Mammals: An Overview PDF

Mammals, with their highly developed nervous systems and numerous adaptations, occupy almost every environment on Earth that supports life. They are not a large group, containing about 4600 species, as compared with approximately 9000 avian species, 24,600 fish species and 800,000 insect species. Ho...

Mammals, with their highly developed nervous systems and numerous adaptations, occupy almost every environment on Earth that supports life. They are not a large group, containing about 4600 species, as compared with approximately 9000 avian species, 24,600 fish species and 800,000 insect species. However, class Mammalia is arguably the most biologically differentiated group within the animal kingdom, being exceedingly diverse in terms of size, shape, form, and function. Size ranges from the Kitti's hog-nosed bats discovered in Thailand, 2008 weighing 1.5g to blue whales, which exceed 130 tonnes. Despite their adaptability, and in some cases because of it, mammals have been influenced by the presence of humans more than any other group of animals. We have domesticated numerous mammals for food and clothing, as beasts of burden, as pets and for medical research. For example, in 2005, according to the United Nations, there were 1.3 billion domestic cattle, 960 million pigs, 54 million horses, 808 million goats and one billion sheep, although the most numerous domestic animal is the chicken at 16 billion individuals. In 2013 the United Nations Department of Economic and Social affairs, Population Division put forward a 2010 estimate that the world population was 6.916 billion. These statistics are continuously changing but they serve to demonstrate the overall statistical trend. We have also introduced alien mammals into new habitats, occasionally with successful results, but usually with poor and even disastrous results. Although history shows us distinct examples (e.g. the extinction of Aurochs in 1627, Steller's sea cow in 1768, Quagga in 1883, and the Tasmanian wolf in 1936) we continue to over crop valuable wild stocks of mammals. The whaling industry has threatened itself with total collapse by exterminating its own resource, where competing segments of an industry are intent only on reaping what they can today with no thoughts of supplies for tomorrow. In some cases, the destruction of a valuable mammalian resource has been deliberate, such as the officially sanctioned policy of the American government of exterminating the food source of the Native American peoples, the bison. Although commercial hunting has declined, the ever-increasing human population with the accompanying habitat destruction is still causing major problems for the world's mammals. A growing number of species and sub species are considered endangered by the National Union for the Conservation of Nature and Natural Resources (IUCN), and includes most cetaceans, wild cats, otters and non-human primates. An international ban on commercial whaling took effect in 1986; however, countries such as Japan are still killing hundreds of animals yearly under the guise of scientific research. Origin and evolution The evolutionary descent of mammals from the earliest amniote ancestors to modern examples is perhaps the best known and fully documented of all vertebrates. The fossil record provides evidence for over 150 million years of endothermic, furry mammals from their exothermic hairless ancestors. Skull structures and teeth are by far the most abundant fossils found, and it is largely from these structures that the evolution of mammals is described. The structure of the skull roof allows the identification of three major groups of amniotes, which diverged in the Carboniferous period of the Palaeozoic era; the synapsids, anapsids and diapsids (see Module 6). The synapsid group, which includes mammals as its descendants, has a pair of openings in the skull roof for jaw muscle attachments. The earliest synapsids radiated extensively into diverse herbivorous and carnivorous forms, usually collectively referred to as pelycosuars, and were the most common amniotes in the early Permian. Pelycosuars have a morphological resemblance to modern lizards, although they are not closely related. From one group of early carnivorous synapisds arose the therapsids, which went on to become the dominant large terrestrial animals during the latter half of the Permian. They have dominated the world twice: once in the Permian and once in the Cenozoic (current era). They were by far the most diverse and abundant animals of the Middle and Late Permian. After flourishing for many millions of years, these successful animals were all but wiped out by the Permian Triassic mass extinction about 250 million years ago. Therapsids had an efficient erect gait with upright limbs positioned beneath the body. Since stability was reduced by raising the body off the ground, the cerebellum, (muscular coordination centre of the brain) assumed a greater role. Therapsid changes in morphology of the skull and mandibular (jaw) adductor muscles are associated with increased feeding efficiency. Previously, pelycosuars and therapsids have been referred to as 'mammal-like reptiles', but this terminology is no longer commonly used, as they are not part of the reptilian lineage. Only a few therapsids (not pelycosaurs) survived the Permian extinction. They went on to be successful in the new early Triassic landscape; they included the cynodont Cynognathus (see left), which appeared later in the early Triassic. Cynodonts evolved several features that supported