Do Fish Sleep And Whats That On Your Eyelash
National Wildlife answers ten pressing questions about the animal kingdom
- Doug Stewart
- Apr 01, 1994
Questions? Do we get questions! To help satisfy our readers' apparently unquenchable curiosity about things zoological, we've asked regular contributor Doug Stewart to answer 10 frequently asked questions about the animal kingdom. Herewith his findings:
Do fish sleep?
Since fish can't shut their eyes, one might think they don't sleep, but that's like assuming we humans don't sleep because we can't shut our ears to drown out sound. Though light may play across a fish's retina, that doesn't mean the creature's brain is paying much attention. Most species of fish take time out during the day or (more often) at night to stop moving and to rest-sleep, after a fashion. Some float in place. Others lie on the bottom. By dawn, schools usually regroup and resume their synchronized swimming.
Members of several species go to the trouble of preparing a sleeping place. Wrasses may burrow into sand until all that's visible is a small mound that rises and falls with their breathing. Before nodding off each evening, some parrot-fish encase themselves in mucus to keep eels from detecting them. When the fish are in this condition, scuba divers can pick up the immobile creatures in their hands.
Which animal's venom is most lethal to humans?
Drop for drop, probably no animal venom in the world is more lethal to humans than that of Australia's blue-ringed octopus. It nips its prey with a small, parrotlike beak, then literally spits its venom into the wound. A small bite from a 6-inch octopus can kill a healthy adult human in minutes.
Octopuses in general are energetic and resourceful predators, and many use venom to paralyze their underwater prey while encircling them with their arms. The venom of the blue-ringed octopus, however, is especially nasty. A form of poison with the ominous name of tetrodotoxin, it kills by shutting down the central nervous system. In human victims, a dry mouth may be followed quickly and painlessly by unconsciousness and respiratory arrest.
A variant of tetrodotoxin found, oddly enough, in the eggs and skin of the western newt (a resident of western U.S. coastal states) serves purely as a defense. Since heating the chemical causes it to break down, the newt poses little danger to humans, notes herpetologist Sherman Minton of Indiana University, because "no one's likely to eat a western newt raw."
Among animals in the United States, the bicolored sea snake, which can be found in Hawaiian waters, may have the most toxic venom. Overall, the bites of sea snakes are more deadly than those of their terrestrial cousins. Tracking wounded prey is more difficult underwater than on land, so a lethal chomp on first encounter saves a sea snake a lot of chase time. Moreover, reptiles need air to breathe, so they can't let their undersea battles drag on too long.
Toxicity aside, land snakes are far more likely than sea snakes to cause trouble for humans. The large size, proximity to humans and occasional aggressiveness of Asian cobras, African black mambas and Australian taipans make these snakes particularly bad company. And though the fearsome king cobra's venom may not match the toxicity of the blue octopus's drop for drop, these huge snakes pack enough of the stuff to kill an adult elephant. Why this should be so-given that cobras can't chew, let alone bite off chunks of their victims, and so must feed on prey small enough to be swallowed whole-is a mystery.
Ironically, most snake experts would rather submit to a cobra bite than to the less lethal bites of rattlers or cottonmouths. Cobras, like the blue-ringed octopus, deal in neurotoxins, which affect the nervous system; if you have antivenin on hand, you needn't suffer a whit. Rattlers and cottonmouths, on the other hand, deliver hemotoxins. These aid the snakes by, among other things, "pre-digesting" tissue around the bite, a process unappreciated by the tissue's owner. "It's said to feel like a red-hot sledgehammer pounding on you all night," says Harry Green, a herpetologist at the University of California, Berkeley, "and that's when you've taken the antivenin."
Which animal is most endangered?
One is an awfully lonely number, and that's how many Spix's macaws are believed to exist in the wild. At its death, the genes of the solitary, Brazilian male won't disappear entirely, however: It recently mated with a female of another endangered species, a blue-winged macaw. Meanwhile, 15 Spix's macaws are known to survive in zoos.
In the United States, the once-plentiful 'o'o, a bird native to the Hawaiian island of Kauai, has also dwindled to a single male. Wildlife biologists have tried to keep tabs on the little black bird, observing it warble, build nests and in general make itself as attractive as possible to a mate that would never appear. Since 1987, however, the 'o'o hasn't been seen. The species may be gone for good.
There are many other candidates for world's most endangered animal, some of which haven't turned up, dead or alive, for decades. And one can only guess how many species have died out without ever being discovered by humans. Take the example of the largely unsurveyed Amazon rain forest, which contains at least half the plant and animal species on Earth. There, 92,000 square miles, or 5.6 percent of the original forest, have been logged.
What creatures pose the greatest danger to humans?
No contest: not snakes, not octopuses, but flies. And among flies, mosquitos are the deadliest. By itself, the malarial parasite carried by mosquitos probably kills a million people a year in Africa alone. Over the course of history, malarial protozoa may have killed more people than all the wars ever fought. Mosquitos also carry yellow fever and at least 100 different viruses. Altogether, these diseases make at least 300 million people sick every year.
