Read an Excerpt
Fact or Fiction: Science Tackles 58 Popular Myths
By Scientific American
Scientific AmericanCopyright © 2013 Scientific American
All rights reserved.
The Animal Kingdom
Chocolate Is Poisonous to Dogs
by Alison Snyder
A small dog should be belly-up after eating a handful M&M's, at least according to conventional wisdom. But watching "Moose," a friend's five-pound Chihuahua, race around a living room after his sweet snack makes one wonder: Is chocolate truly poisonous to dogs?
Dogs and humans have similar tastes. Like us, they seek out sweets and have no problem indulging. But unlike humans, our canine companions experience dangerous effects from eating chocolate — it can poison them and in some cases is lethal. The hazard, however, is probably overblown, says Tim Hackett, a veterinarian at Colorado State University. Chocolate's danger to dogs depends on its quantity and quality. Large dogs can usually handle a small amount of chocolate whereas the same helping could cause problems for Moose and his pintsize kin.
Chocolate is processed from the bitter seeds of the cacao tree, which contain a family of compounds known as methylxanthines. This class of substances includes caffeine and the related chemical theobromine. Both molecules bind to receptors on the surfaces of cells and block the natural compounds that normally attach there. Low doses of methylxanthines can lead to vomiting or diarrhea in dogs, and euphoria in humans. Chocolate contains a significant amount of theobromine and smaller amounts of caffeine. If a large quantity of theobromine or caffeine is ingested, some dogs will experience muscle tremors or even seizures. These chemical constituents of chocolate can cause a dog's heart to race up to twice its normal rate, and some dogs may run around as if "they drank a gallon of espresso," according to Hackett. Moose, it seems, was on a "theobromine high."
Dogs are capable of handling some chocolate, but it depends on the animal's weight and the type of chocolate it eats. Unsweetened baking chocolate contains more than six times as much theobromine as milk chocolate, although amounts vary between cocoa beans as well as different brands of chocolate. Less than four ounces of milk chocolate is potentially lethal for Moose and other small dogs, according to the ASPCA Animal Control Poison Center.
Around every confection-centered holiday — Valentine's Day, Easter and Christmas — at least three or four dogs are hospitalized overnight in the animal medical center at Colorado State. But in 16 years as an emergency and critical care veterinarian, Hackett has seen just one dog die from chocolate poisoning, and he suspects it may have had an underlying disease that made it more vulnerable to chocolate's heart-racing effect.
Dogs that eat a small amount of chocolate should be able to filter the methylxanthines through their body and avoid veterinary treatment. But more acutely poisoned dogs are generally treated by inducing vomiting and administering activated charcoal to absorb any methylxanthines remaining in the gut or that may be circulating through the dog's digestive system
Ultimately, Moose survived his cocoa snack. But no matter how you bake it, wrap it or melt it, chocolate and Moose don't mix.
--Originally published: Scientific American Online, February 2, 2007.
Komodo Dragons Show That Virgin Births Are Possible
by Philip Yam
Indonesian dragons can breed without the benefit of masculine companionship. Researchers reported in Nature that the only two sexually mature female Komodo dragons in all of Europe laid viable eggs without insemination from a male. One Komodo, named Flora, lives at the Chester Zoo in England and has never been kept with a male; yet in 2006 she laid a clutch of 11 eggs, eight of which survived and are now settled in zoos around the world. Earlier in 2006, a now deceased female named Sungai from the London Zoo laid a clutch of 22 eggs, four of which yielded normal male dragons--even though Sungai hadn't had a date in two and a half years.
Some reptiles can hold onto sperm for several years, so initially researchers considered that Sungai's eggs had a father. But genetic analysis ruled that out, unless the father were somehow genetically identical to her. (Sungai did later mate with a male and laid a normally fertilized clutch, so don't think she died a virgin.)
These "virgin births" raised eyebrows because this asexual method of reproduction, called parthenogenesis, is rare among vertebrates: only about 70 backboned species can do it (that's about 0.1 percent of all vertebrates). Biologists have known that some lizards can engage in parthenogenesis, but nonetheless seeing it among Komodo dragons surprised zookeepers.
Despite having only a mother, the offspring are not clones. That's because an unfertilized egg has only half the genes of the mother. The sperm is supposed to provide the other half. In parthenogenesis, the mother's half-set of chromosomes doubles up to generate the full complement. Hence, the offspring derives all its genes from the mother, but they are not a duplicate of her genome.
Komodos have a curious twist in their sex determination as well. Although we think of females being XX (that is, having two X chromosomes) and males as being XY, it's the other way around in these giant monitor lizards. Two identical sex chromosomes make a male Komodo, and two different ones make a female. Biologists label the Komodo's sex chromosomes as W and Z, so ZZ makes a male and WZ makes a female. Birds, some insects and a few other lizard species also rely on this sex-determination system. (Embryos of some reptiles--notably crocodiles and turtles--don't have any sex chromosomes; rather, the incubation temperature dictates their gender.)