a high metabolic rate including increased and specialised jaw musculature (allowing a strong bite), several skeletal changes (giving greater mobility), turbinate bones in the nasal cavity (aiding the retention of body heat), and a secondary bony palate (enabling the animal to breathe whilst holding prey or chewing food). This innovation is particularly important later during evolution, as it allows the young to breathe and suckle at the same time. The earliest mammals (about 200 million years ago) were almost certainly endothermic, although it is thought that their body temperature would be lower than modern placental mammals. Hair was essential for insulation, and the presence of hair implies that sebaceous and sweat glands may have evolved at this time to lubricate the hair and promote heat loss. During the evolutionary succession from early therapsid to cynodont to mammal, the main lower jawbone, the dentary, replaced the adjacent bones. Thus, the lower jaw gradually became just one large bone, with several of the smaller jawbones migrating into the inner ear and allowing sophisticated hearing. Whether through climate change, vegetation change, ecological competition, or a combination of factors, most of the remaining large cynodont and dicynodonts had disappeared even before the Triassic-Jurassic extinction event. Their places were taken by the diapsid archosaurs (dinosaurs), which dominated the terrestrial ecosystem for the rest of the Mesozoic Era. The remaining Mesozoic synapsids were small, insectivorous animals. Synapsid's evolution into mammals is believed to be triggered by moving to a nocturnal niche, one of the few niches that the increasing dinosaurs didn\'t dominate. Proto-mammals with higher metabolic rates were able to keep their bodies warm at night, and were more likely to survive. This meant consuming food (insects) more rapidly. These proto-mammals were able to change direction more quickly in order to catch small prey at a faster rate. Rather than out-running predators, they out manoeuvred them. During the Cretaceous period, about 54 million years ago, modern mammals began to diversify rapidly. This great radiation was partly attributed to the numerous habitats left vacant by the extinction of the vertebrate groups. It was also attributed to their adaptability, agility, intelligence, ability to give birth to live young, giving significant parental care including feeding. Thus, they could dispense with the vulnerability of having eggs in nests. The diagram below summarises the evolution of the mammalian ancestors at the same time as vertebrate groups. The animals illustrated are designed to give a flavour of the range of types of mammal and other vertebrates in each time period, and are not meant to be a comprehensive list. Pulse 1 (Permian): Pelycosaurs. Unspecialised, medium to large amniotes (20kg-100kg) with sprawling posture, ectothermic, and tropical distribution. Pulse 2 (early to middle Triassic): Therapsids. Probably higher metabolic rate than pelycosaurs: \'improved\' posture and jaw musculature. A wider geographical distribution (tropical and temperate zones) and size range 10kg-500kg. Pulse 3 (late Triassic): Cynodont therapsids. Some degree of endothermy: diaphragm, secondary palate, differentiated dentition, masseter muscle in jaw and size range 0.5kg-30kg Pulse 4 (latest Triassic to mid-early Cretaceous): early true mammals. Endothermic, widespread geographically, low diversity and size range 0.03kg-0.5kg Pulse 5 (late early Cretaceous to latest Cretaceous): First therian mammals. Concurrent with the radiation of flowering plants. Split of therians into placentals and marsupials. Multituberculates (non-therians) also diversify and obtain more complex cheek teeth. Size range up to 5kg. Pulse 6 (early Palaeocene to middle Eocene): \'Archaic\' therians and marsupials. First true carnivores and semi-aquatic herbivores. Up to 1000kg. Pulse 7 (late Eocene to Recent): Modern therians, marsupials confined to Australasia and South America by Middle Miocene. Mammals inhabit diverse habitats, radiation of specialised groups, e.g. bats, whales, and hominoids. 0.002kg-5000kg (terrestrial), 100,000kg (aquatic). © OXL/CN/AN 2022 119 Student No: PD24-51917-ZOCIE15 Email: walkergrace116\@gmail.com Name: Grace Walker Structural and functional adaptations Mammalian skin and its derivatives distinguish mammals as a group. Mammalian skin is generally thicker than in other vertebrate classes, although it consists of the same two layers, the epidermis and the dermis. Epidermal depth varies according to whether the area has a dense hair covering, or is in a position of increased wear, e.g. paws. Hair is especially characteristic of mammals. All mammals possess some hair, although in whales, it is reduced to a few bristles around the mouth. A hair grows from a proliferation of cells in a dermal follicle. As the hair grows, new cells are pushed up away from their source of nourishment and die; they are filled with keratin that also constitutes nails, claws, hooves and horn. Mammals characteristically have two types of hair forming their pelage (fur), a dense soft underhair for insulation and a coarse and longer 'guard' hair for protection and colouration. In aquatic mammals (fur seals, otters, etc.) the guard hairs form an impenetrable barrier, leaving the underhair and the skin dry. Hairs die when they reach a certain length and are pushed out by new growth. In most mammals, there are periodic moults of the entire coat, sometimes with accompanying colour and density changes to