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Metabolism refers to the many chemical reactions that take place in living organisms to fuel the biological processes of life, from growth and repair to reproduction. Chains of chemical reactions, or 'metabolic pathways' continuously occur within living cells, catalysed by highly effective and specific enzymes. Metabolic pathways may be 'catabolic', involving breakdown of substances in order to release energy, or 'anabolic' in which molecular units are combined to build complex polymers such as nucleic acids, proteins, carbohydrates and lipids. Certain aspects of metabolism, from the generation of internal heat to fermentative digestion offer clear and surprising examples of convergent evolution (see the full list of Topics below)!
One of the most remarkable of convergences is the independent evolution of endothermy, best known in mammals and birds but extending across the tree of life from bumble bees to arum lilies. Endothermy is characterised by a suite of features relating to elevated metabolic rate, internal heat production and high oxygen consumption. Endothermy allows maintenance of a stable internal temperature in spite of variable environmental conditions and most importantly it is correlated with increased stamina and capacity for prolonged bouts of aerobic activity. Molecular and anatomical adaptations are shared between mammals and birds (e.g. heart and lung specialisations, nasal respiratory turbinates, membrane and tissue composition, mitochondrial density and function), allowing them to be more active and inhabit more extreme environments that do ectotherms. In addition to typical endothermy in birds and mammals, a few aquatic ectotherms also elevate their body temperature using internal heating mechanisms. "Warm-blooded" mako and white lamnid sharks minimise heat loss using a counter-current heat exchange system in the form of a network of vessels ('rete mirabile') in which heat from warm arterial blood is captured by adjacent veins before arteries enter the gills, allowing veins to carry the heat back into the body). Tuna use a similar 'rete mirabile' to conserve body heat and prevent heat loss at the gills, and both tuna and sharks are capable of very rapid 'thunniform' swimming, powered by specialised muscles deep within the body, where they are kept warm and well-supplied with energy for prolonged chases. Tuna and swordfish are both deep divers, and to cope with the colder water at depth swordfish control heat loss from the brain and eyes, allowing fast eye movement and reactions when hunting. Interestingly, a number of insects (bees and moths) display behavioural adaptations to generate heat in their flight muscles and social bees can cool their hive by 'fanning' with their wings and warm it by gathering together and 'shivering' (in an analogous way to mammals) in order to produce heat.
Many endothermic animals conserve energy by entering phases of controlled low body temperature, in the form of daily torpor (significant temperature fall during sleep) or more prolonged hibernation. Torpor occurs in many birds and mammals (e.g. tenrec, opossums, carnivorous marsupials) and grades in extremity towards true hibernation. Hibernation is known in one nocturnal bird, Phalaenoptilus nuttallii and many small placental mammals from distinct groups, including bats, some rodents (e.g. squirrels), insectivores (e.g. hedgehogs), lemurs (e.g. mouse lemurs and the Fat-tailed Dwarf Lemur). In addition, echidnas are the only monotreme known to hibernate and hibernating marsupials include pygmy possums, feathertail gliders (Acrobatidae) and Dromiciops australis.
Fascinatingly, certain plants are able to generate heat by mitochondrial "thermogenesis", a biochemical process that has strong molecular parallels with heat generation in endothermic mammals. Most thermogenic plants belong to the Araceae family, where heat spreads volatile scents to attract (and temporarily trap) pollinating insects. Araceae such as the famous Titan Arum (Amorphopahllus titanum) reproduce via a columnar 'spadix' made of many florets. Female florets at the base of the spadix may produce a pleasant perfume but male and sterile stamens typically give off a pungent odour, foul-smelling to us but irresistible to many insects. The spadix of Lysichiton americanus from Western North America even produces enough heat to melt snow before it spreads volatile scents. Several other plant families are thermogenic, for example the Nymphaceae (water lilies), which have an arum-like system of trapping insects overnight to ensure successful cross-pollination.
"Hummingbirdoid" hawk-moths or sphinx-moths are strikingly convergent with the hummingbirds of the Neotropics. Hummingbirds and "hummingbirdoid" moths share the same body shape and are both able to hover in a precise spot while probing flowers for nectar. Like other birds hummingbirds are endothermic and interestingly the hawk-moths are also capable of raising their body temperature to cope with the high metabolic activity required for hovering flight. Nectar-feeding is convergent in birds from all parts of the world, from hummingbirds to sunbirds (which can also hover) and honeyeaters, as well as being observed in several species of bats. Shared adaptations are morphological (e.g. grooved or tubular brush-tipped tongues) as well as metabolic, to deal with the high-sugar nectivorous diet. Notably, nectivorous birds acquire essential protein by regular consumption of small arthropods.
Herbivorous vertebrates have to survive on low-quality, fibrous plant material that is often rich in cellulose, a polysaccharide that no vertebrate enzyme can break down. Gut fermentation is a solution to this problem that has been adopted convergently in many independent lineages of mammals as well as some plant-eating birds and reptiles. Diverse populations of symbiotic microbes including bacteria, methanogenic archaea, ciliate protistans and fungi live in a specialised fore-gut chamber (rumen or crop) or hind-gut chamber (colon or caecum), where plant material remains in transit for prolonged periods. Microbial enzymes digest or 'ferment' plant material, breaking it down primarily into volatile fatty acids (VFAs) that can be converted into energy. Fore-gut fermentation offers additional benefits in that microbes can detoxify harmful plant compounds before they reach the stomach and also any microbes that are swallowed can be digested as a supplementary food source. Hindgut fermenters include various mammals (e.g. horses, elephants, rhinos, rabbits, koalas and some rodents), a few birds (e.g. grouse) and some reptiles, including iguanid lizards, the green turtle and probably herbivorous dinosaurs (e.g. Triceratops, Brachiosaurus and hadrosaurs). Foregut fermentation is best known in 'ruminant' mammals, which include cattle, goats, antelope, giraffes and others in the sub-order Ruminantia as well as camels, llamas and alpacas in the sub-order Tylopoda. Similar fore-gut adaptations are also found in sloths, Colobine monkeys, the tree rat Thallomys nigricauda and a number of marsupials, including macropods (e.g. kangaroos and wallabies) and potoroids (e.g. potoroos and rat-kangaroos). Intriguingly, fore-gut fermentation analogous to that in ruminants is found in a few specialised birds, including the South American hoatzin, Neotropical green-rumped parrotlet and sub-Saharan speckled mousebird.