The most diversified animals

There are about 4,600 species of mammals. This is a relatively small number compared to the 9,600 species of birds or 35,000 fish species and almost nothing in comparison to about 100,000 species of mollusks or some 10,000,000 species of crustaceans and insects. Even such groups as extant reptiles (with 6,000 species) and frogs (with about 5,200 species) are more diversified at the species level. Nevertheless, in diversity of body sizes, locomotory types, habitat adaptations, or feeding strategies, the mammals greatly exceed all that is common in other classes.

Only birds and arthropods may approach such variety. However, at least in diversity of body size, the mammals clearly surpass even them. The body mass of the largest extant terrestial mammal—the African elephant Loxodonta africana— with shoulder height of 11.5 ft (3.5 m), reaches to 6.6 tons (6,000 kg). The extinct rhinocerotid Baluchitherium was about 18 ft (5.5 m) and 20 tons (18,000 kg), respectively. The largest animal to ever appear—the blue whale (Bal-aenoptera musculus)—with up to 98 ft (30 m) in length, reaches 220 tons (200,000 kg). In contrast to dinosaurs or elesmo-branchians, which also produced quite large forms, the average mammal is a small animal the size of a rat, and the smallest mammals such as a pygmy white-toothed shrew (Suncus etrus-cus) or Kitti's hog-nosed bat (Craseonycteris thonglongyai) have a body length of just 1.2-1.6 in (3-4 cm) and weigh only 0.050.07 oz (1.5-2 g).

Mammals colonized almost all habitats and regions on the Earth. They now feed on flying insects hundreds of meters above the ground; jump through foliage in the canopy of a tropical forest; graze in lowland savannas and high mountain alpine meadows; hunt for fish under the ice cover of arctic seas; burrow the underground labyrinths to feed on diverse plant roots, bulbs, or insects; cruise the world's oceans, or dive there to depths of 1.8 mi (3 km) in the hunt for giant squid. Some even sit by a computer and write articles like this.

About 4,600 species of mammals are arranged in approximately 1,300 genera, 135 families, and 25 orders. Rodents with 1,820 species, 426 genera and 29 families are far the largest order, while in contrast, 8 orders include less than 10 species, and four of them are even monotypic (Microbiotheria, Noto-ryctemorphia, Tubulidentata, Dermoptera). Although interrelationship among individual orders is still the subject of a vivid debate, three major clades of mammals are quite clear: monotremes (2 families, 3 genera, 3 species), marsupials (7 orders, 16 families, 78 genera and 280 spp.), and eutherian or placentals (17 orders, 117 families, 1,220 genera, 4,300 spp.), the latter two clades are together denoted as Theria.

The essential differences among the three major clades of mammals are in mode of their reproduction and patterns of embryonic development. Monotremes (platypus and echidnas), restricted to the Australian region, show only little difference from their ancestral amniote conditions. They deliver eggs rich in yolk, and incubate them for 10 to 11 days. Young hatch from the egg in a manner similar to birds. Monotremes also retain the reptile conditions in the morphology of the reproductive system: the ovary is large and short oviducts come via paired uteri to a broad vagina, which opens with the urinary bladder and rectum into a common cloaca. Except for monotremes, all mammals are viviparous with intrauterine embryonic development and have quite small eggs, poor in yolk (particularly in eutherians).

There are essential differences between marsupials and eutherians in the earliest stages of embryonic development, as well as in many other characteristics. The reproductive tract in a female marsupial is bifurcated (with two vaginas), and also the tip of the penis in a male marsupial is bifurcated. Many marsupials have a marsupium, the abdominal pouch for rearing young, supported with the marsupial epipubic bones that are present in both sexes. The marsupial in-trauterine development is very short and the embryo is attached to the uterine endometrium by the choriovitelline (yolk) placenta that lacks the villi penetrating deeper in the wall of uterus (except in bandicoots). The marsupial new-borns are very small and little developed, and birth is non-traumatic. In contrast, the lactation period is much longer than in eutherians (only bats and some primates have proportionally long lactation periods). Nevertheless, the mother's total investment by the time of weaning young is roughly equal in both clades, but its distribution is different. The marsupial strategy is much less stressful for a mother and allows an extensive variation in tactics of reproduction. For instance, in the kangaroo, a mother can have three generations of young at one time: the young baby returning to drink low-protein but high-fat milk, the embryo-like young attached to a nipple nourished with high-protein but low-fat milk, and an embryo in the uterus for which development is delayed until the second-stage young is released.