aid camouflage and changing environmental conditions. In other mammals, such as humans, moulting occurs continuously. Mammalian hair has been extensively modified for different purposes, e.g. bristles of pigs, spines as armour (porcupines, hedgehogs), vibrissae (whiskers) in most mammals as sensory organs and camouflage/warning colours, e.g. spotted cats, zebra stripes, arctic hare/fox and skunk stripes. Several types of horns or hornlike structures are found in mammals. True horns are found in members of the family Bovidae (e.g. sheep and cattle), and are hollow sheaths of keratinised epidermis enclosing a core of bone arising from the skull. True horns are neither shed, nor branched, but grow continuously and occur in both sexes. Antlers occur in both sexes of the family Cervidae (deer). They are branched, shed annually and usually only occur on males. During the growth phase, they are covered in a 'velvet' highly vascularised skin, the blood supply is constricted before the breeding season and the velvet is shed. The antlers are shed after the breeding season, further antlers begin to grow soon afterwards, and each successive year, antlers have more extensive growth. Annual antler growth places a strain on mineral metabolism; an older moose must accumulate 50 or more pounds of calcium salts from its herbivorous diet. Rhinoceros horns consist of hair-like keratinised filaments that arise from dermal papillae cemented to, but not attached to, the skull. Nearly all mammals are endothermic. Most mammals also have hair to help keep them warm. Like birds, mammals can forage or hunt in cold weather and climates where reptiles and large insects cannot. Endothermy requires plenty of food energy, so weight for weight; mammals need to eat more food than reptiles. A rare exception, the naked mole rat, is exothermic ('cold-blooded'). Birds are also endothermic, so endothermy is not a defining mammalian feature. Of all vertebrates, mammals have the greatest variety of integumentary glands. Most can be categorised as sweat, scent, sebaceous or mammary, all are of epidermal origin. Sweat glands are tubular highly coiled glands occurring over the majority of the skin surface of most mammals, they are not present in other vertebrates. There are two types, eccrine glands, which secrete watery fluid to cool the skin, and apocrine glands, which are larger and open into a hair follicle. They develop in puberty and are limited in humans to the armpits, the pubic mound, breasts, the prepuce, the scrotum and external auditory canals. Apocrine secretions are milky fluids, white or yellowish in colour that dry on the skin to form a film. They are involved in reproduction. Sebaceous glands are usually associated with hair follicles, and keep the hair pliable and glossy. In humans, they are concentrated on the scalp and face. Scent glands are present in nearly all mammals, although their function and location varies greatly. They are used for territorial marking, communication within a species, for warning and for defence. Scent glands are located in orbital, metatarsal and interdigital regions (deer), behind the eyes and on the cheek (pica, woodchuck), in the penis (beavers, canines), in the tail base (wolves and foxes), at the back of head (dromedary), and in the anal region (weasels, skunks). Mammary glands, which occur on all female mammals, are in a rudimentary form on males. They develop by the thickening of the epidermis to form a milk line along each side of the abdomen in the embryo. On certain parts of this ridge, glands appear whilst the intervening areas disappear. Mammary glands increase in size at maturity, becoming considerably larger during pregnancy and subsequent nursing. In humans, adipose tissue begins to accumulate around the mammary glands at puberty to form the breast. For the majority of mammals, mammary glands have a single function, that of feeding the young. In humans, they have an additional sexual role. In most mammals, milk is secreted from the mammary gland via nipples (or teats); however, monotremes lack nipples and simply secrete milk into a depression on the mother's belly, where the young lap it up. Diet and feeding Mammals exploit a huge number of food sources, some are specialist feeders, and others are opportunistic feeders that thrive on a diversified diet. Food choices and physical structure are closely linked. More than any other attribute, dentition reveals the life habits of their owners. With few exceptions, (e.g. monotremes, anteaters, and certain whales) all mammals have teeth and modifications are correlated to diet. Unlike reptiles, mammals do not replace teeth throughout their lives, most only have two sets, firstly deciduous (or milk teeth), which are replaced by permanent teeth when the skull has grown large enough to accommodate a full set. As mammals evolved during the Mesozoic, major changes occurred in teeth and jaws. From the uniform teeth of the first synapsids, mammalian teeth have become differentiated for specialist activities such as cutting seizing, gnawing, grazing, tearing, holding, grinding and chewing. Animals exhibiting this differentiated dentition are known as heterodonts. Mammalian teeth can be categorised into four types, incisors (for snipping and biting), canines (for piercing), premolars and molars for shearing, slicing, crushing or grinding (the photo shows the dentition of a grey wolf). The feeding (trophic) structures of a mammal (teeth and jaws, tongue, and alimentary canal) are adapted to a particular feeding habit. Mammals are usually