Biomechanics is the study of the structure and motion of the mechanical aspects of biological systems, at any level from whole organisms to organs and cell organelles, using the methods of mechanics. The word "biomechanics" and the related "biomechanical" come from the Ancient Greek βίος bios "life" and μηχανική, mēchanikē "mechanics", to refer to the study of the mechanical principles of living organisms their movement and structure. Biological fluid mechanics, or biofluid mechanics, is the study of both gas and liquid fluid flows in or around biological organisms. An studied liquid biofluids problem is that of blood flow in the human cardiovascular system. Under certain mathematical circumstances, blood flow can be modelled by the Navier–Stokes equations. In vivo whole blood is assumed to be an incompressible Newtonian fluid. However, this assumption fails. At the microscopic scale, the effects of individual red blood cells become significant, whole blood can no longer be modelled as a continuum.
When the diameter of the blood vessel is just larger than the diameter of the red blood cell the Fahraeus–Lindquist effect occurs and there is a decrease in wall shear stress. However, as the diameter of the blood vessel decreases further, the red blood cells have to squeeze through the vessel and can only pass in single file. In this case, the inverse Fahraeus -- the wall shear stress increases. An example of a gaseous biofluids problem is that of human respiration. Respiratory systems in insects have been studied for bioinspiration for designing improved microfluidic devices; the main aspects of Contact mechanics and tribology are related to friction and lubrication. When the two surfaces come in contact during motion i.e. rub against each other, friction and lubrication effects are important to analyze in order to determine the performance of the material. Biotribology is a study of friction and lubrication of biological systems human joints such as hips and knees. For example and tibial components of knee implant rub against each other during daily activity such as walking or stair climbing.
If the performance of tibial component needs to be analyzed, the principles of biotribology are used to determine the wear performance of the implant and lubrication effects of synovial fluid. In addition, the theory of contact mechanics becomes important for wear analysis. Additional aspects of biotribology can include analysis of subsurface damage resulting from two surfaces coming in contact during motion, i.e. rubbing against each other, such as in the evaluation of tissue engineered cartilage. Comparative biomechanics is the application of biomechanics to non-human organisms, whether used to gain greater insights into humans or into the functions and adaptations of the organisms themselves. Common areas of investigation are Animal locomotion and feeding, as these have strong connections to the organism's fitness and impose high mechanical demands. Animal locomotion, has many manifestations, including running and flying. Locomotion requires energy to overcome friction, drag and gravity, though which factor predominates varies with environment.
Comparative biomechanics overlaps with many other fields, including ecology, developmental biology and paleontology, to the extent of publishing papers in the journals of these other fields. Comparative biomechanics is applied in medicine as well as in biomimetics, which looks to nature for solutions to engineering problems. Computational biomechanics is the application of engineering computational tools, such as the Finite element method to study the mechanics of biological systems. Computational models and simulations are used to predict the relationship between parameters that are otherwise challenging to test experimentally, or used to design more relevant experiments reducing the time and costs of experiments. Mechanical modeling using finite element analysis has been used to interpret the experimental observation of plant cell growth to understand how they differentiate, for instance. In medicine, over the past decade, the Finite element method has become an established alternative to in vivo surgical assessment.
One of the main advantages of computational biomechanics lies in its ability to determine the endo-anatomical response of an anatomy, without being subject to ethical restrictions. This has led FE modeling to the point of becoming ubiquitous in several fields of Biomechanics while several projects have adopted an open source philosophy; the mechanical analysis of biomaterials and biofluids is carried forth with the concepts of continuum mechanics. This assumption breaks down when the length scales of interest approach the order of the micro structural details of the material. One of the most remarkable characteristic of biomaterials is their hierarchical structure. In other words, the mechanical characteristics of these materials rely on physical phenomena occurring in multiple levels, from the molecular all the way up to the tissue and organ levels. Biomaterials are classified in two groups and soft tissues. Mechanical deformation of hard tissues may be analysed with the theory of linear elasticity.
