Caltech physicists achieve first bona fide quantum teleportation

PASADENA—Physicists at the California Institute of Technology, joined by an international collaboration, have succeeded in the first true teleportation of a quantum state.

In the October 23 issue of the journal Science, Caltech physics professor H. Jeff Kimble and his colleagues write of their success in transporting a quantum state of light from one side of an optical bench to the other without it traversing any physical medium in between.

In this sense, quantum teleportation is similar to the far-fetched "transporter" technology used in the television series Star Trek. In place of the actual propagation of a light beam, teleportation makes use of a delicate quantum mechanical phenomenon known as "quantum entanglement," the quintessential ingredient in the emerging field of quantum information science.

"In our case the distance was only a meter, but the scheme would work just as well over much larger distances," says Professor Samuel Braunstein, a coauthor from the University of Wales in Bangor, United Kingdom, who, with Kimble, conceived the scheme. "Our work is an important step toward the realization of networks for distributing quantum information—a kind of 'quantum Internet.'"

Teleportation of this kind was first proposed theoretically by IBM scientist Charles H. Bennett and colleagues in 1993. The Caltech experiment represents the first time quantum teleportation has actually been performed with a high degree of "fidelity." The fidelity describes how well a receiver, "Bob," can reproduce quantum states from a sender, "Alice."

Although quantum teleportation was recently announced by two independent labs in Europe, neither experiment achieved a fidelity that unambiguously required the use of quantum entanglement between Alice and Bob.

"True quantum teleportation involves an unknown quantum state entering Alice's apparatus and a similar unknown state emerging from Bob's remote station," says Kimble. "Moreover, the similarity of input and output, as quantified by the fidelity, must exceed that which would be possible if Alice and Bob only communicated by classical means—for instance, by normal telephone wiring.

"Although there has been wonderful progress in the field, until now there has not been an actual demonstration of teleportation that meets these criteria."

In the experiment, the Caltech team generated exotic forms of light known as "squeezed vacua," which are split in such a way that Alice and Bob each receive a beam that is the quantum mechanical "twin" of the other. These EPR beams, named after the historic Einstein-Podolsky-Rosen (EPR) paradox of 1935, are among the strangest of the predictions of quantum mechanics. It was their theoretical possibility that led Einstein to reject the idea that quantum mechanics might be a fundamental physical law.

A trademark of quantum mechanics is that the very act of measurement limits the controllability of light in ways not observed in the macroscopic world: even the most delicate measurements can cause uncontrollable disturbances. Nevertheless, in certain circumstances, these restrictions can be exploited to do things that were unimaginable in classical physics.

Here, photons from the EPR beams delivered to Alice and Bob can share information that has no independent existence in either beam alone. Through this "entanglement," the act of measurement in one place can influence the quantum state of light in another.

Once Alice and Bob have received their spatially separate but entangled components of the EPR beams, Alice performs certain joint measurements on the light beam she wishes to teleport together with her half of the EPR "twins." This destroys the input beam, but she then sends her measurement outcomes to Bob via a "classical" communication channel. Bob uses this classical information to transform his component of the EPR beam into an output beam that closely mimics the input to Alice, resurrecting at a distance the original unknown quantum state.

A unique feature of Kimble's experiment is a third party called "Victor," who "verifies" various aspects of the protocol performed by Alice and Bob. It is Victor who generates and sends an input to Alice for teleportation, and who afterward inspects the output from Bob to judge its fidelity with the original input.

"The situation is akin to having a sort of 'quantum' telephone company managed by Alice and Bob," says Kimble. "Having opened an account with an agreed upon protocol, a customer (here Victor) utilizes the services of Alice and Bob unconditionally for the teleportation of quantum states without revealing these states to the company. Victor can further perform an independent assessment of the 'quality' of the service provided by Alice and Bob."

The experiment by the Kimble group shows that the strange "connections" between entities in the quantum realm can be gainfully employed for tasks that have no counterpart in the classical world known to our senses.

