Biology: quotes, facts and reflections - Page 2

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WonderNature

The kākāpō, a flightless, night-active parrot of New Zealand, courts with sound: in the rare years when it breeds, males dig shallow bowls in the ground and boom from them through the night, a deep call that can carry for more than a kilometre.

The kākāpō is the world's only flightless parrot and the heaviest, with males weighing up to about 4 kilograms. It once lived across New Zealand, but hunting and the dogs and rats brought by Polynesian settlers, then the cats, stoats and rats that came with Europeans, drove it close to extinction. Today every surviving bird has a name and lives on predator-free islands or in fenced sanctuaries, watched over by the Kākāpō Recovery programme. Females breed only in years when rimu and some other native trees produce a huge crop of fruit, known as masting, which comes every two to four years. In those seasons rangers check nests closely and sometimes raise chicks by hand.

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WonderNature

Great frigatebirds can sleep while soaring over the ocean, sometimes with one half of the brain and sometimes with both. In flight, though, they sleep only about 40 minutes a day, against more than 12 hours a day on land.

The finding comes from a 2016 study led by Niels Rattenborg of the Max Planck Institute for Ornithology in Germany. His team fitted great frigatebirds nesting in the Galápagos Islands with small devices that recorded brain activity during flights lasting up to ten days. Most sleep came in bursts of about twelve seconds, often while the birds circled upward in rising warm air. When only one half of the brain slept, the eye linked to the awake half usually looked in the direction of flight. Frigatebirds hunt far out at sea, yet their feathers are not waterproof, so they cannot rest on the water. How they cope with so little sleep is still not known.

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WonderNature

Leafcutter ants do not eat most of the leaves they harvest; they process them into a growing medium for a domesticated fungus that feeds the colony.

Leafcutter ants grow a single fungus, Leucoagaricus gongylophorus, which is not known to live anywhere outside their nests. Ants began farming fungi around 50 to 60 million years ago; leafcutters are a much younger branch of that lineage, and their crop is passed from nest to nest rather than gathered from the wild. A young queen leaving to found a colony carries a small pellet of it in a pouch inside her mouth. Many colonies also have a third partner: bacteria called Pseudonocardia, grown on the ants' bodies, make antibiotics that help keep down Escovopsis, a mould that can wipe out the fungus garden. A mature nest of Atta leafcutters can house several million ants in chambers reaching several metres underground.

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StillnessNature

Biological soil crusts are living communities of cyanobacteria, lichens, fungi, mosses and algae that bind dryland soil and influence water and nutrient cycles.

Scientists often call these crusts biocrusts. The cyanobacterium Microcoleus vaginatus is usually among the first to settle: its sticky filaments glue sand grains together and prepare the ground for the lichens, mosses and algae that follow. Some crust organisms also fix nitrogen, taking it from the air and turning it into a form plants can use, which matters in poor desert soils. Growth is slow in dry ground, so a crushed crust can take years to regain its first layer and decades or even centuries to rebuild fully. In US national parks on the Colorado Plateau, such as Arches and Canyonlands, signs ask visitors to stay on the trails with the slogan 'Don't bust the crust'.

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StillnessNature

Cloud forests catch water straight from passing cloud: tiny droplets collect on leaves, mosses and branches, then drip to the ground, adding a significant share of the forest's water.

Scientists call this fog drip or horizontal precipitation, because the water arrives sideways on the wind rather than falling as rain. Epiphytes, plants such as mosses, ferns and orchids that grow on other plants instead of in soil, are especially good at it: their tangled surfaces multiply the area that catches droplets. In some cloud forests, fog supplies a significant share of the water that reaches the ground in the dry season, when rain is scarce. Monteverde in Costa Rica is one of the best-studied examples, and research published there in 1999 linked warming seas to clouds forming higher up the mountains, leaving less fog in the forest. Clearing the trees removes this water catcher along with the forest.

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WonderNature

Rafflesia arnoldii lives almost entirely inside a tropical vine as a parasite, emerging briefly to produce a vast carrion-scented flower.

Rafflesia arnoldii grows in the rainforests of Sumatra and Borneo. It is named after Sir Stamford Raffles and the surgeon and naturalist Joseph Arnold, who recorded it in Sumatra in 1818; local people knew it long before. It is a holoparasite: it has no green tissue at all and takes all its water and food from its host, a Tetrastigma vine of the grape family, living as threads of cells inside the vine's stem. A bud swells for many months before opening into a flower up to about a metre across and weighing several kilograms, the largest single flower of any plant. Its smell of rotting meat draws the carrion flies that pollinate it, and the bloom lasts only about a week. Several Rafflesia species are threatened as forests are cleared.

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WonderNature

Elephants generate and detect low-frequency ground vibrations, using sensitive feet and trunks to receive seismic information carried through soil.