On the other hand, soft tissues undergo large deformations and thus their analysis rely on the finite strain theory and computer simulations. The interest in continuum biomechanics is spurred by the need for realism in the development of medical simulation
A carnivore, meaning "meat eater", is an organism that derives its energy and nutrient requirements from a diet consisting or of animal tissue, whether through predation or scavenging. Animals that depend on animal flesh for their nutrient requirements are called obligate carnivores while those that consume non-animal food are called facultative carnivores. Omnivores consume both animal and non-animal food, apart from the more general definition, there is no defined ratio of plant to animal material that would distinguish a facultative carnivore from an omnivore. A carnivore at the top of the food chain, not preyed upon by other animals, is termed an apex predator. "Carnivore" may refer to the mammalian order Carnivora, but this is somewhat misleading: many, but not all, Carnivora are meat eaters, fewer are true obligate carnivores. For example, while the Arctic polar bear eats meat most species of bears are omnivorous, the giant panda is herbivorous. There are many carnivorous species that are not members of Carnivora.
Outside the animal kingdom, there are several genera containing carnivorous plants and several phyla containing carnivorous fungi. Carnivores are sometimes characterized by their type of prey. For example, animals that eat insects and similar invertebrates are called insectivores, while those that eat fish are called piscivores; the first tetrapods, or land-dwelling vertebrates, were piscivorous amphibians known as labyrinthodonts. They gave rise to insectivorous vertebrates and to predators of other tetrapods. Carnivores may alternatively be classified according to the percentage of meat in their diet; the diet of a hypercarnivore consists of more than 70% meat, that of a mesocarnivore 30–70%, that of a hypocarnivore less than 30%, with the balance consisting of non-animal foods such as fruits, other plant material, or fungi. Obligate or "true" carnivores are those. While obligate carnivores might be able to ingest small amounts of plant matter, they lack the necessary physiology required to digest it.
In fact, some obligate carnivorous mammals will only ingest vegetation for the sole purpose of its use as an emetic, to self-induce vomiting of the vegetation along with the other food it had ingested that upset its stomach. Obligate carnivores include the axolotl, which consumes worms and larvae in its environment, but if necessary will consume algae, as well as all felids which require a diet of animal flesh and organs. Cats have high protein requirements and their metabolisms appear unable to synthesize essential nutrients such as retinol, arginine and arachidonic acid. Characteristics associated with carnivores include strength and keen senses for hunting, as well as teeth and claws for capturing and tearing prey. However, some carnivores do not hunt and are scavengers, lacking the physical characteristics to bring down prey. Carnivores have comparatively short digestive systems, as they are not required to break down the tough cellulose found in plants. Many hunting animals have evolved eyes facing forward.
This is universal among mammalian predators, while most reptile and amphibian predators have eyes facing sideways. Predation predates the rise of recognized carnivores by hundreds of millions of years; the earliest predators were microbial organisms, which grazed on others. Because the fossil record is poor, these first predators could date back anywhere between 1 and over 2.7 Gya. The rise of eukaryotic cells at around 2.7 Gya, the rise of multicellular organisms at about 2 Gya, the rise of mobile predators have all been attributed to early predatory behavior, many early remains show evidence of boreholes or other markings attributed to small predator species. Among more familiar species, the first vertebrate carnivores were fish, amphibians that moved on to land. Early tetrapods were large amphibious piscivores; some scientists assert that Dimetrodon "was the first terrestrial vertebrate to develop the curved, serrated teeth that enable a predator to eat prey much larger than itself." While amphibians continued to feed on fish and insects, reptiles began exploring two new food types: tetrapods and plants.
Carnivory was a natural transition from insectivory for medium and large tetrapods, requiring minimal adaptation. In the Mesozoic, some theropod dinosaurs such as Tyrannosaurus rex were obligate carnivores. Though the theropods were the larger carnivores, several carnivorous mammal groups were present. Most notable are the gobiconodontids, the triconodontid Jugulator, the deltatheroideans and Cimolestes. Many of these, such as Repenomamus and Cimolestes, were among the largest mammals in their faunal assemblages, capable of attacking dinosaurs. In the early-to-mid-Cenozoic, the dominant predator forms were mammals: hyaenodonts, entelodonts, ptolemaiidans and mesonychians, representing a great diversity of eutherian carnivores
Vertebrates comprise all species of animals within the subphylum Vertebrata. Vertebrates represent the overwhelming majority of the phylum Chordata, with about 69,276 species described. Vertebrates include the jawless fishes and jawed vertebrates, which include the cartilaginous fishes and the bony fishes; the bony fishes in turn, cladistically speaking include the tetrapods, which include amphibians, reptiles and mammals. Extant vertebrates range in size from the frog species Paedophryne amauensis, at as little as 7.7 mm, to the blue whale, at up to 33 m. Vertebrates make up less than five percent of all described animal species; the vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution, though their closest living relatives, the lampreys, do. Hagfish do, possess a cranium. For this reason, the vertebrate subphylum is sometimes referred to as "Craniata" when discussing morphology. Molecular analysis since 1992 has suggested that hagfish are most related to lampreys, so are vertebrates in a monophyletic sense.