"Taking quantum teleportation from a purely theoretical concept to an actual experiment brings the quantum world a little closer to our everyday lives," says Christopher Fuchs, a Prize Postdoctoral Scholar at Caltech and a coauthor. "Since the earliest days of the theory, physicists have treated the quantum world as a great mystery. Maybe making it part of our everyday business is just what's been needed for making a little sense of it."

This demonstration of teleportation follows other work the Kimble group has done in recent years, including the first results showing that individual photons can strongly interact to form a quantum logic gate. Kimble's work suggests that the quantum nature of light may someday be exploited for building a quantum computer, a machine that would in certain applications have computational power vastly superior to that of present-day "classical" computers.

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Robert Tindol
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Multilayered silicon could bea breakthrough for electronic technology

PASADENA—Researchers at the California Institute of Technology have found a way to stack silicon layers on chips in a way that could lead to significant new advances in silicon-based electronic devices.

In the October issue of the Journal of Vacuum Science and Technology B, Caltech's Fletcher Jones Professor of Applied Physics Thomas McGill and his colleagues report on their work growing a novel silicon structure through a process known as molecular beam epitaxy.

The process begins with an existing silicon wafer, onto which an insulating layer of cerium dioxide just a few atoms thick is grown. Finally, a single crystal of silicon can be grown back onto the cerium dioxide.

The end result is a three-dimensional device using cerium dioxide as an insulator with crystalline silicon on top. Beginning with this top layer of silicon, the wafer is then ready to begin the process again. In this manner, layer upon layer of devices may be grown one after another on the same chip.

"The implications are very significant," says McGill. "For years there have been predictions that progress will eventually stop in silicon electronics because the devices will have been shrunk as much as they can.

"But this new technology could allow you to get the functionality increase by stacking instead of shrinking," he says.

McGill says the group has stacked only a single extra layer of silicon so far. However, the key is the demonstration that the cerium oxide is indeed acting as an insulator, and that the silicon on top is single crystalline and suitable for further growth.

"In principle, you can stack forever," McGill says.

According to Caltech grad student Joel Jones, another member of the team, this technique is especially interesting because it also allows the fabrication of a new group of novel silicon devices.

"We've already fabricated a primitive tunnel switched diode from the multilayered chips," Jones explains. "This is a single device that exhibits memory. At a given voltage, you can have two different stable currents depending on how you've switched the device."

This phenomenon is called negative differential resistance, allowing two current states of different amperage to exist at the same voltage.

Similar effects can be found in other devices enabled by this new technique, including resonant tunneling diodes. These devices can be exploited for novel memory storage, as well as used to enhance the performance of numerous other microelectronic circuits.

"The silicon industry is a $100 billion industry," says McGill. "This could be a major contributor in 10 to 15 years."

In addition to McGill and Jones, the authors of the paper are Edward Timothy Croke, Carol M. Garland, and Ogden Marsh.

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Robert Tindol
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Galileo data shows Jupiter's lightning associated with low-pressure regions

MADISON, Wisconsin—Images of Jupiter's night side taken by the Galileo spacecraft reveal that the planet's lightning is controlled by the large-scale atmospheric circulation and is associated with low-pressure regions.

The new findings were reported October 13, 1998 by Andrew Ingersoll at the 30th annual meeting of the American Astronomical Society's Division for Planetary Sciences.

"Lightning is an indicator of convection and precipitation," says Ingersoll, a professor of planetary science at the California Institute of Technology and member of the Galileo Imaging Team. "These processes are the main sources of atmospheric energy, both on Earth and on Jupiter."

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In a terrestrial hurricane, Ingersoll explains, the low pressure at the center draws air in along the ocean surface, where it picks up moisture. Energy is relased when the moisture condenses and falls out as rain.

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On Jupiter, energy is transferred from the warm interior of the planet to the visible atmosphere in a similar process. The new findings show that lightning occurs in the low-pressure regions on Jupiter, too.

"On both planets, the air spins counterclockwise around a low in the northern hemisphere and clockwise around a low in the southern hemisphere," Ingersoll says. "The lows are called cyclones and the highs are called anticyclones."