In Komodo females, each egg contains either a W or a Z. Parthenogenesis hence leads to embryos that are either WW or ZZ. Eggs that consist of WW material are not viable and die off (just as YY is not a viable combination); in contrast, ZZ does work. So all the Komodo hatchlings have been and will be male (ZZ).
Evidently, in the case of these Komodos, the doubling of the egg genes occurred when, in essence, another egg, rather than sperm, did the job of fertilization. Oogenesis, the biological process of making an egg cell, typically also yields a polar body--a mini ovum of sorts, containing a duplicate copy of egg DNA. Normally, this polar body shrivels up and disappears. In the case of the Komodos, though, polar bodies evidently acted as sperm and turned ova into embryos.
The ability to reproduce both sexually and parthenogenetically probably resulted from the Komodo dragon's isolated natural habitat, living as it does on islands in the Indonesian archipelago. Researchers have seen other species resort to parthenogenesis when isolated, such as damselflies in the Azores. The ability, researchers speculate, may have enabled the dragons to establish new colonies if females had found themselves washed up alone on neighboring shores, as might happen during a storm.
High school biology texts tend to gloss over parthenogenesis, typically mentioning the process as rare and restricted to mostly small invertebrates. But the phenomenon has emerged from the backwaters in recent years, primarily as a tool for science. Some scientists hope to exploit the phenomenon to get around ethical concerns surrounding embryonic stem cell research. They can fool an unfertilized human egg to divide by pricking it, thereby simulating the penetration of sperm. Such deceived eggs continue dividing into the blastocyst stage of 50 to 100 cells before petering out naturally.
In principle, it may be possible to keep that cell dividing. In 2004, as a means to elucidate the details of how fertilized eggs develop, scientists in Japan engaged in some genetic trickery to create a fatherless mouse. Such a developmental process probably didn't happen in the little town of Bethlehem two millennia ago--the mistranslation of "young girl or maid" into "virgin" explains the story a lot better. But as the Komodo dragons' astonishing parthenogenesis feat shows, nature has plenty to teach us about making do without a mate.
--Originally published: Scientific American Online December 28, 2006.
A Cockroach Can Live without Its Head
by Charles Q. Choi
Cockroaches are infamous for their tenacity, and are often cited as the most likely survivors of a nuclear war. Some even claim that they can live without their heads. It turns out that these armchair exterminators (and their professional brethren) are right. Headless roaches are capable of living for weeks.
To understand why cockroaches — and many other insects — can survive decapitation, it helps to understand why humans cannot, explains physiologist and biochemist Joseph Kunkel at the University of Massachusetts Amherst, who studies cockroach development. First off, decapitation in humans results in blood loss and a drop in blood pressure hampering transport of oxygen and nutrition to vital tissues. "You'd bleed to death," Kunkel notes.
In addition, humans breathe through their mouth or nose and the brain controls that critical function, so breathing would stop. Moreover, the human body cannot eat without the head, ensuring a swift death from starvation should it survive the other ill effects of head loss.
But cockroaches do not have blood pressure the way people do. "They don't have a huge network of blood vessels like that of humans, or tiny capillaries that you need a lot of pressure to flow blood through," Kunkel says. "They have an open circulatory system, which there's much less pressure in."
"After you cut their heads off, very often their necks would seal off just by clotting," he adds. "There's no uncontrolled bleeding."
The hardy vermin breathe through spiracles, or little holes in each body segment. Plus, the roach brain does not control this breathing and blood does not carry oxygen throughout the body. Rather, the spiracles pipe air directly to tissues through a set of tubes called tracheae.
Cockroaches are also poikilotherms, or cold-blooded, meaning they need much less food than humans do. "An insect can survive for weeks on a meal they had one day," Kunkel says. "As long as some predator doesn't eat them, they'll just stay quiet and sit around, unless they get infected by mold or bacteria or a virus. Then they're dead."
Entomologist Christopher Tipping at Delaware Valley College in Doylestown, Pa., has actually decapitated American cockroaches (Periplaneta americana) "very carefully under microscopes," he notes. "We sealed the wound with dental wax, to prevent them from drying out. A couple lasted for several weeks in a jar."
Insects have clumps of ganglia — nerve tissue agglomerations — distributed within each body segment capable of performing the basic nervous functions responsible for reflexes, "so without the brain, the body can still function in terms of very simple reactions," Tipping says. "They could stand, react to touch and move."
And it is not just the body that can survive decapitation; the lonely head can thrive, too, waving its antennae back and forth for several hours until it runs out of steam, Kunkel says. If given nutrients and refrigerated, a roach head can last even longer.