As for houseflies, just one can carry as many as 6 million disease-causing bacteria. Many of the germs hitch rides on the fly's hairy body and sticky foot pads. Its digestive tract carries far more. The latter often emerge to begin their own colonies as a result of the fly's habit of dissolving its food by vomiting on it. Aggravating matters is the fly's sheer plenitude and its tendency to keep moving, often in an unhygienic commute between animal wastes and human food.
Flies have been considered a source of pestilence since antiquity. (Reports the Bible: "There came great swarms of flies into the house of Pharaoh and into his servants' houses, and in the land of Egypt the land was ruined by reason of the flies." -Exodus 8:24.) Fly-borne diseases include cholera, tuberculosis, polio, plague, dysentery and numerous eye diseases.
What are the oldest and newest forms of animal life now living?
Even the appearance of the newest form of animal life predates that of biologists with notebooks, so no one will ever know for sure. But a shellfish called Lingula-a long worm, really, with two small shells at one end to protect its vital organs-could well be the animal species whose fossil record stretches back furthest in time.
Lingula evolved from an ancient ancestor called Lingulella, both part of a phylum called brachiopods. Comparing the two versions, "We can hardly detect any difference in fossil shells that go back 500 million years," says Norman Newell, retired curator of invertebrates at the American Museum of Natural History. Over time, animals change as the world around them changes. In Lingulella's case, its shallow, stagnant ocean-bottom habitat has evidently not varied much since the Palezoic Era. "If an animal's ecological niche doesn't change," says Newell, "the organism itself is not under pressure to change."
As for the newest arrival among the world's creatures, a plausible choice is a species of Hawaiian fly, Drosophila silvestris. The insects live in rain forests on the big island of Hawaii, which burst from the Pacific Ocean as a mound of smoking lava only a half million years ago. Hawaiian Drosophila may all be descendants of a single pregnant female that managed to fly the 30 miles from Maui soon after vegetation had begun to cover the island.
Today, nearly 100 species of Drosophila are native to the big island. Ken Kaneshiro, director of the Hawaiian Evolutionary Biology Program at the University of Hawaii, attributes this rapid speciation to the island's ever active volcanoes. Fresh lava continually cuts off patches of rain forest called kipuka. Despite the aerial adventurousness of their long-ago matriarch, most Hawaiian Dropsophila like to stay put, not even crossing a barren lava flow a few hundred yards wide to a new patch of forest. "In just 100 years," says Kaneshiro, "there can be significant genetic differences in the populations on either side." He's already identified six subspecies of Drosophila silvestris on the island, each in its own kipuka. "We're seeing evolution happen right under our noses," says Kaneshiro.
What animals have the shortest and longest lives?
If you can bring yourself to pity a bug, pity the poor white mayfly, which typically lives an hour and a half once it emerges from its nymphal shell. Few mayflies of any kind live longer than a day. Belonging to the aptly named order Ephemeroptera, they devote their brief lives to a single desperate mission: finding a mate. Bereft of mouths and stomachs, they don't even stop to eat. What they lack in longevity, however, they make up for in gregariousness. Adult mayflies sometimes gather and die in such swarms that snowplows are needed to clear paths through their lifeless little bodies.
At the other extreme, the record holder is the arctic clam, one of which lived quietly underwater for some 220 years. That is, if you accept growth rings in lieu of a birth certificate. (Not all biologists do.) If you insist on better documentation, the oldest animal ever was a Marion's tortoise that died on the island of Mauritius in 1918. The reptile had been captured -- already full-grown - in the Seychelles in 1766, nine years before the American Revolution began, and it died 152 years later as World War I came to a close.
As elderly as this creature was, it officially outlived by only 32 years the oldest fully documented mammal in history, a Japanese gentleman named Shigechiyo Izumi (1865-1986).
What creature has the sharpest sense of smell?
The silkworm moth's is unrivaled. In experiments in the early 1960s in Germany, male silkworm moths (Bombyx mori) guided only by scent were able to zero in on females 7 miles away. Not that female silkworm moths are particularly odorous: Their abdominal glands carry one ten millionth of a gram of their love scent, or pheromone, and they release only a tiny portion of that into the air at one time. When miles of air intervene, it's unlikely that more than a few molecules wind up striking the frondlike antennae of a distant love-struck suitor. (The male's sensory ability is species-specific, by the way. Aside from female silkworm moth scent, male silkworm moths have no sense of smell whatsoever.)
Do all animals dream?
Fish probably don't, but many other creatures undoubtedly do. One bit of circumstantial evidence is a dog's well-known tendency to growl, whimper, even simulate copulation while fast asleep. A more clinical piece of evidence comes from sleep laboratories: Humans routinely dream during nightly periods of REM (rapid eye movement) sleep, and many animal species-mammals and birds -also experience REM sleep.