On Jupiter the cyclones are amorphous, turbulent regions that are spread out in the east-west direction. In the Voyager movies they spawn rapidly expanding bright clouds that look like huge thunderstorms. The Galileo lightning data confirm that convection is occurring there.

"We even caught one of these bright clouds on the day side and saw it flashing away on the night side less than two hours later," says Ingersoll.

In contrast, the Jovian anticyclones tend to be long-lived, stable, and oval-shaped. The Great Red Spot is the best example (it is three times the size of Earth and has been around for at least 100 years), but it has many smaller cousins. No lightning was seen coming from the anticyclones.

"That probably means that the anticyclones are not drawing energy from below by convection," says Ingersoll. "They are not acting like Jovian hurricanes."

Instead, the anticyclones maintain themselves by merging with the smaller structures that get spun out of the cyclones. "That's what we see in the Voyager movies, and the Galileo lightning data bear it out. Whether the precipitation is rain or snow is uncertain," says Ingersoll.

"Models of terrestrial lightning suggest that to build up electrical charge, both liquid water and ice have to be present. Rain requires a relatively wet Jupiter, and that's a controversial subject.

"Water is hard to detect from the outside because it is hidden below the ammonia clouds. And the Galileo probe hit a dry spot where we didn't expect much water."

Fortunately the Galileo imaging system caught glimpses of a cloud so deep it has to be water, according to findings to be reported at the conference by Dr. Don Banfield of Cornell University and an imaging team affiliate. Banfield showed images of the water cloud near the convective centers in the cyclonic regions.

These results appear in the September issue of Icarus, the International Journal of Solar System Studies.

"We know the water is there, and we know where it's raining," says Ingersoll. "This is a big step toward understanding how Jupiter's weather gets its energy."

 

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Robert Tindol
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Caltech Question of the Month: Is there any such thing as "earthquake weather"?

Submitted by Maureen Castro, Oakland, California, and answered by Kate Hutton, Seismologist, Caltech.

There is a popular notion that earthquakes happen more often in certain kinds of weather. Unfortunately, the description of the preferred weather varies geographically and with the person providing the description.

Traditionally, earthquake weather was still and sultry. According to Aristotle, earthquakes were caused by winds trapped underground. Less wind above the surface must mean more below, and hence earthquakes were more likely. In California, however, which frequently has hot weather, earthquakes are often associated with the Santa Ana condition. In particular, the Northridge quake, "the earthquake" that is currently on people's minds, happened on an unseasonably warm day. Case proven, right?

In actuality, earthquakes are caused by the accumulation of strain in the crust due to the motion of tectonic plates. Most hypocenters (the place where the quake starts, directly below the epicenter) occur at least five miles below the surface, for large quakes. What is going on in the atmosphere simply does not affect conditions at that depth. Weather doesn't even affect the temperature in a well-designed wine cellar!

Besides, we have to remember that Alaska has several times the number of earthquakes that we do here. In Alaska, earthquake weather is cold and snowy!

As an aside, weather could very easily influence the scale of the disaster caused by an earthquake. Consider the effect of Santa Ana winds on fires and the effect of rain on the ubiquitous California mudslide in your worst-case planning scenarios.

Crust of Tibetan Plateau is being squeezed by India and Asia, new study shows

PASADENA—Geophysicists have discovered why there are high plains and mountains in the Himalayas for trekkers to trek on. According to new data, the soft crust of the Tibetan Plateau is being squeezed like an accordion between the harder crusts of India and Asia.

According to Caltech professor of geophysics Donald Helmberger and his doctoral student Lupei Zhu, the results show for the first time that a portion of crust can be squeezed and thickened if plate tectonics is forcing a harder section of crust into another hard section. Before the current study, geophysicists were unsure whether the plateau was formed by actions in the mantle or more shallow movements of the crust.

In the August 21 issue of the journal Science, the researchers show that the northward tectonic motion of India is forcing the softer and younger crust of the Tibetan Plateau into the Qaidam Basin to the north. Like India, the crust of the Qaidam Basin is also old and hard.

