相关背景内容： | Bioluminescence is the production and emission of light by a living organism. It is a form of chemiluminescence. Bioluminescence occurs widely in marine vertebrates and invertebrates, as well as in some fungi, microorganisms including some bioluminescent bacteria and terrestrial invertebrates such as fireflies. In some animals, the light is produced by symbiotic organisms such as Vibrio bacteria.
The chemical reaction in bioluminescence involves the light-emitting pigment luciferin and the enzyme luciferase. The enzyme catalyzes the oxidation of luciferin. In some species, the type of luciferin requires cofactors such as calcium or magnesium ions, and sometimes also the energy-carrying molecule adenosine triphosphate (ATP). In evolution, luciferins vary little: one in particular, coelenterazine, is found in nine different groups of animals (phyla), though some of these obtain it through their diet. Conversely, luciferases vary widely in different species. Bioluminescence has arisen over forty times in evolutionary history.
The uses of bioluminescence include counter-illumination camouflage, mimicry of other animals whether for offensive or defensive purposes, and signalling to other individuals of the same species, such as to attract mates.
Carbon dioxide (C02), adenosine monophosphate (AMP) and phosphate groups (PP) are released as waste products. Luciferase catalyzes the reaction, which may be mediated by cofactors such as calcium or magnesium ions, and for some types of luciferin also the energy-carrying molecule adenosine triphosphate (ATP). The reaction can occur either inside or outside the cell. In bacteria such as Vibrio, the expression of genes related to bioluminescence is controlled by an operon called the Lux operon.
Protein folding structure of the luciferase of the firefly Photinus pyralis. The enzyme is a much larger molecule than luciferin.
In evolution, luciferins generally vary little: one in particular, coelenterazine, is the light emitting pigment for nine phyla (ancient groups of very different organisms), including polycystine radiolaria, Cercozoa (Phaeodaria), protozoa, comb jellies, cnidaria including jellyfish and corals, crustaceans, molluscs, arrow worms and vertebrates (ray-finned fish). Not all these organisms synthesize coelenterazine: some of them obtain it through their diet. Conversely, luciferase enzymes vary widely and tend to be different in each species. Overall, bioluminescence has arisen over forty times in evolutionary history.
Luciferin-luciferase reactions are not the only way that organisms produce light. The parchment worm Chaetopterus (a marine Polychaete) makes use of the photoprotein aequorin instead of luciferase. When calcium ions (Ca2+) are added, the aequorin's rapid catalysis creates a brief flash quite unlike the prolonged glow produced by luciferase. In a second, much slower, step luciferin is regenerated from the oxidised (oxyluciferin) form, allowing it to recombine with aequorin, in readiness for a subsequent flash. Photoproteins are thus enzymes, but with unusual reaction kinetics. |
相关背景内容： | Continental drift is the movement of the Earth's continents relative to each other, thus appearing to drift across the ocean bed. The speculation that continents might have 'drifted' was first put forward by Abraham Ortelius in 1596. The concept was independently and more fully developed by Alfred Wegener in 1912, but his theory was rejected by some for lack of a mechanism (though this was supplied later by Holmes) and others because of prior theoretical commitments. The idea of continental drift has been subsumed by the theory of plate tectonics, which explains how the continents move.
Evidence for the movement of continents on tectonic plates is now extensive. Similar plant and animal fossils are found around the shores of different continents, suggesting that they were once joined. The fossils of Mesosaurus, a freshwater reptile rather like a small crocodile, found both in Brazil and South Africa, are one example; another is the discovery of fossils of the land reptile Lystrosaurus in rocks of the same age at locations in Africa, India, and Antarctica. There is also living evidence—the same animals being found on two continents. Some earthworm families (e.g. Ocnerodrilidae, Acanthodrilidae, Octochaetidae) are found in South America and Africa, for instance.
The complementary arrangement of the facing sides of South America and Africa is obvious, but is a temporary coincidence. In millions of years, slab pull and ridge-push, and other forces of tectonophysics, will further separate and rotate those two continents. It was this temporary feature which inspired Wegener to study what he defined as continental drift, although he did not live to see his hypothesis generally accepted.
Widespread distribution of Permo-Carboniferous glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia was one of the major pieces of evidence for the theory of continental drift. The continuity of glaciers, inferred from oriented glacial striations and deposits called tillites, suggested the existence of the supercontinent of Gondwana, which became a central element of the concept of continental drift. Striations indicated glacial flow away from the equator and toward the poles, based on continents' current positions and orientations, and supported the idea that the southern continents had previously been in dramatically different locations, as well as being contiguous with each other. |
相关背景内容： | Social animals are those animals which interact highly with other animals, usually of their own species (conspecifics), to the point of having a recognizable and distinct society. Many animals are social to the extent that mothers and offspring bond, and males and females interact to mate, but the term "social animal" is usually only applied when there is a level of social organization that goes beyond this, with permanent groups of adults living together, and relationships between individuals that endure from one encounter to another. The most commonly known example of a social animal is a domestic dog. In the case of social animals, not having those social interactions can be detrimental to the animal's development; they are crucial. These interactions and socializing help to develop emotional stability and flexibility for the span of the animal's life.
While some birds are essentially territorial or live in small family groups, other birds may form large flocks. The principal benefits of flocking are safety in numbers and increased foraging efficiency. Defence against predators is particularly important in closed habitats like forests, where ambush predation is common and multiple eyes can provide a valuable early warning system. This has led to the development of many mixed-species feeding flocks, which are usually composed of small numbers of many species; these flocks provide safety in numbers but increase potential competition for resources. Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases.
Birds sometimes also form associations with non-avian species. Plunge-diving seabirds associate with dolphins and tuna, which push shoaling fish towards the surface. Hornbills have a mutualistic relationship with dwarf mongooses, in which they forage together and warn each other of nearby birds of prey and other predators. |