Others consider them a sister group of vertebrates in the common taxon of craniata. The word vertebrate derives from the Latin word vertebratus. Vertebrate is derived from the word vertebra, which refers to any of the bones or segments of the spinal column. All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal, with a hollow tube of nervous tissue above it and the gastrointestinal tract below. In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the anus opens to the exterior before the end of the body; the remaining part of the body continuing after the anus forms a tail with vertebrae and spinal cord, but no gut. The defining characteristic of a vertebrate is the vertebral column, in which the notochord found in all chordates has been replaced by a segmented series of stiffer elements separated by mobile joints. However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the sturgeon and coelacanth.
Jawed vertebrates are typified by paired appendages, but this trait is not required in order for an animal to be a vertebrate. All basal vertebrates breathe with gills; the gills are carried right behind the head, bordering the posterior margins of a series of openings from the pharynx to the exterior. Each gill is supported by a cartilagenous or bony gill arch; the bony fish have three pairs of arches, cartilaginous fish have five to seven pairs, while the primitive jawless fish have seven. The vertebrate ancestor no doubt had more arches than this, as some of their chordate relatives have more than 50 pairs of gills. In amphibians and some primitive bony fishes, the larvae bear external gills, branching off from the gill arches; these are reduced in adulthood, their function taken over by the gills proper in fishes and by lungs in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish being irrevocably lost early in the evolution of tetrapods.
While the more derived vertebrates lack gills, the gill arches form during fetal development, form the basis of essential structures such as jaws, the thyroid gland, the larynx, the columella and, in mammals, the malleus and incus. The central nervous system of vertebrates is based on a hollow nerve cord running along the length of the animal. Of particular importance and unique to vertebrates is the presence of neural crest cells; these are progenitors of stem cells, critical to coordinating the functions of cellular components. Neural crest cells migrate through the body from the nerve cord during development, initiate the formation of neural ganglia and structures such as the jaws and skull; the vertebrates are the only chordate group to exhibit cephalisation, the concentration of brain functions in the head. A slight swelling of the anterior end of the nerve cord is found in the lancelet, a chordate, though it lacks the eyes and other complex sense organs comparable to those of vertebrates.
Other chordates do not show any trends towards cephalisation. A peripheral nervous system branches out from the nerve cord to innervate the various systems; the front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The prosencephalon and rhombencephalon, further differentiated in the various vertebrate groups. Two laterally placed eyes form around outgrowths from the midbrain, except in hagfish, though this may be a secondary loss; the forebrain is well developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders. Vesicles of the forebrain are paired, giving rise to hemispheres like the cerebral hemispheres in mammals; the resulting anatomy of the central nervous system, with a single hollow nerve cord topped by a series of vesicles, is unique to vertebrates. All invertebrates with well-developed brains, such as insects and squids, have a ventral rather than dorsal system of ganglions, with a split brain stem running on each side of the mouth or gut.
Vertebrates originated about 525 million years ago during the Cambrian explosion, which saw
Paleobiology is a growing and comparatively new discipline which combines the methods and findings of the life science biology with the methods and findings of the earth science paleontology. It is referred to as "geobiology". Paleobiological research uses biological field research of current biota and of fossils millions of years old to answer questions about the molecular evolution and the evolutionary history of life. In this scientific quest, macrofossils and trace fossils are analyzed. However, the 21st-century biochemical analysis of DNA and RNA samples offers much promise, as does the biometric construction of phylogenetic trees. An investigator in this field is known as a paleobiologist. Paleobotany applies the principles and methods of paleobiology to flora green land plants, but including the fungi and seaweeds. See mycology and dendrochronology. Paleozoology uses the methods and principles of paleobiology to understand fauna, both vertebrates and invertebrates. See vertebrate and invertebrate paleontology, as well as paleoanthropology.