Researchers have identified parts of the brain stem in cats that seem to inhibit muscular movement during sleep. When these centers are damaged, sleeping cats appear to act out their dreams quite dramatically-sometimes stalking and pouncing on imaginary prey. Until cats learn to talk, we'll never know for sure what's going on in their snoozing little cat minds, but it would certainly be odd if they weren't dreaming.
Jonathan Winson, a neuroscientist at Rockefeller University in New York, speculates that dreaming is what has allowed higher animals to evolve. Back when marsupial and placental mammals first evolved 40 or 50 million years ago, he theorizes, "REM sleep took on the task of updating an animal's waking experiences"-matching new and old memories, rehearsing responses to new situations and the like. "It took over some of the load of information processing that's placed on the cortex."
This form of off-line processing, to use the computer term, could let mammalian species become smarter without being burdened with impossibly large brains, Winson believes. Rats are clever, he points out, yet their prefrontal cortex is tiny. By comparison, the prefrontal cortex of the echidna, a primitive anteater that predates most other mammals, is huge and convoluted. The echidna, he notes, is one of the few mammals that's totally incapable of REM sleep (which probably means it doesn't dream).
What animal has the most sophisticated sense of hearing?
The bat's sense of hearing is astoundingly acute. And it does far more than listen for the buzzing of insects in order to find a meal. If that were the case, why would a bat swoop in on wet cotton balls tossed silently into the air of a pitch-black room? Rather, the winged mammal uses echolocation for a variety of purposes, judging the position of objects from the timing, strength, and direction of its own ultrasonic voice bouncing off an airborne target. A bat closing in on a fleeing moth can emit 200 bursts of sound in a second (not audible squeaks but high-pitched chirps and clicks) and analyze the returning signals as the moth takes evasive action. All the while, the bat manages to avoid crashing into obstacles-or other bats-in its path.
This, and the fact that bats won't actually chomp down on a cotton ball (or collide with it) but instead veer away at the last moment, suggests they do more than simply locate objects in the dark. Somehow they're seeing with their ears. Recent studies suggest that bats' brains may be converting the echoes they hear into three-dimensional images of their surroundings. Bats can apparently distinguish a moth's shape from that of a leaf the same size. They can also gauge a moth's speed through the air, even how fast its wings are beating-all by split-second analysis of echoes they're hearing.
One puzzle to researchers has been how a bat can see near and far objects as part of the same scene when the echoes of each arrive at different times. (We would face the same problem matching foregrounds to backgrounds if light traveled as sluggishly as sound, about a foot per millisecond.) The answer turns out to be an amazing adaptation in the bat's brain: delay-tuned neurons.
The neurons, or brain cells, that help in echolocation fire only in response to echoes from objects a set distance away. Nothing else will trigger them, says Brown University neurobiologist and bat expert Steven P. Dear. "But even those neurons that are tuned only to objects a foot away don't all fire at once," he says. The built-in delays compensate for early arrivals among the echoes, those bouncing off the nearest objects, and thus let the bat's cortex build up three-dimensional pictures. "It solves the foreground-background problem," says Dear. "If you're going to chase a bug through branches full of leaves, you've got to tell what's the bug and what's the branches."
As wondrously complex as a bat's system of echo analysis may be, it is not necessarily the animal world's most complex. Whales and dolphins also use echolocation, and the amount of brain tissue they can devote to the task is far greater. They're harder to study in controlled settings than bats are, however, so less is known about their hearing.
What's that on your eyelash? and what's that in your bed?
What, these? They're mites. Demodex folliculorum, a tiny eight-legged relative of the spider, makes its home at the base of our eyelashes. There, the creatures hatch, defecate, copulate and die, sometimes a half dozen mites to a lash, though we wouldn't know it. They don't hurt a bit, and as far as anyone knows, they're perfectly benign. Never much longer than a hair is wide, follicle mites feed by puncturing the cells of our eyelash follicles with two tiny needles, then pumping out cellular fluid as needed. Better yet, they love mascara.
"You can find them yourself," says William B. Nutting, a zoologist at the University of Massachusetts who's been studying mites since the 1940s. Drop a fresh-plucked eyelash on a drop of water, he suggests, and examine its thickened base with a large magnifying glass. "It's kind of fun," he says.
Follicle mites are vastly outnumbered by their off-body brethren, which feed on the minuscule flakes of dead skin that cascade from our bodies day and night (at the rate of tens of thousands of tiny fragments per minute). Probably every human dwelling the world over has them, though people with allergies to the tiny creatures can keep their numbers down by eliminating carpeting and wrapping mattresses in plastic. A typical double-bed mattress contains as many as two million mites (not to be confused with much larger bedbugs). Sweet dreams!
Massachusetts freelance writer Doug Stewart reports that he unearthed 10 new questions for every one he tried to answer.