Since the seismic data shows there is a dramatic change in the thickness of the crust at the edge of the Qaidam Basin, the researchers infer that the softer crust is being literally forced into a hard vertical wall beneath the surface.

Therefore, the crust of the Tibetan Plateau is being crammed up and thickened in the collision. In addition to providing uplift, the action is also grinding the materials laterally. The horizontal fault lines observed in the region also support this interpretation.

"This gives a different perception about how strong an old crust can be," says Helmberger. "There's a very sharp change in the thickness of the crust, from about 40 kilometers at the Qaidam Basin to about 60 kilometers at the Tibetan Plateau.

"Physically, this means the crust beneath the Qaidam Basin is like a solid wall," he adds. "This thing below the basin is cold and old and very tough."

Zhu and Helmberger's results come from raw data collected by the Institute of Geophysics at Beijing and the University of South Carolina during the joint PASSCAL project in the early 1990s. Zhu did some of the field work before entering Caltech in 1993 as a graduate student.

Zhu says he would like to return to Tibet for additional data at other sites, and both he and Helmberger think the work could herald a new understanding of how the crust figures into plate tectonics. 

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Robert Tindol
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Mechanism of cell suicide determined by Caltech, MIT researchers

PASADENA—Biologists at MIT and Caltech have uncovered the chemical details of a mechanism that cells use to commit suicide. The work appears in the August 28 issue of the journal Science.

According to David Baltimore, president of Caltech and a Nobel Prize-winning biologist, his lab at MIT has succeeded in describing how roundworms known as nematodes kill off unwanted cells. The work is especially interesting, Baltimore says, because human beings have very similar proteins to those causing cell suicide in nematodes and, in fact, his lab can often substitute human proteins for the same results.

"All cells contain the machinery to commit suicide," Baltimore said prior to publication of the paper. "You can see this in a wide variety of events, such as a tadpole's resorption of its tail, local ischemia in a stroke victim's brain, and tissue destruction after a heart attack.

"Cell suicide is also one of the great protections against cancer." According to the current paper in Science, a common type of apoptosis, or cell suicide, involves three stable proteins found in nematode cells. These proteins are normally quiet, but can be readily triggered by death signals in such a way that the cell digests itself.

The three proteins are known as CED-3, CED-4, and CED-9. None of these proteins alone will kill cells, the research shows, but the three interact in such a way that CED-4 can signal CED-3 to begin the destruction process, while CED-9 acts as an inhibitor to CED-4.

The general outline of this particular pathway of apoptosis was discovered by MIT professor Robert Horvitz some years ago, but the details have never been understood until now, Baltimore says.

"We did all of this with proteins from a nematode where the pathway was first found, but the proteins all have human homologs," Baltimore says. These are Apaf-1, which is very similar to CED-4; Bcl-2, which is a homolog of CED-9; and mammalian cysteine protease zymogens that are analogous to CED-3.

Therefore, the cascade of reactions in nematode cells could very well resemble the manner in which the human body can cause cancerous cells to self-destruct. The work was supported by the National Institutes of Health. In addition to Baltimore, the authors are Xiaolu Yang and Howard Y. Chang. Yang is currently at the University of Pennsylvania.

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Robert Tindol
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New Study Shows How Axons Find Their Way Home

Pasadena--Like a commuter trying to get to work during rush hour, a growing axon must thread its way through a throng of other axons that are headed in many different directions in the developing brain. Axons are the wire like extensions of nerve cells that carry electrical signals from one place to another in the brain, and during development they must navigate across long distances (many centimeters) to reach their correct address within the brain. If the axon gets lost, brain circuits cannot form normally, and, like the commuter showing up at the wrong office, the axon may not be able to do its job. So how do axons find their way? A report published in the July 24 issue of the journal Science. by Drs. Susan Catalano and Carla Shatz of the University of California at Berkeley, sheds light on how axons home in on their correct targets.

