the subclass synapsida, diverged almost at the base of the family tree of that class. Their relationship to the existing reptilian orders is thus exceedingly remote (Fig. 10.160).
