ScienceWeek

ScienceWeek

070909

1. Planetary Science: On Saturn's Spin

2. Behavior: The Social Origin of Mind

3. Psychology: Nonhuman Primates Demonstrate Humanlike Reasoning

4. Solar System: Lethal Billiards

5. Zoology: On the Moray Eel

6. Genome Biology: A Genome Within a Genome

1.

Science 7 September 2007: Vol. 317. no. 5843, pp. 1330 - 1331 DOI: 10.1126/science.1147793

Planetary Science: On Saturn's Spin

Planetary Science: The Case of Saturn's Spin

Morris Podolak

Planet formation theorists, like detectives, have to gather clues and put them together to explain an event that happened in the past. Sometimes the clue is so subtle that it requires a chain of reasoning to see how it contributes to unraveling the mystery. As Sherlock Holmes said, "It has long been an axiom of mine that the little things are infinitely the most important" (1). The work reported by Anderson and Schubert on page 1384 of this issue is a case in point (2). These authors have found that Saturn rotates somewhat more quickly than had been thought. This apparently minor result may have profound implications for our understanding of giant planet origins.

The early solar system began as a gas disk around the Sun, and two scenarios for formation of the giant planets seem possible. One scenario, the core accretion hypothesis (3), argues that the solids in the outer solar system, in the form of planetesimals, were gradually accreted into a planetary core on the order of 15 Earth masses. Such a massive core could then attract a large amount of hydrogen and helium from the surrounding disk to form a gas giant planet. Although some of the core might be mixed back into the accreting gas as it collapsed onto the planet, we would still expect Jupiter and Saturn to have cores on the order of 10 Earth masses.

The other scenario, the disk instability hypothesis (4), argues that the gas disk itself was unstable and that density fluctuations became large enough that some portion of the disk collapsed under its own gravity. This collapsing clump would eventually evolve into a gas giant planet. Some of the solids that were in the gas would eventually settle into a core (5), but that core is expected to be small, on the order of a few Earth masses.

Although most theorists favor the core accretion scenario, both hypotheses have strengths and weaknesses. One possible way to resolve the issue is to investigate the structure of the two gas giants in our solar system. Are the cores of Jupiter and Saturn closer to 15 Earth masses or to zero? To answer this question, we have to model the internal structure and composition of these planets.

The standard approach is to assume a reasonable composition with some free parameters. Typically, one assumes that there is a heavy-element core of undetermined mass (one free parameter), surrounded by an envelope of hydrogen, helium, and some fraction of heavier material mixed in (a second free parameter). The details are not very sensitive to the exact choice of additional heavy material, but this too adds freedom in fixing the composition. The pressure inside the body can be computed by assuming that at each depth the pressure exactly balances the weight of the overlying layers. There is good reason to believe that the envelopes in Jupiter and Saturn are convecting, so that the temperature gradient follows an adiabat (i.e., a sequence of changes in pressure and temperature but with no heat exchanged). Thus, if the temperature at, say, the 1-bar pressure level is known, the temperature throughout the envelope can be determined. All that remains is to use the best physics available to determine the density of the material, given its pressure and temperature. In this way, the density can be found as a function of depth.

One of the free parameters can be fixed by forcing the mean density of the planet to match the observed value, but the others are harder to tie down. Because Jupiter and Saturn rotate rapidly, they have a pronounced oblateness. This departure from a spherical shape means that the gravitational potential of these planets differs slightly from the usual r-1 law (where r is radius). The strength of the potential at a fixed distance from the center will vary as the angle from the spin axis changes. The dependence on this angle is expressed by a series of coefficients called gravitational moments. These moments can be measured by following the motion of a satellite in the planetary gravitational field. They can also be computed from the internal density distribution and the rotation rate of the body.

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2.

Science 7 September 2007: Vol. 317. no. 5843, pp. 1326 - 1327 DOI: 10.1126/science.1144284

Books

Behavior: The Social Origin of Mind

Alison Jolly

Baboon Metaphysics: The Evolution of a Social Mind

by Dorothy L. Cheney and Robert M. Seyfarth

University of Chicago Press, Chicago, 2007. 358 pp. $27.50, £16. ISBN 9780226102436.