Micropaleontology applies paleobiologic principles and methods to archaea, bacteria and microscopic pollen/spores. See microfossils and palynology. Paleovirology examines the evolutionary history of viruses on paleobiological timescales. Paleobiochemistry uses the methods and principles of organic chemistry to detect and analyze molecular-level evidence of ancient life, both microscopic and macroscopic. Paleoecology examines past ecosystems and geographies so as to better comprehend prehistoric life. Taphonomy analyzes the post-mortem history of an individual organism in order to gain insight on the behavior and environment of the fossilized organism. Paleoichnology analyzes the tracks, trails, burrows and other trace fossils left by ancient organisms in order to gain insight into their behavior and ecology. Stratigraphic paleobiology studies long-term secular changes, as well as the bed-by-bed sequence of changes, in organismal characteristics and behaviors. See stratification, sedimentary rocks and the geologic time scale.
Evolutionary developmental paleobiology examines the evolutionary aspects of the modes and trajectories of growth and development in the evolution of life – clades both extinct and extant. See adaptive radiation, evolutionary biology, developmental biology and phylogenetic tree; the founder or "father" of modern paleobiology was Baron Franz Nopcsa, a Hungarian scientist trained at the University of Vienna. He termed the discipline "paleophysiology." However, credit for coining the word paleobiology. He proposed the term in 1904 so as to initiate "a broad new science" joining "traditional paleontology with the evidence and insights of geology and isotopic chemistry."On the other hand, Charles Doolittle Walcott, a Smithsonian adventurer, has been cited as the "founder of Precambrian paleobiology." Although best known as the discoverer of the mid-Cambrian Burgess shale animal fossils, in 1883 this American curator found the "first Precambrian fossil cells known to science" – a stromatolite reef known as Cryptozoon algae.
In 1899 he discovered the first acritarch fossil cells, a Precambrian algal phytoplankton he named Chuaria. Lastly, in 1914, Walcott reported "minute cells and chains of cell-like bodies" belonging to Precambrian purple bacteria. 20th-century paleobiologists have figured prominently in finding Archaean and Proterozoic eon microfossils: In 1954, Stanley A. Tyler and Elso S. Barghoorn described 2.1 billion-year-old cyanobacteria and fungi-like microflora at their Gunflint Chert fossil site. Eleven years Barghoorn and J. William Schopf reported finely-preserved Precambrian microflora at their Bitter Springs site of the Amadeus Basin, Central Australia. In 1993, Schopf discovered O2-producing blue-green bacteria at his 3.5 billion-year-old Apex Chert site in Pilbara Craton, Marble Bar, in the northwestern part of Western Australia. So paleobiologists were at last homing in on the origins of the Precambrian "Oxygen catastrophe." Acta Palaeontologica Polonica Biology and Geology Historical Biology Palaios Palaeogeography, Palaeoclimatology, Palaeoecology Paleobiology Paleoceanography History of biology History of paleontology History of invertebrate paleozoology Molecular paleontology Taxonomy of fossilised invertebrates Treatise on Invertebrate Paleontology Derek E.
G. Briggs and Peter R. Crowther, eds.. Palaeobiology II. Malden, Massachusetts: Blackwell Publishing. ISBN 0-632-05147-7 and ISBN 0-632-05149-3; the second edition of an acclaimed British textbook. Robert L. Carroll. Patterns and Processes of Vertebrate Evolution. Cambridge Paleobiology Series. Cambridge, England: Cambridge University Press. ISBN 978-0-521-47809-0 and ISBN 0-521-47809-X. Applies paleobiology to the adaptive radiation of quadrupeds. Matthew T. Carrano, Timothy Gaudin, Richard Blob, John Wible, eds.. Amniote Paleobiology: Perspectives on the Evolution of Mammals and Reptiles. Chicago: University of Chicago Press. ISBN 0-226-09478-2 and ISBN 978-0-226-09478-6; this new book describes paleobiological research into land vertebrates of the Mesozoic and Cenozoic eras. Robert B. Eckhardt. Human Paleobiology. Cambridge Studies in Biology and Evolutionary Anthropology. Cambridge, England: Cambridge University Press. ISBN 0-521-45160-4 and ISBN 978-0-521-45160-4; this book connects archeology to the field of paleobiology.