Still, in roaches, "the body provides a huge amount of sensory information to the head and the brain cannot function normally when denied these inputs," explains neuroscientist Nick Strausfeld of the University of Arizona, who specializes in arthropod learning, memory and brain evolution. For instance, although cockroaches have a fantastic memory, "when we've tried to teach them when they had bits of them missing, it's hopeless. We have to keep their bodies completely intact."
Cockroach decapitation may seem macabre, but scientists have conducted many experiments with headless roach bodies and bodiless roach heads. Decapitating roaches deprives their bodies of hormones from glands in their heads that control maturation, helping researchers investigate metamorphosis and reproduction. And studies of bodiless roach heads shed light on how their neurons work. Plus, it provides just one more testament to the cockroach's enviable endurance.
--Originally published: Scientific American Online March 15, 2007.
UV Light Puts Spiders "in the Mood"
by John Matson
Ultraviolet (UV) light — the band of electromagnetic radiation nestled between visible light and x-rays — seems to cast a particularly amorous glow on the animal world. For instance, the budgie, an Australian parrot, is known to respond negatively to potential mates whose plumage has been stripped of its UV-induced fluorescence (wherein ultraviolet light is absorbed and light of a different, usually visible, wavelength is emitted). And although we humans cannot see UV light, as birds and many other animals can, we have incorporated lamps that produce it into some of our modern courtship rituals — just ask anyone who has ever hit the tanning beds in hopes of snaring a mate or any teen whose idea of setting the mood involves shining a black light on a Pink Floyd poster (which then, like the plumage of a budgie, fluoresces visible light).
The most illuminating example of the potential of ultraviolet romance, though, just might come from a jumping spider. As described in a January 26 paper in Science, researchers have shown that the Cosmophasis umbratica spider not only needs UV light (a constituent of sunlight) to instigate normal mating behavior, but that males and females of the species respond to it in physiologically distinct ways.
C. umbratica males feature scales on their face and body that reflect ultraviolet light, whereas the females do not. (Jumping spiders possess UV receptors in their retinas, so they can detect its emission or reflectance.) The females do, however, possess something the males lack: the ability to fluoresce bright green under UV illumination. Having recognized this distinction, the researchers decided to examine what role this gender-specific physiology plays in mating. So they blocked the ultraviolet wavelengths and observed what might be the arachnid equivalent of a cold shower. "It kind of ruined their sex life, really," says Michael Land, professor of neurobiology at the University of Sussex in England and one of the authors of the Science paper.
The sexual preferences of this species are easily observable, because an interested C. umbratica "has, like many jumping spiders, a fairly colorful mating dance," Land notes. "The males do this kind of Highland fling in front of the females." Females, for their part, have their own come-hither protocol: "They either stay still or they go for a little run and then come back again," he says. Under UV-blocked light, the authors report, "a large proportion of the same pairs that successfully interacted in the presence of UV failed to show intersexual behavior in its absence." Most males "failed to court the female when she lacked fluorescence," and most females similarly snubbed males not reflecting UV.
While the aforementioned budgies also disdain suitors in the absence of UV-induced cues, "it just seems to be rather like having a shirt at the disco under UV light, which glows if you wash it in the right washing powder," Land says. "But that's both sexes and just seems to be a property of the yellow pigments. So this is different. This is a sexual badge, if you like." Whether that badge acts as an aphrodisiac or merely a prerequisite identifier remains unclear. "I don't know how you distinguish between the two. Because if they identify [another spider] as the wrong species, this is obviously not going to be very aphrodisiacal," Land explains. The two roles, he adds, are "obviously going to go together, and it's almost impossible, I think, to disentangle them."
Even if the presence of UV had no effect on C. umbratica courtship, and even if the spiders were unified in their UV-induced signaling — that is, if both sexes reflected UV or both fluoresced green under UV — the species would still be in elite company: "Ultraviolet reflectance is not particularly common in animals," says Thomas Cronin, a vision researcher and professor of biological sciences at the University of Maryland, Baltimore County. And as for fluorescence, Cronin says, "it's not thought to be that common; we don't have very many examples of it." Still, he adds, "it would be easy to miss, because you kind of have to look for it rigorously."
Land says his colleagues have "big, big plans" to do just that — to look at other species and determine whether this type of signaling is unique to C. umbratica or more widespread. Whatever these further experiments reveal, this much is already clear: one species, for ultraviolet light at least, seems to have reached some sort of consensus on the age-old lights on–lights off debate.
--Originally published: Scientific American Online March 29, 2007.
Excerpted from Fact or Fiction: Science Tackles 58 Popular Myths by Scientific American. Copyright © 2013 Scientific American. Excerpted by permission of Scientific American.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.