A key agent of eutherian reproduction is the highly specialized organ supporting a prolonged embryonic development—the chorioallantoic placenta. Eutherian newborns are large and despite considerable variation over particular clades, are potentially capable of an independent life soon after birth. Large herbivores such as elephants, perissodactyls, and artio-dactyls, as well as cetaceans, sirenians, hyraxes, and some primates, deliver single, fully developed newborns with open eyes, ears, and even the ability to walk immediately after birth. Such a newborn is called precocial in contrast to the altricial newborns of insectivores, bats, rodents, or carnivores, which are hairless, blind, and fully dependent on intensive mother's care. Both developmental strategies may, of course, appear within one clade as in lagomorphs (large litters and altricial young in a rabbit versus small litters and precocial young in a hare). Variations in reproductive strategies are closely interconnected with numerous behavioral adaptations and adaptations in social organization and population dynamics, all of which contribute significantly to mammalian diversity.

Recent molecular data strongly support the essential role of geographic factors in phylogenetic history and in taxo-nomic diversity of mammals. Thus, there is very strong support for the African clade Afrotheria, which is composed of the tenrecid and potamogalid insectivores, golden moles, macroscelids, aardvark, hyraxes, proboscideans, and sirenia. Also, the extensive covergences between Australian marsupials and particular eutherian clades and/or the paleontological data on mammalian evolution on particular continents suggest that on each continent, the adaptive radiation produced quite similar life forms: small to medium sized insectivores, rodent-like herbivores, large herbivores, and their predators. The niche of large herbivores seems to be particularly attractive (at least 18 different clades attained it) but at the same time, it is perhaps the most dangerous (13 of them are extinct).

Nearly one fourth of all mammals fly. This is pertinent to a number of species, the number of genera, and perhaps for the number of individuals as well. Bats, with more than 1,000 species in 265 genera, are the most common mammals in

The manatee (Trichechus manatus) is primarily herbivorous. Here a mother nurses her young. (Photo by Jeff Foott. Bruce Coleman, Inc. Reproduced by permission.)

many tropical and subtropical habitats. Mostly active at night, bats hunt for various kinds of aerial prey (a basic strategy of the clade) or feed on fruit, nectar, or pollen. Some bats feed on frogs, reptiles, or other bats, and in the tropics of South America, the total biomass of bats exceeds that of all other mammals. Several Old World bats, such as false vampires, feed on small vertebrates, while others feed on fish plucked from the water surface. Frugivorous and nectarivorous bats are the essential agents for pollination and seed dispersal of many tropical plants, including banana and mango. Bats are often very social and form large colonies, including the largest assemblies known in mammals, such as the maternity colony of about 36 million Mexican free-tailed bats in Bracken Cave in Texas.

However, most of the extant mammals (nearly a half of all genera) maintain the basic mammalian niche. They are terrestrial, mostly nocturnal or crepuscular, and forage for different food resources that are available on the ground. In a tropical forest this may be seeds and fruits falling down from the canopy and the invertebrate or vertebrate animals feeding on them. In the subtropics and temperate regions, the significance of this habitat increases as the soil surface becomes the most significant crossroads of ecosystem metabolism. In a temperate ecosystem, the soil is the major conveyer of the energetic flow and an important source of free energy that is available in a variety of food resources. It is no wonder that in the temperate regions terrestrial mammals form more than half of the local mammalian taxa (while it is one third or less in the tropics) and that their densities exceed those of all remaining mammalian species. Among them we find the groups that are the most progressive and most rapidly diversifying clades of the extant mammals (such as shrews or muroid rodents). Terrestrial mammals are, as a rule, quite small animals, and are often the r-strategists. They have short life spans, large litter sizes, several litters per year, and rapidly attain sexual maturity, sometimes even a few weeks after birth. Most of the small ground mammals dig underground burrows for resting. This reduces not only the risk of predation, but due to stable microclimatic conditions of the underground habitat, it also reduces metabolic stress by ambient temperature or by daytime changes in other weather conditions. Many mammals also tend to spend a considerable part of their active life underground, including food gathering. Those that combine it with terrestrial foraging are called semifossorial—most of the 57 genera of semifossorial mammals are rodents. Those that are entirely adapted to an underground way of life and often do not come above ground at all are called fossorial. The fossorial adaptations, which make them all quite similar in general appearance, are seen in 35 genera of 13 different clades and evolved convergently in all major geographic regions (Australian marsupial mole,