The biologist Caitlin O'Connell-Rodwell began studying this in African elephants in Namibia in the 1990s. Her team found that an elephant's low rumbles and stamps also send vibrations through the ground, and that other elephants react when such signals are played back through the soil. Researchers estimate that these ground waves may carry for several kilometres, in some conditions further than the same call through the air. Elephants seem to pick them up in two ways: through pressure-sensitive nerve endings called Pacinian corpuscles in the feet and trunk, and through vibration travelling up the leg bones to the inner ear. Elephants have been seen leaning forward or lifting a foot as if listening. How much the ground channel can carry is still being studied.

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WonderNature

Common vampire bats form enduring social bonds and may regurgitate blood meals to hungry roost-mates, including non-relatives who have shared before.

A vampire bat that fails to feed can starve within about two to three nights, so a shared meal can be the difference between life and death. The biologist Gerald Wilkinson described the sharing in detail in 1984, from wild bats in Costa Rica, and showed that bats tend to feed those that have fed them before, a rare mammal example of reciprocal help between non-relatives. Later studies led by Gerald Carter found that new partnerships grow step by step, starting with grooming before any blood is shared. In a 2019 study, bats that had formed bonds while housed together in captivity kept many of those bonds after release into the wild, where they were tracked with tiny proximity sensors.

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WonderNature

The leaf-eating hoatzin relies on microbes in an enlarged foregut to ferment tough vegetation, an unusual digestive strategy among birds.

The hoatzin lives in swampy forests along rivers in the Amazon and Orinoco basins of South America. It is the only bird known to digest leaves mainly by fermentation at the front of its gut, much as a cow does in its rumen. Bacteria in its huge crop and lower oesophagus break down tough plant fibre before food reaches the stomach. The fermenting crop gives the bird a manure-like smell, which earned it the English nickname 'stinkbird' and means it is seldom hunted for food. The oversized crop also leaves little room for flight muscles, so hoatzins are clumsy fliers. Chicks have two claws on each wing, which help them climb back into the branches after dropping into the water to escape a predator.

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WonderNature

The hooded pitohui, a songbird of New Guinea, can carry batrachotoxin, a nerve poison also found in some poison frogs, in its skin and feathers. Levels vary from bird to bird and are thought to come from its diet.

People in New Guinea have long known the bird is unpleasant to eat, and Western naturalists recorded this as early as 1895; 'pitohui' is said to be a Papuan term for 'rubbish bird'. Its poison drew scientific attention in 1990, when researchers preparing the birds' skins for museum collections felt numbness and burning in their hands. In 1992 a team including the ornithologist Jack Dumbacher reported homobatrachotoxin, a close relative of batrachotoxin, in the skin and feathers, making pitohuis among the first poisonous birds documented by science. The likely source was later traced to small Choresine beetles, found in pitohuis' stomachs and carrying the same family of toxins. The poison probably helps deter predators and parasites such as feather lice.

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WonderNature

Bombardier beetles mix defensive chemicals in a reinforced chamber and eject a near-boiling spray as rapid pulses that help regulate pressure and heat.

The beetle stores two ingredients, hydroquinones and hydrogen peroxide, in a reservoir in its abdomen. When threatened, it squeezes them into a tough-walled reaction chamber containing enzymes (proteins that speed up chemical reactions), which set off a violent reaction that heats the mixture to about 100 degrees Celsius and turns part of it to vapour. A 1990 study showed that the spray leaves in about 70 pulses, at roughly 500 pulses a second, rather than as one jet. In 2015 researchers filmed living beetles with high-speed X-ray imaging and showed how the chamber's flexible walls and valves create this rhythm, which also keeps the beetle from scalding itself. Some species can swivel the tip of the abdomen to aim the spray almost anywhere.

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WonderBiology

Cut certain planarian flatworms into pieces, and each small fragment can regrow into a whole worm within days to weeks, complete with a head and tail at the right ends.

Laboratory work focuses mostly on Schmidtea mediterranea and species of Dugesia. As early as 1898 the biologist Thomas Hunt Morgan found that a piece as small as 1/279 of a worm could regrow a whole animal; later estimates put the smallest working fragment at around 10,000 cells. The new tissue comes from neoblasts, adult stem cells spread throughout the body. In 2011 Peter Reddien's laboratory showed that a single transplanted neoblast could rebuild the tissues of a worm whose own stem cells had been destroyed by radiation. Chemical signals between cells, including the Wnt pathway that humans also have, tell each wound whether to grow a head or a tail. Planarians are now a leading model for research into how tissues might be made to repair themselves.