Douglas H. Erwin. Extinction: How Life on Earth Nearly Ended 250 Million Years
A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, petrified wood, coal, DNA remnants; the totality of fossils is known as the fossil record. Paleontology is the study of fossils: their age, method of formation, evolutionary significance. Specimens are considered to be fossils if they are over 10,000 years old; the oldest fossils are around 3.48 billion years old to 4.1 billion years old. The observation in the 19th century that certain fossils were associated with certain rock strata led to the recognition of a geological timescale and the relative ages of different fossils; the development of radiometric dating techniques in the early 20th century allowed scientists to quantitatively measure the absolute ages of rocks and the fossils they host. There are many processes that lead to fossilization, including permineralization and molds, authigenic mineralization and recrystallization, adpression and bioimmuration.
Fossils vary in size from one-micrometre bacteria to dinosaurs and trees, many meters long and weighing many tons. A fossil preserves only a portion of the deceased organism that portion, mineralized during life, such as the bones and teeth of vertebrates, or the chitinous or calcareous exoskeletons of invertebrates. Fossils may consist of the marks left behind by the organism while it was alive, such as animal tracks or feces; these types of fossil are called trace ichnofossils, as opposed to body fossils. Some fossils are called chemofossils or biosignatures; the process of fossilization varies according to external conditions. Permineralization is a process of fossilization; the empty spaces within an organism become filled with mineral-rich groundwater. Minerals precipitate from the groundwater; this process can occur in small spaces, such as within the cell wall of a plant cell. Small scale permineralization can produce detailed fossils. For permineralization to occur, the organism must become covered by sediment soon after death, otherwise decay commences.
The degree to which the remains are decayed when covered determines the details of the fossil. Some fossils consist only of skeletal teeth; this is a form of diagenesis. In some cases, the original remains of the organism dissolve or are otherwise destroyed; the remaining organism-shaped hole in the rock is called an external mold. If this hole is filled with other minerals, it is a cast. An endocast, or internal mold, is formed when sediments or minerals fill the internal cavity of an organism, such as the inside of a bivalve or snail or the hollow of a skull; this is a special form of mold formation. If the chemistry is right, the organism can act as a nucleus for the precipitation of minerals such as siderite, resulting in a nodule forming around it. If this happens before significant decay to the organic tissue fine three-dimensional morphological detail can be preserved. Nodules from the Carboniferous Mazon Creek fossil beds of Illinois, USA, are among the best documented examples of such mineralization.
Replacement occurs. In some cases mineral replacement of the original shell occurs so and at such fine scales that microstructural features are preserved despite the total loss of original material. A shell is said to be recrystallized when the original skeletal compounds are still present but in a different crystal form, as from aragonite to calcite. Compression fossils, such as those of fossil ferns, are the result of chemical reduction of the complex organic molecules composing the organism's tissues. In this case the fossil consists of original material, albeit in a geochemically altered state; this chemical change is an expression of diagenesis. What remains is a carbonaceous film known as a phytoleim, in which case the fossil is known as a compression. However, the phytoleim is lost and all that remains is an impression of the organism in the rock—an impression fossil. In many cases, however and impressions occur together. For instance, when the rock is broken open, the phytoleim will be attached to one part, whereas the counterpart will just be an impression.
For this reason, one term covers the two modes of preservation: adpression. Because of their antiquity, an unexpected exception to the alteration of an organism's tissues by chemical reduction of the complex organic molecules during fossilization has been the discovery of soft tissue in dinosaur fossils, including blood vessels, the isolation of proteins and evidence for DNA fragments. In 2014, Mary Schweitzer and her colleagues reported the presence of iron particles associated with soft tissues recovered from dinosaur fossils. Based on various experiments that studied the interaction of iron in haemoglobin with blood vessel tissue they proposed that solution hypoxia coupled with iron chelation enhances the stability and preservation of soft tissue and provides the basis for an explanation for the unforeseen preservation of fossil soft tissues. However, a older study based on eight taxa ranging in time from the Devonian to the Jurassic found that reasonably well-preserved fibrils that represent collagen were preser
Animal locomotion, in ethology, is any of a variety of methods that animals use to move from one place to another. Some modes of locomotion are self-propelled, e.g. running, jumping, hopping and gliding. There are many animal species that depend on their environment for transportation, a type of mobility called passive locomotion, e.g. sailing, rolling or riding other animals. Animals move for a variety of reasons, such as to find food, a mate, a suitable microhabitat, or to escape predators. For many animals, the ability to move is essential for survival and, as a result, natural selection has shaped the locomotion methods and mechanisms used by moving organisms. For example, migratory animals that travel vast distances have a locomotion mechanism that costs little energy per unit distance, whereas non-migratory animals that must move to escape predators are to have energetically costly, but fast, locomotion; the anatomical structures that animals use for movement, including cilia, wings, fins, or tails are sometimes referred to as locomotory organs or locomotory structures.