He who understands baboon would do more towards metaphysics than Locke. -- Charles Darwin, Notebook M (1838).

Imagine as a life's work toting loudspeaker, tapes, batteries, and video camera around the Okavango Delta, occasionally being treed by lions or warned off by elephants. You skulk into long grass to conceal the speaker, then wait and wait until the supposed author of the playback call is safely out of sight and the intended hearer facing at least 90° away. It may take you a whole year to complete one series of experiments--all with the goal of confusing a baboon.

Baboon Metaphysics is the distillation of a big chunk of academic lives: the wife-and-husband team of Dorothy Cheney and Robert Seyfarth plus a flock of their students and friends. It is exactly what such a book should be--full of imaginative experiments, meticulous scholarship, limpid literary style, and above all, truly important questions. Baboon confusion turns out to be one of the strongest tools available for illuminating a primate's metaphysics as well as our own. What are the components of intelligence? How does intelligence evolve to meet the challenge of life in a social group? Is behavioral foresight possible without real empathy and without a "theory of mind"? How can factual information be communicated without language? Can we impute consciousness to another species? Philosophers argue in the abstract; Cheney and Seyfarth offer data.

The "social intelligence hypothesis" proposes that the major influence in the evolution of primate intelligence has been the challenge of life in a social group. Of course an environment of fruit trees, floods, and lions has its own challenges, but dealing with the Primates may not be as unique as we would like to believe: dolphins, dogs, and pinyon jays have many of the capacities of a baboon. It becomes clear, though, that human minds are fundamentally those of a social primate. Baboons offer insight into how we arrived at our own kind of mind.

First and simplest, every baboon knows the voices of everyone in the 80-strong troop. Each call is tagged with the individual's identity. If the playback calls sound like an ordinary social interaction, the hearer is little interested, but if calls violate expectations she will look toward the speaker for much longer. If she assumes that a call has something to do with herself, she changes her behavior toward the supposed caller. Long suites of logically constructed experiments make it clear that baboons categorize others by both rank and matriline: these are rule-governed classes on separate axes. Baboons foresee others' behavior with great sophistication. However, they seem to lack empathy toward others' emotions or awareness of others' knowledge. Cheney and Seyfarth write: "Baboons' theory of mind might best be described as a vague intuition about other animals' intentions.… There are hints that learned contingencies alone cannot explain all aspects of baboon behavior, but we cannot yet conclude that baboons regard other baboons--even tacitly--as intentional beings with goals, motives, likes, and dislikes."

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3.

Science 7 September 2007: Vol. 317. no. 5843, p. 1308 DOI: 10.1126/science.317.5843.1308

Psychology: Nonhuman Primates Demonstrate Humanlike Reasoning

Elizabeth Pennisi

Monkeys may see, hear, and speak no evil, but they do seem to understand a person's intentions. We constantly judge the actions of those around us, assessing what others are trying to do, and why, to decide the best course of action for ourselves. Experiments reported on page 1402 now suggest that this supposedly unique human attribute is shared by chimps and at least two monkey species. The finding suggests that this skill and the enabling neuronal circuitry date back at least 40 million years, predating the evolution of the unique social system or language of humans. It promises to fuel the debate about the cognitive divide between humans and our primate cousins.

"It's stunning evidence for [nonhuman primates'] understanding goal-directed behavior," says Melissa Gerald, a primatologist at the University of Puerto Rico in San Juan who runs a macaque research center on a nearby island, Cayo Santiago. However, she and others are not completely convinced, citing apparent flaws in the study design and analysis or concerns about anthropomorphic interpretations of the findings.

In 2002, György Gergely of the Hungarian Academy of Sciences' Institute for Psychology in Budapest shocked his fellow psychologists by asserting that 14-month-old infants could figure out what another person was trying to do and whether their behavior made sense. He assessed this ability by looking at mimicry behavior. In this experiment, an infant watched someone turn on a light with the touch of her head, not her hands. When the tester's hands were full, the infants did not mimic the head movement and instead used their hands to turn the light on. They seemed to realize that the tester had to use her head because she couldn't use her hands, although hands would work better for the task. But when the tester used her head when her hands were empty, the infants followed suit, apparently concluding that there must be a good reason to use the head in this situation. The infants not only recognized the tester's goal but also thought through the best way to achieve that goal themselves. "The infants are extremely sensitive to how efficient an action is," says Gergely.