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WonderBiology

Tortoiseshell cats are almost always female. Early in development, each embryo cell randomly switches off one of its two X chromosomes, and all its descendants, pigment cells included, keep that choice. One X carries an orange coat-colour gene, the other a non-orange version, so the switched-off X sets each patch's colour.

The process is called X-inactivation, or lyonisation, after the British geneticist Mary Lyon, who proposed it in 1961 from her studies of coat colour in mice. Female mammals have two X chromosomes and males one; switching one X off in each female cell keeps the dose of X-chromosome genes the same in both sexes. Because the choice is made while the embryo is still a small ball of cells and is then copied to every descendant cell, the colour appears in patches rather than a fine speckle. A male cat can be tortoiseshell only if he carries an extra X chromosome (XXY), a rare condition that usually leaves him sterile. Calico cats show the same mechanism, plus a separate gene that adds white patches.

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WonderBiology

Cultivated sweet potatoes carry DNA that passed into them naturally, long ago, from a soil bacterium called Agrobacterium, and some of those bacterial genes are still active. No laboratory was involved: the crop is a naturally genetically modified plant.

Agrobacterium is known for inserting part of its own DNA into plants, making them produce nutrients for it; since the 1980s, genetic engineers have used the same trick to add genes to crops. In 2015 a team led by Tina Kyndt and Godelieve Gheysen of Ghent University, working with the International Potato Center, reported bacterial DNA in the sweet potato genome. One insertion, containing four bacterial genes, was present in all 291 cultivated varieties they tested but not in the plant's close wild relatives. That pattern suggests early farmers may have unknowingly favoured plants carrying it, perhaps for a trait it affected, although which trait, if any, is still unknown. The case is often mentioned in debates about genetically modified food.

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WonderOcean

Some sea slugs keep the chloroplasts of the algae they eat, the tiny green parts that turn sunlight into food, working inside their own bodies. The stolen chloroplasts can go on photosynthesising for days or, in some species, months.

These slugs belong to a group called the Sacoglossa, and the habit is known as kleptoplasty, from the Greek for 'thief' and 'formed'. They pierce algal cells, suck out the contents and store the chloroplasts in cells lining the gut. The best-known example, the emerald green Elysia chlorotica of North America's Atlantic coast, can keep chloroplasts from the alga Vaucheria litorea working for months. Scientists once thought the slug had taken algal genes into its own DNA to maintain them, but later genetic studies did not confirm this, and how the chloroplasts last so long is still debated. So is how much they feed the slug: in some experiments, starved slugs kept in the dark survived about as long as those kept in light.

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WonderBiology

The closest living relatives of animals are single-celled organisms called choanoflagellates. Their cells look strikingly like certain cells in sponges, and they offer clues to how the first animals evolved from single cells.

A choanoflagellate beats a single whip-like tail, a flagellum, to draw water through a collar of tiny finger-like projections that trap bacteria to eat. Choanoflagellates are not our ancestors but a sister lineage that split from the animal line hundreds of millions of years ago. Their genomes, first sequenced in 2008, contain genes for proteins such as cadherins, which animals use to stick cells together, and tyrosine kinases, which pass signals between cells. One species, Salpingoeca rosetta, can switch from living alone to forming small rosette-shaped colonies, and in 2012 Nicole King's laboratory at the University of California, Berkeley, found that a molecule released by a particular bacterium triggers the switch.

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WonderOcean

Every night, vast numbers of sea animals rise from the depths towards the surface to feed, then sink back before dawn. This daily journey, called diel vertical migration, carries carbon down into the deep ocean.

The migration was first noticed by accident. In 1942 the scientists C. F. Eyring, R. J. Christensen and R. W. Raitt, working with sonar aboard the USS Jasper, found a false seafloor 300 to 500 metres down by day that sat shallower at night. The echo came from millions of animals, above all small fish of the dim middle depths whose gas-filled swim bladders reflect sound. The layer is now called the deep scattering layer. The animals rise under cover of darkness to feed on plankton and descend at dawn to hide from predators that hunt by sight. By mass it is often described as the largest migration on Earth, and it moves carbon downwards as the animals breathe, excrete and are eaten at depth.

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StillnessOcean

Marine snow is a continuous fall of organic particles, including plankton remains, mucus, faecal pellets and other debris, from upper waters into the deep sea.

Much of this material is eaten or broken down by bacteria on the way, and only a small fraction settles on the seabed, yet it is the main food supply for many deep-sea animals, far below where sunlight lets algae grow. Flakes grow as particles clump together and can sink tens to hundreds of metres a day. The name is often traced to the naturalist William Beebe, who saw particles drifting in his lights during bathysphere dives off Bermuda in the 1930s. Scientists measure the fall with sediment traps and underwater cameras, because it is a key part of the ocean's biological carbon pump: carbon taken up by plankton at the surface sinks, and what reaches the seabed can stay there for centuries or longer.