Holoarctic true moles, the African golden moles, and 10 groups of rodents in Holoarctic, Ethiopian, and Neotropical regions). Compared to their relatives, the fossorial mammals are all the K-strategists, some with pronounced tendencies to complex organization (mole rats).

The mammals also evolved another way to inhabit terrestrial habitats. It is called scansorial adaptation and is typical of large herbivores with an enormous locomotory capacity, enabling them to exploit distant patches of optimal resources and react actively to seasonal changes in them. In many instances these are social animals living in large nomadic herds. Kangaroos, the large macropodid marsupials of Australia, exhibit this scansorial adaptation. They move rapidly around their terrestrial habitat by hopping bipedally on their long, powerful hind legs, using their long tails for balance.

Locomotory modes are entirely different in the 156 genera of mammals that forage in arboreal habitats. Essentially arboricolous are primates, dermopterans, and tree shrews, as well as many marsupials, rodents, bats, and some edentates and carnivores. Typical for most of them are long forelimbs and a long tail, often prehensile. Other arboricolous mammals have a haired membrane between their legs, enabling them to glide between tree trunks. The mammals equipped for such gliding flight include flying lemurs (Dermoptera), several groups of rodents (flying squirels, African anomalurids), and three genera of marsupials.

Roughly 107 genera and 170 species are aquatic or semi-aquatic and mostly fish-eating. Three grades can be distinguished here: (1) terrestrial animals that enter aquatic habitats only temporarily for feeding only (African otter shrews, Old World water shrews, desmans, water opossum, more clades of rodents, including large rodents such as beaver and capy-bara, and several clades of carnivores, particularly otters); (2) marine mammals that spend most of their life in aquatic habitats but come to shore for breeding (all pinnipedian carnivores, such as seals, sea lions and walruses, and sea otters); and (3) the exclusively aquatic mammals incapable of surviving outside of the aquatic environment—sirenians and cetaceans. The latter group is quite diversified, and includes 78 species in 41 genera that can be subdivided into two major clades: Mysticeti, whales that filter marine plankton with baleen plates hanging from roof of the mouth cavity, and Odontoceti, dolphins and toothed whales, which echolocate and feed on fish or squid (including the giant deep-sea ar-chiteuthids as in the sperm whale). Cetaceans evolved various sophisticated adapatations for prolonged diving into deep oceanic waters, such very economic ways of gas exchange that include a reduced heart rate during diving and more oxygen-binding hemoglobin and myoglobin in blood than in other mammals. Cetaceans, though closely related to non-ruminant artiodactyls and recently included together with them in a common order, Cetartiodactyla, diverge from the common picture of "what is a mammal?" perhaps most of all.

The extreme diversity in feeding adaptations is among the most prominent characteristics of mammals. Feeding specializations such as grazing grass or herbal foliage, palynovory (eating pollen of plants), myrmecophagy (specialized feeding on ants and termites), and sanguivory (feeding on blood of birds and mammals, in five species of true vampires) are not known from any other vertebrates. At the same time, all the feeding adaptations occurring in other vertebrate clades occur also among mammals.

In all mammals, the efficiency of a feeding specialization depends upon the appropriate morphological, physiological, and behavioral adaptations. First, it concerns the design of the teeth and dentition. The generalized heterodont dentition and the tribosphenic molar teeth designed for an insectivorous diet (as retained in various marsupials, insectivores, tree shrews, prosimian primates, and bats) can be easily modified to the carnivorous diet. A carnivorous diet further demands enlarging the size of the canines and arrangements that increase the shearing effect of cheek teeth. A lower position of the jaw joint increases the powered action of temporal muscles at the anterior part of dentition, and in extremely specialized carnivores such as cats, the dentition is then considerably shortened and reduced except for canines and the carnasial cheek teeth (the last upper premolar and the first lower molar, generally the largest teeth of carnivores). There is no problem with digesting the tissues of vertebrates and thus no special arrangements of the alimentary tract are needed.