Traditionally, scientists studying the mechanisms of axon navigation thought in terms of molecular guidance cues. Molecules located in specific places in the brain can tell a growing axon "grow here," "don't grow there," or "make a left turn here." The collective distribution of these molecules in the developing brain forms a pathway that the axon can follow to get to the right place. But Catalano and Shatz suspected that the situation might be more complicated than that. The brain is too complicated, and the genome too small, for there to be a molecular address at every possible target location in the brain. They suspected that there might be another potential source of guidance cues for the growing axons: electrical activity itself. They decided to block electrical activity within the developing brain with a neurotoxin made by the Japanese puffer fish, and their suspicions were confirmed: in the absence of activity many axons fail to find their way to the correct address. Instead they become confused and wander into other regions they normally bypass. Dr. Susan Catalano, now at the California Institute of Technology, offers this analogy: "If the growing axon is like a car, then the highway pavement and traffic signals would be like the guidance molecules. Demonstrating that neural activity is critical for axon navigation is like adding a Global Positioning System into the mix; its a whole new level of information that the axon can potentially use to guide its way toward the appropriate target."

Catalano and Shatz studied axons that grow out from nerve cells located in a brain structure called the thalamus. During development these axons must navigate toward their correct target, the neocortex. The thalamus is a vital way station within the brain; all of the information coming from the sensory organs (such as the eyes, ears, and skin surface) passes through the thalamus on its way to the neocortex. The neocortex is the highly folded layer of neurons on the surface of the brain that is responsible for such functions as language processing; in other words, it is the brain structure that makes us uniquely human. The connections from the thalamus to the cortex are not randomly organized: specific groups of nerve cells within the thalamus (called nuclei) connect up to specific areas of the neocortex. This precise organization, or "map", is critical for proper brain function. In order to form this circuit correctly during development, groups of axons coming from specific places within the thalamus must navigate across the vast expanse of neocortex. They must bypass incorrect areas of the neocortex and choose just the right area to connect with, but without electrical activity, the axons become lost.

How might electrical activity produce this effect? While that is not currently known, clues can be found in studies of other regions in the brain. Previous work from Dr. Shatz's lab has shown that very early in development when the axons from the eye are still navigating toward their targets in the brain, waves of electrical activity sweep across the retina. This means that axons that are nearest neighbors are electrically active at the same time. Simultaneous activity could alter the molecular environment of the pathway through which the axons grow and allow cohorts of axons to keep together during navigation.

Ever since the pioneering work of Nobel laureates David Hubel and Torsten Wiesel, it has been known that the pattern of electrical activity carried by different sets of axons can influence the physical shape of the axons themselves. During the last phases of development, axons from the thalamus form many branches as they spread out through the neocortex to make their final sets of connections. These branches are literally shaped like the branches of a tree, and hence are called the "terminal arbor." Changes in the axon's pattern of electrical activity can change the shape of the tree that forms; less activity results in a shrunken, knarled axon tree. Surrounding axons with normal levels of activity form many more branches that grow into the shrunken tree's territory, just like their counterparts in nature that grow into the sunlit space created when a neighbor falls.

While the role of electrical activity in the final stages of thalamic axon branch formation had been well established, the possibility that the same process might be crucial in early development during axon navigation remained uninvestigated until now. The clinical implications of this are potentially alarming: drugs such as nicotine, which can affect electrical activity within the brain, have the potential to disrupt circuit formation in a developing infant's brain at very early stages, when the major circuits of the brain are being formed. The possibility that developing brains are vulnerable to disruption by activity-altering agents at such early times suggests important areas for future research.

Brain cells attuned to visual nearness and farness interact to allow judgments of size, research shows

PASADENA—Evolution has been benevolent to humans and other primates in providing us with eyes that can judge the size of nearby objects.

With a visual feature known as "size constancy," we can pretty accurately judge whether the furry thing walking across our field of view is the size of a mouse or the size of a lion, regardless of its distance and whether we recognize the object. Where survival of the species is concerned, the advantage of having size constancy is pretty obvious: it helps us identify dinner, but at the same time helps us stay off someone else's menu.