classified according to one of four basic trophic categories, insectivores, carnivores, omnivores and herbivores. Insectivores such as shrews, moles, anteaters and most bats are small. Most small invertebrate are included in this type of diet, including insects. However, larger animals also occasionally eat this sort of prey, so the definition of this category can be seasonally variable according to available food sources. Herbivores are mammals that feed on grasses and other vegetation, and can be further categorised into browsers and grazers such as hoofed animals (horse, deer, antelope, cattle, sheep and goats) and gnawers such as rodents, and lagomorphs (rabbits and hares). Other adaptations to a vegetable diet are necessary in addition to specialist dentition. Cellulose, the structural carbohydrate of plants, is a tough material, which few enzymes can break down. Herbivorous mammals have commensal bacteria and protozoa in fermentation chambers in their guts, which can metabolise cellulose into fatty acids, sugars and starches, which the host animal can absorb. Hare rabbits and some rodents additionally eat their faecal pellets (coprophagy) giving the food a second passage through the digestive system to maximise absorption of nutrients. Carnivorous mammals feed mainly on herbivores, and include species such as foxes, dogs, cats, and stoats. Carnivores are equipped with biting and piercing teeth, and powerful, clawed limbs for catching and killing prey. Meat protein is easier to digest than vegetative matter, as the digestive tract is shorter and simpler than that of herbivores. In general, carnivores lead more active lives than herbivores; catching prey requires more intelligence than eating plants, whereas herbivores have developed agility and defensive measures to avoid the predators. Humans however, have changed the balance between the carnivore and herbivore. Carnivores have suffered extensively from the presence of humans, and have been virtually eliminated from many habitats, whereas small herbivores such as rabbits have thrived despite human efforts to eradicate them from certain areas. We have largely removed the natural predators of these 'pests', only to be unable to replace them with another measure of control. Omnivores include pigs, racoons, many rodents, bears and most primates (including humans). They use both animal and plant material as food. Many carnivorous forms will also eat plant material if their usual prey is scarce, e.g. foxes will eat frozen apples and berries. Other mammals, thought to be herbivorous, (such as some rodents) will also take insects on occasion. Many mammals store food during periods of plenty. This behaviour is common in rodents (e.g. squirrels, hamsters, chipmunks). Some store all the food in one location, (hamsters have underground burrows) or store individual food items in different places (scatter hoarding e.g. grey squirrels). Other species store large amounts of body fat to help them through times of food scarcity, such as hibernating bears, and reproducing female whales. Migration Unlike birds, few mammals make regular seasonable migrations, instead tending to centre their activities in a defined and limited home range. An interesting example of mammalian migration is from the caribou of northern Canada and Alaska, which make a 160km to 1100km migration twice yearly from autumn/winter from feeding grounds in southerly forests to barren tundra breeding grounds in the spring. Oceanic seals and whales carry out the longest mammalian migrations. Grey whales migrate between Alaska in the summer and winter off California, a journey of 18,000km yearly. Somewhat surprisingly, bats do not copy the flighted bird groups, as few migrate. Most deal with harsher winter conditions by hibernating, not by moving to warmer climates. Flight and echolocation Gliding and flying evolved in several groups of mammals, including marsupials, rodents, flying lemurs and bats. Bats occupy an ecological niche not often taken by birds, that of the nocturnal environment. Their success is due to two attributes, flight and the ability to navigate in the dark, that is, echolocation. When in flight, bats emit short pulses (5msec-10msec in duration) in a narrow directed beam from the nose or mouth. Each pulse is frequency modulated, so it is higher in pitch at the beginning. When hunting up to 200 pulses a second can be emitted during prey capture. Normal navigational pulses occur at about ten pulses per second. Pulses are emitted so that the echo is heard before the next pulse is emitted. Some frequent prey species (moths) have co-evolved ultrasonic detectors to prevent predation by bats. Not all bats are able to echolocate, 170 species of fruit-eating bat find food by sight and smell. Other insectivorous mammals such as shrews have also developed a crude form of echolocation, but many aquatic mammals use it extensively, for example, totally blind sperm whales have been found with full stomachs. Toothed whales (e.g. dolphin) can determine the size, shape, speed, direction, and density of objects by echolocation, and know the position of every member of their pod. Reproduction There are three different patterns of reproduction in mammals. Considered the most primitive, are the egg laying (oviparous) monotremes. The duck-billed platypus has one breeding season each year; ovulated eggs (usually two) are fertilised in the oviduct. Eggs continue to develop in the uterus where they are nourished by yolk deposited prior to ovulation and by secretions from the mother. A thin leathery