In contrast, herbivores, especially those specialized in feeding on green plant mass, require a modified jaw design. This kind of food is everywhere and easily accessible as a rule, but it is extremely difficult to digest for several reasons. One is that this diet is very poor in nutritive content and must be consumed in very large volumes; it must also be broken down mechanically into small particles. Hence, the dentition is overburdened by wear of occluding teeth and their abrasion with hard plant tissue. Efficiency of feeding depends directly on the design of the tooth crown, on the size of total area for effective occlusion, and the efficiency of masticatory action. Large teeth with flat surfaces and high crowns resistant to intensive wear are particularly required.

The major problem with a diet of plants is that mammals (as well as other animals) do not produce enzymes that break down cellulose. They must rely on symbiotic microorganisms residing in their alimentary tract, evolve an appropriate housing for them, and ensure a sufficient time for proper food fermentation. The mammals evolved several ways to fulfill these requirements. One is the foregut fermentation (digastric digestion system) characteristic of ruminant artiodactyls (bovids, cervids), kangaroos, and colobus monkeys. The fermentation chambers are situated in spacious folds of the stomach; from these fermentation chambers the partially fermented food can be regurgitated and chewed during a rest period, which also prolongs the movement of food through the gut. The microorganisms detoxify alkaloids by which growing plants defend against herbivores prior to digestion, but are very sensitive to tanins contained in the dry plant tissues. The foregut fermenters avoid dry plants but feed on growing parts of plants, selectively cut with the tongue and lips (ruminants even lack the upper incisors).

Perissodactyls, rodents, lagomorphs, hyraxes, and elephants evolved hindgut fermentation (monogastric digestion system), where fermenting microorganisms are housed in the caecum and large intestine. Food is not regurgitated and all mechan ical disintegration of food must be performed at one mastication event. Except for caeca, the passage of food through the gut is almost twice as fast as in the foregut fermenters. Hindgut fermenters can survive on a very low-quality food, if it is available in large quantity. They can effectively separate the tanins and dry plant mass, both of which decrease the efficiency of the foregut fermenting. Correspondingly, the foregut and hindgut fermenters prefer different parts of plants and can both forage in the same habitats without any actual competition. The latter are, of course, under more intense pressure to evolve further adaptations to compensate for the energetic disadvantages of their digestion. One of them is extreme enlargement of caeca (as in rodents); another is considerable increase in the height of cheek teeth (maximized in several clades of lagomorphs and rodents, in which cheek teeth are hypselodont, or permanently growing). The third way is an increase in body size. This enlarges the length of the alimentary tract and prolongs the passage of food through it, while at the same time it reduces the rate of metabolism. The behavioral reduction of metabolic rate by a general decrease of activity level as in foliovore (leaf-eating) sloths or the koala produces the same results.

The gradual increase in body size is a feature of mammalian evolutionary dynamics, as it was repeatedly demonstrated by the fossil record of many clades. This is seen in most eutherians (not only in the herbivorous clades), but is much less apparent in marsupials. It seems that in addition to the common factors promoting a larger body size (a reduced basal metabolic rate, smaller ratio of surface area to body mass, and smaller heat transfer with ambient environment), something else comes into play, something which has to do with the essential differences of both the clades. This is the enormous stress of the eutherian way of reproduction. While intrauterine development is short and a litter weight is less than 1% of the mother body mass in a marsupial, the eutherian female must endure a very long pregnancy and the traumatic birth of a litter that in small eutherians such as insectivores, rodents, or bats, may weigh 50% of the mother's body mass.

With enlarging body size, the stress of pregnancy and parturition is reduced as the size of a newborn is relatively smaller (compared with 3-5% of a mother's mass in large mammals and 10-20% in smaller mammals). With a reduction of litter size, it further provides a chance to refine the female investment and deliver fully developed precocial young, as in ungulates or cetaceans. This aspect of mammalian adaptation and diversity should remind us that perhaps the ways in which a female does manage the stress of eutherian reproduction (the factor that magnified the strength of selection pressure) became the most influential source of viability of our clade.

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