Saturday, October 17, 2009


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The intense exchanges that human mothers share with their newborn infants may have some pretty deep roots, suggests a study of rhesus macaques reported online on October 8th in Current Biology, a Cell Press publication.

The new findings show that mother macaques and their infants have interactions in the first month of life that the researchers say look a lot like what humans tend to do.

"What does a mother or father do when looking at their own baby?" asked Pier Francesco Ferrari of the Università di Parma in Italy. "They smile at them and exaggerate their gestures, modify their voice pitch—the so-called "motherese"—and kiss them. What we found in mother macaques is very similar: they exaggerate their gestures, "kiss" their baby, and have sustained mutual gaze."

In humans, those communicative interactions go both ways, research in the last three decades has shown. Newborns are sensitive to their mother's expressions, movements, and voice, and they also mutually engage their mothers and are capable of emotional exchange.

"For years, these capacities were considered to be basically unique to humans," the researchers said, "although perhaps shared to some extent with chimpanzees." The new findings extend those social skills to macaques, suggesting that the infant monkeys may "have a rich internal world" that we are only now beginning to see.

The researchers closely observed 14 mother-infant pairs for the first two months of the infants' lives. They found that mother macaques and their babies spent more time gazing at each other than at other monkeys. Mothers also more often smacked their lips at their infants, a gesture that the infants often imitated back to their mothers.

The researchers also saw mothers holding their infant and actively searching for the infant's gaze, sometimes holding the infant's head and gently pulling it towards her face. In other instances, when infants were physically separated from their mothers, the parent moved her face very close to that of the infant, sometimes lowering her head and bouncing it in front of the youngster. Interestingly, those exchanges virtually disappeared when infants turned about one month old.

Why so soon, you might ask?

"It's quite puzzling," Ferrari said, "but we should consider that macaque development is much faster that of humans. Motor competences of a two-week-old macaque could be compared to an eight- to twelve-month-old human infant. Thus, independence from the mother occurs very early… what happens next in the first and second month of life is that infants become more interested in interacting with their same-age peers."

The findings offer new insight into the origins of such mother-infant behavior. "Our results demonstrate that humans are not unique in showing emotional communication between mother and infant," the researchers wrote. "Instead, we can trace the evolutionary foundation of those behaviors, which are considered crucial for the establishment of social exchange with others, to macaques. Mutual gaze, neonatal imitation, infant gestures, and exaggerated facial gesturing by mothers are distinctive signs in macaques, as well as in humans, of interpersonal communication and perhaps even a mutual appreciation of others' intentions and emotions."

Cell Press


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Some stars are lonely behemoths, with no surrounding planets or asteroids, while others sport a skirt of attendant planetary bodies. New research published in The Astrophysical Journal Letters explains why the composition of the stars often indicates whether their light shines into deep space, or whether a small fraction shines onto orbiting planets.

When a star forms, collapsing from a dense cloud into a luminous ball, it and the disk of dust and gas orbiting it reflect the composition of that original cloud and the elements within it. While some clouds are poor in heavier elements, many have a wealth of these elements. These are the dirty stars that are good solar system hosts.

"When you observe stars, the ones with more heavy elements have more planets," says co-author Mordecai-Mark Mac Low, Curator of Astrophysics at the American Museum of Natural History. "In other words, what's in the disk reflects what's in the star. This is a common sense result." Observation of distant solar systems shows that exoplanets, or planets that orbit stars other than the Sun, are much more abundant around stars that have a greater abundance of elements heavier than helium, like iron and oxygen. These elements are the ones that can turn into rocks or ice.

The new simulations by Mac Low and his colleagues Anders Johansen (Leiden Observatory in the Netherlands) and Andrew Youdin (Canadian Institute of Theoretical Astrophysics at the University of Toronto) compute just how planets and other bodies form as pebbles clump into mini-planets referred to as planetesimals. Their current work hinges on their previously published research (in Nature in 2007) that explains why rocks orbiting a star within the more slowly-revolving gas disk are not quickly dragged into the star itself because of the headwinds they feel. Like bicyclists drafting behind the leader in the Tour de France, the rocks draft behind each other, so that in orbits with more rocks, they feel less drag and drift towards the star more slowly. Rocks orbiting further out drift into those orbits, until there are so many that gravity can form them into mini-planets. This concentration of orbiting rocks in a gas disk is called a "streaming instability" and is the theoretical work of co-author Youdin. "It's a run-away process. When a small group of rocks distorts the flow of gas, many others rush to line up like lazy cyclists and matter accumulates very quickly," he says.