The term "locomotion" is formed in English from Latin loco "from a place" + motio "motion, a moving". Animals move through, or on, four types of environment: aquatic, terrestrial and aerial. Many animals—for example semi-aquatic animals, diving birds—regularly move through more than one type of medium. In some cases, the surface they move on facilitates their method of locomotion. In water, staying afloat is possible using buoyancy. If an animal's body is less dense than water, it can stay afloat; this requires little energy to maintain a vertical position, but requires more energy for locomotion in the horizontal plane compared to less buoyant animals. The drag encountered in water is much greater than in air. Morphology is therefore important for efficient locomotion, in most cases essential for basic functions such as catching prey. A fusiform, torpedo-like body form is seen in many aquatic animals, though the mechanisms they use for locomotion are diverse; the primary means by which fish generate thrust is by oscillating the body from side-to-side, the resulting wave motion ending at a large tail fin.
Finer control, such as for slow movements, is achieved with thrust from pectoral fins. Some fish, e.g. the spotted ratfish and batiform fish use their pectoral fins as the primary means of locomotion, sometimes termed labriform swimming. Marine mammals oscillate their body in an up-and-down direction. Other animals, e.g. penguins, diving ducks, move underwater in a manner, termed "aquatic flying". Some fish propel themselves without a wave motion of the body, as in the slow-moving seahorses and Gymnotus. Other animals, such as cephalopods, use jet propulsion to travel fast, taking in water squirting it back out in an explosive burst. Other swimming animals may rely predominantly on their limbs. Though life on land originated from the seas, terrestrial animals have returned to an aquatic lifestyle on several occasions, such as the aquatic cetaceans, now distinct from their terrestrial ancestors. Dolphins sometimes ride on the bow waves created by boats or surf on breaking waves. Benthic locomotion is movement by animals that live on, in, or near the bottom of aquatic environments.
In the sea, many animals walk over the seabed. Echinoderms use their tube feet to move about; the tube feet have a tip shaped like a suction pad that can create a vacuum through contraction of muscles. This, along with some stickiness from the secretion of mucus, provides adhesion. Waves of tube feet contractions and relaxations move along the adherent surface and the animal moves along; some sea urchins use their spines for benthic locomotion. Crabs walk sideways; this is because of the articulation of the legs. However, some crabs walk forwards or backwards, including raninids, Libinia emarginata and Mictyris platycheles; some crabs, notably the Portunidae and Matutidae, are capable of swimming, the Portunidae so as their last pair of walking legs are flattened into swimming paddles. A stomatopod, Nannosquilla decemspinosa, can escape by rolling itself into a self-propelled wheel and somersault backwards at a speed of 72 rpm, they can travel more than 2 m using this unusual method of locomotion.
Velella, the by-the-wind sailor, is a cnidarian with no means of propulsion other than sailing. A small rigid sail catches the wind. Velella sails always align along the direction of the wind where the sail may act as an aerofoil, so that the animals tend to sail downwind at a small angle to the wind. While larger animals such as ducks can move on water by floating, some small animals move across it without breaking through the surface; this surface locomotion takes advantage of the surface tension of water. Animals that move in such a way include the water strider. Water striders have legs that are hydrophobic, preventing them from interfering with the structure of water. Another form of locomotion is used by the basilisk lizard. Gravity is the primary obstacle to flight; because it is impossible for any organism to have a density as low as that of air, flying an
An ice age is a long period of reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth is in the Quaternary glaciation, known in popular terminology as the Ice Age. Individual pulses of cold climate are termed "glacial periods", intermittent warm periods are called "interglacials", with both climatic pulses part of the Quaternary or other periods in Earth's history. In the terminology of glaciology, ice age implies the presence of extensive ice sheets in both northern and southern hemispheres. By this definition, we are in an interglacial period—the Holocene; the amount of heat trapping gases emitted into Earth's Oceans and atmosphere will prevent the next ice age, which otherwise would begin in around 50,000 years, more glacial cycles. In 1742, Pierre Martel, an engineer and geographer living in Geneva, visited the valley of Chamonix in the Alps of Savoy. Two years he published an account of his journey.