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WonderOcean

The barreleye Macropinna microstoma has tubular eyes beneath a transparent, fluid-filled shield, and can rotate them from looking upward to looking forward.

The barreleye lives about 600 to 800 metres down in the northern Pacific and grows to around 15 centimetres. It was described in 1939, but for decades it was known only from specimens hauled up in nets, which destroyed the soft shield, and its eyes were thought to be fixed, pointing straight up. Bruce Robison and Kim Reisenbichler of the Monterey Bay Aquarium Research Institute filmed living barreleyes off California with a remotely operated submersible and, in a 2008 paper, showed the green eyes turning inside the intact dome. The two dark spots above the mouth that look like eyes are in fact its organs of smell. Robison has suggested that the fish may steal food from siphonophores, stinging relatives of jellyfish, with the shield protecting its eyes.

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WonderOcean

A siphonophore, a relative of jellyfish, grows from a single fertilised egg into a colony of connected parts. Each part, called a zooid, takes on one job: swimming, feeding, defence or reproduction.

Siphonophores are cnidarians, the group that includes jellyfish and corals. The fertilised egg becomes a single founding zooid, which buds off all the others, so every part is genetically identical. Each zooid corresponds to a whole polyp or small jellyfish in related animals, but here it is specialised and cannot live alone, so the colony works like one body with organs. The best-known siphonophore is the Portuguese man o' war, whose painful sting comes from its feeding and defensive zooids. Many others are fragile, glassy animals of the deep sea. In April 2020 researchers filming in the Ningaloo Canyons off Western Australia found a giant Apolemia colony coiled in a spiral whose outer ring was estimated at about 47 metres, among the longest animals ever recorded.

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WonderOcean

When attacked, hagfish release mucus and microscopic protein threads that expand through seawater into extraordinary volumes of defensive slime.

Hagfish are eel-shaped, jawless fish that live on the seafloor. More than 70 species are known, and fossils about 300 million years old already look much like them. They often feed by burrowing into the carcasses of dead fish and whales. Their slime comes from glands along the body, which release mucus and tightly coiled protein threads. In seawater the threads unravel in a fraction of a second, and a small amount of secretion can turn litres of water into slime, enough to clog the gills of an attacking fish; underwater cameras have filmed sharks letting go and swimming off. Each thread is far thinner than a human hair yet strong and stretchy, so researchers study it as a model for new fibres. Afterwards the hagfish ties itself in a knot and slides the knot along its body to wipe the slime off.

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WonderOcean

Coccolithophores are tiny single-celled algae that cover themselves in intricate plates of chalk (calcium carbonate). Drifting in sunlit seawater in vast numbers, they link microscopic life to ocean chemistry and the global carbon cycle.

Each plate, called a coccolith, is a few thousandths of a millimetre across; it is built inside the cell and then pushed out to its surface. When coccolithophores die, their plates sink, and over millions of years thick layers of them have hardened into chalk: the White Cliffs of Dover are made largely of coccoliths laid down in the Cretaceous period, roughly 100 to 66 million years ago. The commonest species today, Emiliania huxleyi, forms blooms that turn the sea milky turquoise and can be seen from space. Coccolithophores take up carbon through photosynthesis, and their sinking plates carry carbon to the seabed, where scientists read their fossil remains to reconstruct past climates.

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WonderBiology

Archerfish shoot insects off overhanging leaves with a jet of water. They hit the target even though light bends as it passes from air into water, so the insect is not where it appears to be.

Archerfish live in mangroves and river mouths from India and South-East Asia to northern Australia. To shoot, a fish presses its tongue against a groove in the roof of its mouth and snaps its gill covers shut, forcing out a jet that can bring down prey up to about three metres above the water. Adults usually hit with the first shot, and they stay accurate from a wide range of angles. Young fish miss often at first and improve with practice; a 2006 study led by the German biologist Stefan Schuster found they can also learn by watching an experienced fish shoot. Schuster's team also showed that as soon as a struck insect starts to fall, the fish judges where it will land and swims straight there.

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WonderOcean

Male humpback whales in a population share an evolving song, and novel song types can spread between populations through social learning rather than genes.

Only male humpbacks sing, mostly in the breeding season: a long, repeated sequence of moans, cries and pulses, built from phrases and themes. All the males in one area sing nearly the same song, and it keeps changing as singers add small variations that the others copy. Sometimes the change is sudden. In 1996, researchers off eastern Australia recorded two males singing a song that belonged to the population off western Australia; within two years every singer they recorded had switched to it. Their report in Nature in 2000 called it a cultural revolution, because such a rapid, complete change can only spread by learning. Later studies traced new songs moving eastward, population by population, across the South Pacific.

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