At the time, "I doubt if anyone would have put money on adult chimpanzees being able to do this, let alone monkeys," says Richard Byrne, a psychologist at the University of St. Andrews in Fife, U.K. But Justin Wood, a graduate student at Harvard University, was willing to take the bet.

Wood looked at whether cotton-top tamarins could tell a goal-directed action from a random one. In one test, he either grasped one of two food containers or flopped his hand on top of one, as if by accident, while a monkey looked on. In another, he "pointed" to the container by putting his elbow on it, sometimes while that same hand was free, and sometimes while holding an object with both hands. Wood counted how often the monkeys inspected the designated container.

This protocol was designed to evaluate whether these New World monkeys could tell a rational action (use the elbow because the hands were busy) from one that seemed less intentional. "These are very clever ways of getting at questions that are very basic to our understanding of intentionality," says Gerald.

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4.

Nature 449, 30-31 (6 September 2007) | doi:10.1038/449030a; Published online 5 September 2007

Solar System: Lethal Billiards

Philippe Claeys & Steven Goderis

A huge collision in the asteroid belt 160 million years ago sent fragments bagatelling around the inner Solar System. One piece might have caused the mass extinction that wiped out the dinosaurs 65 million years ago.

What are the chances of the sky falling on our heads? If it's an asteroid hitting Earth that we're talking about, we can't be too sure. Not only do estimates of the current terrestrial meteorite impact rate differ by a factor of five to ten, depending on the approximations used1, but we don't know for certain whether that rate has remained constant or has varied throughout geological time.

A current theory proposes that the impact rate has increased during the past 100 million years or so. On page 48 of this issue, Bottke et al.2 present an intriguing explanation for why this might be, invoking errant fragments from a powerful ancient collision in the asteroid belt between Mars and Jupiter.

Clusters of impact craters and layers of material ejected in meteorite impacts, as well as higher levels of extraterrestrial material in some sedimentary rocks, seem to indicate that, during several glacial periods, the Earth–Moon system has suffered abnormally high rates of bombardment. The late Miocene epoch around 8 million years ago, for example, was marked by an increased flux of interplanetary dust particles between 1 microm and 1 mm across, which might have been produced by a collision within the asteroid belt3. An asteroid or comet shower has similarly been put forward to explain the higher dust-particle flux in the late Eocene around 35 million years ago, an event that seems to be coupled with an unusually high concentration of impact craters4, 5. These include the two largest craters in recent geological history, Popigai in Siberia (100 km in diameter) and Chesapeake Bay off the Maryland coast (around 85 km in diameter).

And we can go even farther back in recording periods of heavy bombardment. The abundant micrometeorites in the 480-million-year-old Ordovician limestones of southern Sweden most probably reached Earth after a significant disruption had occurred in the asteroid belt6. Several impact craters also seem to cluster around this age, although here the geological record is rather poor. Farther back still, recognized ejecta layers are concentrated in two time windows between 2.65 billion and 2.5 billion years ago and 3.47 billion and 3.24 billion years ago7. Finally, the most dramatic series of events is undoubtedly the Late Heavy Bombardment of 3.8 billion years ago, the occurrence of which is inferred from the lunar cratering record8. Although its traces have been erased by geological activity on Earth, extrapolation of the lunar data indicates9 the formation of up to 22,000 terrestrial craters with a diameter of more than 20 km. This catastrophic bombardment probably resulted from colliding asteroids disturbed by changes in the orbits of the giant gas planets10.

Bottke et al.2 have discovered the remnants of another huge collision hidden in the inner region of the main asteroid belt. These comprise the Baptistina asteroid family (BAF), a class of variously sized objects of similar composition and orbital geometry, typified by the 40-km-diameter asteroid known as (298) Baptistina. The authors use a computer simulation to track the orbits of these fragments back to the moment of their formation, and find that the collision must have taken place about 160 million years ago. The best fit to the data is given by an object of 60-km diameter colliding almost vertically with a 170-km-diameter body. This collision, at a velocity of 3 km s-1, generated more than 1,000 large bodies greater than 1 km in diameter (Fig. 1).