But the precise neurological nature of size constancy has never been well understood. If we are seeing our very first lion and the lion is walking away from us, then his image in our field of vision is getting smaller and smaller. Distance cues and stereoscopic vision are at play, but what is really happening in our brains? Is the third dimension added on at a late stage in visual processing, or are the images of lions at varying distances actually analyzed at the very first stage of visual perception?

New research from the California Institute of Technology shows that the latter is the case. Our brains need information for object and three-dimensional scaling, and this information is common to all visual cortical areas of the brain.

In the July 24 issue of Science, Caltech biology professor John Allman and his colleagues write that brain cells involved in vision tend to be apportioned to picking up farness or nearness. In working with rhesus monkeys trained to follow dots of varying size on a moving TV monitor, the researchers have found that the monkeys use their nearness and farness cells in tandem.

"The perception of depth is the product of the interaction of the two opposed tendencies, near and far," says Allman. "There are many systems in the body, and several in the visual system, which work by the precise counterbalancing of two opposed tendencies.

"For color perception, for example, you have opposition between black and white, red and green, and blue and yellow," he adds. "So our results show that depth perception is also a fundamental opposition."

Thus, the basic idea is that ability to judge the size of objects is embedded in the primary visual center as a code of opposed interaction of "nearness" and "farness" cells. Therefore, the neurons are pooled for depth perception; lab work with monkeys earning rewards for correct depth identification bears this out.

In addition to Allman, the authors are Jozsef Fiser of the University of Southern California; and Allan C. Dobbins and Richard M. Jeo of the Caltech Division of Biology.

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Robert Tindol
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Caltech Question of the Month: What is really happening when we have aftershocks after an earthquake?

Submitted by Gloria Hughes, Pasadena, California, and answered by Lucile M. Jones, Seismologist, U.S. Geological Survey/Visiting Research Associate, Caltech.

Earthquakes occur in clusters. In any cluster, the earthquake with the largest magnitude is called the mainshock, anything before it is a foreshock, and anything after it is an aftershock. A mainshock will be redefined as a foreshock if a subsequent event has a larger magnitude. Aftershock sequences follow predictable patterns as a group, although the individual earthquakes are random and unpredictable.

Aftershocks usually occur geographically near the mainshock. The stress on the mainshock's fault changes drastically during the mainshock and that fault produces most of the aftershocks. Sometimes the change in stress caused by the mainshock is great enough to trigger aftershocks on other, nearby faults, and for a very large mainshock sometimes even farther away. As a rule of thumb, we call earthquakes aftershocks if they are at a distance from the mainshock's fault no greater than the length of that fault. For example, the fault rupture length was 15 km (10 miles) in the 1994 Northridge (M6.7) earthquake, and 430 km (270 miles) in the 1906 San Francisco (M7.8) earthquake.

An earthquake large enough to cause damage will probably be followed by several felt aftershocks within the first hour. The rate of aftershocks dies off quickly: the decrease is proportional to the inverse of time since the mainshock. This means the second day has about 1/2 the number of aftershocks of the first day and the tenth has about 1/10 the number of the first day. These patterns describe only the mass behavior of aftershocks; the actual times, numbers and locations of the aftershocks are random. We call an earthquake an aftershock as long as the rate at which earthquakes occur in that region is greater than the rate before the mainshock. How long this lasts depends on the size of the mainshock (bigger earthquakes have more aftershocks) and how active the region was before the mainshock (if the region was seismically quiet before the mainshock, the aftershocks continue above the previous rate for a longer time). It can be weeks or decades.

Bigger earthquakes have more and larger aftershocks. The bigger the mainshock, the bigger the largest aftershock will be, on average. The difference in magnitude between the mainshock and largest aftershock ranges from 0.1 to 3 or more, but averages 1.2 (a M5.5 aftershock to a M6.7 mainshock, for example). There are more small aftershocks than large ones. Aftershocks of all magnitudes die off at the same rate, but because the large aftershocks are already less frequent, the decay can be noticed more quickly. We have large aftershocks months or even years after the mainshock.