shell is secreted around the embryos prior to the eggs being laid. Eggs are laid in a burrow and the young hatch in a relatively undeveloped state about 12 days later. Echidnas incubate their eggs in an abdominal pouch, after hatching young feed on milk from mammary glands. As previously mentioned, monotremes have no nipples, so milk is lapped from the belly fur of the mother. Marsupials are pouched, viviparous mammals that exhibit the second pattern of reproduction present in mammals. Although only the eutherian group are called 'placental' mammals, marsupials do have a primitive type of placenta, (choriovitelline or yolk sac placenta). A marsupial embryo (blastocyst stage) is first encapsulated by shell membranes. Most marsupial embryos do not implant in the uterus as they do in eutherians, but they do erode shallow depressions in the uterine wall in which they lie and absorb nutrient secretions from the mucosa by way of the vascularised yolk sack. Gestation (the intrauterine part of development) is brief and all marsupials give birth to very undeveloped young (effectively still embryos, both anatomically and physiologically). However, birth is followed by a prolonged period of lactation and parental care. Although it is tempting to class marsupials as an intermediate stage between the absence of placentas in monotremes and the persistent chorioallantoic placenta in eutherians, evolutionary evidence does not indicate this. All marsupials and placental mammals have a choriovitelline placenta, and a chorioallantoic placenta is present in most primitive marsupials. This suggests that a chorioallantoic placenta was present in the common ancestor of marsupials and placental mammals, but was later lost in most marsupials. In red kangaroos, the joey is born after a 33-day gestation, it then crawls without assistance to the pouch and attaches to a nipple. The mother immediately becomes pregnant again, but this second embryo's development is arrested at the 100-cell stage. This embryonic diapause lasts about 235 days while the first joey is growing in the pouch. When it leaves the pouch, the second joey begins to develop in utero again. The mother becomes pregnant again. At this point, the mother has one joey in utero, one in pouch and one returning to suckle. This system maximises the breeding potential. If any youngster fails to survive, there is already another waiting to take its place. The third pattern of reproduction is seen in viviparous placental mammals, the eutherians. In this group, reproductive investment is in a long gestation, unlike marsupials that invest in prolonged lactation. The embryo remains in utero, nourished by food supplied initially by a choriovitelline placenta and later by a chorioallantoic type of placenta, an intimate connection between mother and young. Gestation in this group lasts from 16 days in the hamster, 30-36 days in rabbits, 60-65 days in cats and dogs, 280 days in cattle and 22 months in elephants. Usually the larger the animal the longer the gestation, although there are exceptions, for example, baleen whales are pregnant for 12 months, and bats (the size of hamsters) have a gestation period of 4-5 months. The condition of the young at birth also varies greatly, from blind, naked and helpless rodent pups to fully formed and able to run (within minutes of birth) in the case of antelopes. The number of young produced in a season is usually correlated to size and mortality rates. Small mammals that are heavily preyed upon tend to have many young, at frequent intervals. Meadow mice have been known (in exceptionally good food years) to produce as many as 17 litters of 4-9 young in a single year. Most carnivores have one litter of 3-5 young per year; larger animals such as ungulates, elephants and horses have one offspring per pregnancy. An elephant, on average, will produce four calves during her reproductive lifespan of around 50 years. Behaviour The dependence of the young mammal on its mother for nourishment has made possible a period of training. Such training permits the non-genetic transfer of information between generations. The ability of young mammals to learn from the experience of their elders has allowed a behavioural plasticity unknown in any other group of organisms and has been a primary reason for the evolutionary success of mammals. The possibility of training is one of the factors that have made increased brain complexity a selective advantage. Increased associational potential and memory extend the possibility of learning from experience, and the individual can make adaptive behavioural responses to environmental change. Individual response to short-term change is far more efficient than genetic response. Some types of mammals are solitary except for brief periods when the female is in estrus. Others, however, form social groups. Such groups may be reproductive or defensive, or they may serve both functions. In those cases that have been studied in detail, a more or less strict hierarchy of dominance prevails. Within the social group, the hierarchy may be maintained through physical combat between individuals, but in many cases stereotyped patterns of behaviour evolve to displace actual combat, thereby conserving energy while maintaining the social structure. A pronounced difference between sexes (sexual dimorphism) is frequently extreme in social mammals. This is because dominant males tend to be those that are largest or best armed. Dominant males tend to have priority in mating or may have exclusive responsibility for mating within a 'harem.'