He reported that the inhabitants of that valley attributed the dispersal of erratic boulders to the glaciers, saying that they had once extended much farther. Similar explanations were reported from other regions of the Alps. In 1815 the carpenter and chamois hunter Jean-Pierre Perraudin explained erratic boulders in the Val de Bagnes in the Swiss canton of Valais as being due to glaciers extending further. An unknown woodcutter from Meiringen in the Bernese Oberland advocated a similar idea in a discussion with the Swiss-German geologist Jean de Charpentier in 1834. Comparable explanations are known from the Val de Ferret in the Valais and the Seeland in western Switzerland and in Goethe's scientific work; such explanations could be found in other parts of the world. When the Bavarian naturalist Ernst von Bibra visited the Chilean Andes in 1849–1850, the natives attributed fossil moraines to the former action of glaciers. Meanwhile, European scholars had begun to wonder. From the middle of the 18th century, some discussed ice as a means of transport.
The Swedish mining expert Daniel Tilas was, in 1742, the first person to suggest drifting sea ice in order to explain the presence of erratic boulders in the Scandinavian and Baltic regions. In 1795, the Scottish philosopher and gentleman naturalist, James Hutton, explained erratic boulders in the Alps by the action of glaciers. Two decades in 1818, the Swedish botanist Göran Wahlenberg published his theory of a glaciation of the Scandinavian peninsula, he regarded glaciation as a regional phenomenon. Only a few years the Danish-Norwegian geologist Jens Esmark argued a sequence of worldwide ice ages. In a paper published in 1824, Esmark proposed changes in climate as the cause of those glaciations, he attempted to show. During the following years, Esmark's ideas were discussed and taken over in parts by Swedish and German scientists. At the University of Edinburgh Robert Jameson seemed to be open to Esmark's ideas, as reviewed by Norwegian professor of glaciology Bjørn G. Andersen. Jameson's remarks about ancient glaciers in Scotland were most prompted by Esmark.
In Germany, Albrecht Reinhard Bernhardi, a geologist and professor of forestry at an academy in Dreissigacker, since incorporated in the southern Thuringian city of Meiningen, adopted Esmark's theory. In a paper published in 1832, Bernhardi speculated about former polar ice caps reaching as far as the temperate zones of the globe. In 1829, independently of these debates, the Swiss civil engineer Ignaz Venetz explained the dispersal of erratic boulders in the Alps, the nearby Jura Mountains, the North German Plain as being due to huge glaciers; when he read his paper before the Schweizerische Naturforschende Gesellschaft, most scientists remained sceptical. Venetz convinced his friend Jean de Charpentier. De Charpentier transformed Venetz's idea into a theory with a glaciation limited to the Alps, his thoughts resembled Wahlenberg's theory. In fact, both men shared the same volcanistic, or in de Charpentier's case rather plutonistic assumptions, about the Earth's history. In 1834, de Charpentier presented his paper before the Schweizerische Naturforschende Gesellschaft.
In the meantime, the German botanist Karl Friedrich Schimper was studying mosses which were growing on erratic boulders in the alpine upland of Bavaria. He began to wonder. During the summer of 1835 he made some excursions to the Bavarian Alps. Schimper came to the conclusion that ice must have been the means of transport for the boulders in the alpine upland. In the winter of 1835 to 1836 he held. Schimper assumed that there must have been global times of obliteration with a cold climate and frozen water. Schimper spent the summer months of 1836 at Devens, near Bex, in the Swiss Alps with his former university friend Louis Agassiz and Jean de Charpentier. Schimper, de Charpentier and Venetz convinced Agassiz that there had been a time of glaciation. During the winter of 1836/37, Agassiz and Schimper developed the theory of a sequence of glaciations, they drew upon the preceding works of Venetz, de Charpentier and on their own fieldwork. Agassiz appears to have been familiar with Bernhardi's paper at that time.
At the beginning of 1837, Schimper coined the term "ice age" for the period of the glaciers. In July 1837 Ag