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5.

Nature 449, 33-34 (6 September 2007) | doi:10.1038/449033b; Published online 5 September 2007

Zoology: On the Moray Eel

Mark W. Westneat

The toothy visage of a moray eel is a fearsome sight. The discovery that morays can thrust a second pair of jaws out from their throat to wolf down prey whole increases their predatory reputation still further.

Many animals swallow their prey whole. Snakes come to mind, of course; but amphibians, lizards, birds and thousands of fish species can also attack, and gulp down, prey nearly as large as their head — imagine swallowing a peanut-butter sandwich or a salmon whole and you get some idea of how remarkable a feat that is. The methods by which animals do this vary from the suction feeding of fishes1 to the 'unhinged-jaw' mechanism of snakes2 and the inertial feeding of lizards and birds3. (For inertial feeding, picture a pelican thrusting its head upward to use gravity to choke down a large fish.)

These gulping mechanisms, along with most other vertebrate feeding habits that involve killing, dismembering and/or swallowing other animals, have generally been thoroughly investigated. On page 79 of this issue, however, Mehta and Wainwright4 document yet another tactic. They show how moray eels — elongated snake-like fishes that inhabit coral reefs and rocky intertidal habitats worldwide — drag a large item of prey into their gullet by using a second set of grasping jaws that they thrust forward from deep in their throat.

Accessory jaws positioned in the throat are known as pharyngeal jaws, and are quite common among fishes. In many species, some of the bones that support the gills, called the branchial arches, have been modified into feeding tools that can filter prey from the water, crush and grind hard food such as snails or clams, and even grasp and tear softer prey before it is swallowed5.

Perhaps the most widely known pharyngeal jaws are found in freshwater cichlids and marine wrasses and parrotfishes. These fish families possess hard, toothy pharyngeal plates that are thought to have allowed a wide range of feeding habits to develop and promoted their evolutionary diversification6. But Mehta and Wainwright reveal4 an additional class of pharyngeal-jaw mechanism. They aptly term this the 'raptorial pharyngeal jaw', for its ability to reach far forward from its resting position in the pharynx, and grab the prey to transport it back towards the stomach.

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6.

Nature 449, 6 (6 September 2007) | doi:10.1038/449006b; Published online 5 September 2007

Genome Biology: A Genome Within a Genome

Ewen Callaway

Bacterial parasite's DNA discovered in fruitfly host.

Another team of genome researchers at the J. Craig Venter Institute in Rockville, Maryland, which has been investigating the DNA of a rather less salubrious organism, this week reports a surprise discovery: the DNA of fruitfly Drosophila ananassae contains the entire genome of a parasitic bacterium of the Wolbachia genus. Smaller parts of the parasite's genetic material also turned up in worms and wasps.

Bacteria commonly swap DNA with each other. But transfer of bacterial genes into animals was thought to be rare. The new work, published in Science (J. C. Dunning Hotopp et al. Science doi:10.1126/science.1142490; 2007), suggests that gene flow from bacteria to animal hosts happens on a larger scale and more commonly than suspected. And it hints that the bacterial genome may have provided some sort of evolutionary advantage to its host. "You're talking about a significant portion of [the fruitfly] DNA that is now from Wolbachia," says Julie Dunning Hotopp, who led the study. "There has to be some sort of selection to carry around that much extra DNA."

But Dunning Hotopp's former colleague Jonathan Eisen of the University of California, Davis, contests this. "One cannot conclude that some DNA is advantageous simply because it is there," he says.

Up to 75% of insect species are plagued by Wolbachia, which lives inside testes and ovaries and passes from one female generation to another through infected ova. To ensure its spread, Wolbachia can skew insect birth ratios towards females and even prevent infected males from successfully mating with disease-free females. The bacterium's close association with egg cells means there's ample opportunity for bacterial DNA to get permanently sewn into a host's nuclear genome, says Dunning Hotopp.

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