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Robert Tindol
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Parent that takes care of offspring tends to outlive the other parent, study shows

PASADENA--The parent who stays home to take care of the kids may be getting a good deal healthwise. New primate research from the California Institute of Technology shows that a primary caregiver tends to live longer than the other parent.

In a statistical study of 10 primate species, including humans, apes, and various Old and New World monkeys, the Caltech researchers show that the parent that cares for the offspring is significantly longer lived than the mate, regardless of gender. Titi monkey males of South America, for example, which take care of the baby after the mother has given birth, outlive their mates by 20 percent.

"The numbers show that if there is a difference in role, the sex doing the bulk of the care is likely to survive longer," says John Allman, a Caltech biology professor who is lead author of the study.

"This follows from the fact that it takes a lot of energy to raise a big-brained offspring like a human or an ape or a monkey," he adds. "The sex not caring for the infants will not be as crucial for the survival of the species."

The size of the brain is the key, Allman says. Species with big brains mature slowly and have only one baby at time, and these babies depend on their parents for a long time.

"Big brains are very expensive," Allman says. "They are costly in terms of time, energy and anatomical complexity. This reduces the reproductive potential of the parents because extra-special care must be provided to insure that this reduced number survive to reproductive age."

In an article appearing in the current issue of the journal Proceedings of the National Academy of Science, Allman and his coauthors outline their data from the 10 species of primates. To determine the lifespans of males and females that have borne offspring, the researchers analyzed the data from zoo populations, field studies, laboratory research, and human historical and demographic documents.

The researchers were especially interested in reviewing field studies as well as zoo data to ensure that artificial effects were not skewing the data. However there is also data from natural populations of primates that supports this hypothesis.

"Female gorillas, orangutans, and chimpanzees have a proportionally larger survival advantage than human females," Allman says. But the advantage of female gorillas is not so pronounced, and this could very well have to do with the fact that male gorillas play with their offspring and take on certain other nurturing duties.

In fact, the Caltech hypothesis is not only that the caregiver who takes care of the offspring tends to outlive his/her mate, but also that the effect disappears when parents share in caregiving more or less equally.

Perhaps for this reason human males and females also have lifespans that are fairly similar in length. The current figure is about 8 percent, but does not take into consideration the fact that medical care has significantly reduced the death rate from childbirth. Swedish demographic data from the late 1700s, by contrast, shows that females lived about 5 percent longer than their mates in those days, when childbirth was a leading cause of death in women.

The demographic data of the 10 primate species shows remarkable conformity to the hypothesis. In all of the primates studied in which females are the primary caregivers-spider monkeys, gibbons, orangutans and gorillas-females live significantly longer than males. Human female live longer than males, but the difference is smaller than in these primates and the male role is larger although less than the female role in childrearing.

In the two primates studied in which females and males share caregiving more or less equally, there is no difference between the survival rates of the sexes. In the two primate studies in which males have a larger role in caring for offspring, the owl monkey and the titi monkey, males live longer than females. The effect is significant for the owl monkeys, but not for titi monkeys because of the smaller sample available for these animals.

Allman acknowledges that the results are somewhat counterintuitive: many people think that child raising is quite stressful, and if anything should shorten the life of a harried parent. But just the opposite is true.

"There's probably not one single reason that the caregiver outlives the other parent," he says. "Risk taking in males and estrogen in females are probably factors, and there may even be a beneficial hormonal or chemical change that occurs through extending care to another.

"There's evidence that greater longevity can also coincide with the taking care of an elderly parent or even a pet," he concludes.

"So it could be that taking care of others is just good for you."

The other authors of the paper are Andrea Hasenstaub, a junior majoring in mathematics and engineering at Caltech; Aaron Rosin, a former Caltech student who graduated with a degree in biology; and Roshan Kumar, another Caltech graduate, who is now a researcher at the Scripps Research Institute in La Jolla, California.

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Robert Tindol
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