The team was able to build this mechanism—drag leading to clumping—into a three-dimensional simulation of gas and solid rocks orbiting a star. Their results show that when pebbles, made of heavy elements, constitute less than one percent of the gas mass, clumping is weak. But if the fraction of pebbles is increased slightly, the clumping increases dramatically and quickly results in the accretion of sufficient material to make larger-scale planetesimals. These mini-planets work as planetary building blocks, merging over millions of years to form planets. In short, clumping of pebbles, when the fraction of solids in the gas is high enough, is the recipe for mini-planet formation, a crucial intermediate step in forming planets.

"There is an extremely steep transition from not being able to make planets at all to easily making planets, by increasing the abundance of heavy elements just a little," says lead author Johansen. "The probability of having planets almost explodes."

Youdin adds that "There's an inherent advantage in being born rich, in terms of solid rocks. But less advantaged systems, like our own Solar System, can still make planets if they work to marshal their resources and hang onto their solids as the gas evaporates away. So the Sun is middle-class, rather than rich." The Sun's abundance of heavy elements suggests its protoplanetary disk (the disk from which the Solar System formed) had close to the critical ratio of pebbles to gas; if the abundance of heavy elements had been slightly less, planetesimals and planets would have been far less likely to form, and we would not be here to study the question.

American Museum of Natural History


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A new study suggests that high mortality rates in small-bodied people, commonly known as pygmies, may be part of the reason for their small stature. The study, by Jay Stock and Andrea Migliano, both of the University of Cambridge, helps unravel the mystery of how small-bodied people got that way.

The article appears in the October issue of Current Anthropology.

Adult males in small-bodied populations found in Africa, Asia and Australia are less than four feet, 11 inches (150 centimeters) tall, which is about one foot shorter than the average adult male in the U.S. Why people in these populations are so small remains a mystery, but several hypotheses have been proposed.

Some scientists think that small bodies provide an evolutionary advantage under certain circumstances. For example, a smaller body needs less food—a good thing in places where food supplies are inconsistent. Small bodies also may provide an advantage in getting around in thickly forested environments.

Recently, however, a new hypothesis has come to the fore suggesting that reproductive consequences of high mortality rates explain small body size.

If death comes at an early age, then natural selection would favor those who are able to reproduce at an early age. But early sexual maturity comes with a cost. When the body matures early, it diverts resources to reproduction that otherwise would have gone to growth. So small body size could be essentially a side effect of early sexual maturity.

Stock's and Migliano's study provides the first long-term evidence for the mortality hypothesis.

The two researchers looked at over 100 years of data on three small-bodied populations from the Andaman Islands in the Bay of Bengal, south of Burma. When the British established colonies on the islands in the 1850s, these indigenous tribes had very different reactions to their new neighbors. Those reactions would have vast implications for the tribes' mortality rates.

Two tribes, the Onge and the Jarawa, resisted relations with the British, and retreated deep into the forest to avoid them. But the largest group of tribes, the Great Andamanese, befriended the British, some even living in homes built and supervised by colonists. In doing so, the tribesmen were unwittingly exposed to infectious diseases for which they had no defenses. As a result, the Great Andamanese experienced a sharp increase in mortality due to influenza, tuberculosis, measles and syphilis. By 1900 their numbers had dwindled to 600, from 6000 just 50 years before. By the 1960s, only 19 individuals remained.

Using historical records compiled by British researchers at the time, Stock and Migliano found that during the peak period of increased mortality, the Great Andamanese got smaller in stature. From 1879 to 1927, the height of the adult males who were measured decreased at a rate of 4.7 centimeters per 100 years.

Meanwhile, the Onge and the Jarawa, who for the most part isolated themselves from colonists and did not have dramatic increases in mortality, saw no drop in stature. The Jarawa, which have had the most stable mortality rate, remain the tallest of the three tribes today.

The relationships of the tribes with colonists "led to differences in mortality among these tribes, which appears to have been a fundamental determinant of variation in body size," the authors conclude.

This is first time that a link between mortality and body size has been shown using long-term data, the authors say. And it bolsters the idea that the reproductive trade-off associated with a short life could play a role in the evolution of human body size.

University of Chicago




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