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ration. Or, a father’s absence early in lifemay trigger doubts in girls about male reli-ability that hasten sexual activity and repro-duction, as well as promote a preferencefor brief relationships. —B. BOWER

Split Ends Cancers follow shrinkageof chromosomes’ tips

Molecular caps that normally protect theends of chromosomes shrink in many cellsthat later turn cancerous, according to anew study in people.

Just as shoelaces that lose their plastictips unravel, so may chromosomes with bro-ken tips, or telomeres, be more prone tomutations that cause cells to become can-cerous, says Alan K. Meeker of Johns Hop-kins Medical Institutions in Baltimore. Hisnew findings, which are based on studies ina variety of human tissues, support thatpopular hypothesis.

Past studies had confirmed that telom-eres are abnormally short in human can-cer cells. The phenomenon could be aproduct of cancer cells’ rapid replication,

because telomeres commonly shrinkslightly with each round of cell division.However, studies in mice have suggestedthat short telomeres lead to cancer, andnot vice versa. Six years ago, scientistsfound that mice that lack an enzymeinvolved in maintaining telomere lengthare predisposed to develop certain can-cers (SN: 10/11/97, p. 228).

Meeker and his colleagues reportedlast year that, in people, telomere short-ening usually precedes cancer of theprostate and pancreas and may thereforecontribute to the development of the dis-ease in those tissues.

To see whether telomere shortening alsopresaged other human cancers, Meeker andhis team examined dozens of tissue sam-ples from precancerous lesions that sur-geons had removed from people. Samplescame from the bladder, breast, colon, esoph-agus, mouth, and cervix. All of these areorgans that, like the prostate and pancreas,are formed from so-called epithelial tissue.

Using a technique that causes telomereDNA to fluoresce under a microscope, theresearchers scrutinized tissue samples inwhich some cells had a precancerousappearance.

Most of these samples contained chro-mosomes with abnormally short telom-eres, Meeker reported in Washington, D.C.,at the annual meeting of the AmericanAssociation for Cancer Research last week.Eighteen of 23 breast lesions examined and45 of 46 lesions from other parts of the bodyhad some cells with telomeres so short thatthey weren’t visible in microscope images.

Those images nevertheless showed normaltelomeres in neighboring, apparentlyhealthy cells, Meeker says.

Telomeres in lesions were as short asthose commonly seen in full-blown tumors.This similarity indicates that telomereshortening precedes the development ofcancer rather than being a product of it,says Angelo DeMarzo, who worked withMeeker on the new research.

The findings appear to confirm that theerosion of telomeres helps drive thegenetic instability required for cells tobecome malignant cancers, says geneti-cist Ronald A. DePinho of Harvard Med-ical School in Boston.

The earliest clinical applications of thefindings are likely to be diagnostic, DePinhosays. Screening for telomere shrinkageamong people at risk for epithelial-tissuecancers might identify tissues in “extremelyearly stages of malignancy,” he says.

In the future, preventive treatmentsmight maintain telomere length andthus mitigate chromosomal changes inpeople at high risk of cancer, saysDeMarzo. Telomere length could also beused to measure the effectiveness of cer-tain other treatments designed to pre-vent cancer, he says. —B. HARDER

Protective Blanket Atmosphere blocks manysmall stony asteroids

Although Earth and the moon inhabit thesame cosmic neighborhood, our planet hasfar fewer scars from extraterrestrialimpacts because incoming objects burnup in its atmosphere. A new computermodel suggests that Earth’s thin layer of airis an even better shield than previouslythought.

Scientists have identified fewer than 200impact craters on Earth (SN: 6/15/02, p.378), and only for a few do they suspect thetype of object that gouged the hole. Of those,most were blasted out by asteroids, whichcome in rocky and iron-rich varieties, saysPhilip Bland, a planetary scientist at Impe-rial College in London. From observationsof asteroids in space, and analysis of thecomposition of their meteorite remnantson Earth, scientists believe that about 5 per-cent of the asteroids that enter the upperatmosphere are of the iron-rich type.

That proportion roughly matches thedata from Earth’s craters larger than 10kilometers in diameter and of known ori-gin, says Bland. However, 16 of the 17 cratersless than 1.5 km across and with knownimpactor types apparently were blasted byiron-rich bodies. According to Bland and

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SCIENCENEWSThis Week

DARK AND DANGEROUS Normal telomeres glow pink under fluorescent imaging. Thathealthy glow disappears in cells with extremely short telomeres, such as the breast cells(blue) that show signs of becoming malignant.

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Natalia A. Artemieva of the Russian Acad-emy of Sciences in Moscow, the disparitystems from the filtering effect of Earth’satmosphere.

The researchers’ computer simulations,described in the July 17 Nature, calculatethe aerodynamic forces on an object pass-ing through Earth’s atmosphere. Besidesaffecting the body’s motion, these forcespummel it, break it apart, and burn awayits surface, says Bland. Because a mass ofrock doesn’t withstand such pounding aswell as a lump of iron does, most smallrocky asteroids don’t survive their tripthrough the atmosphere. The team’s modelsuggests that even large rocky bodiespartly disintegrate but that remainingchunks can still smash a big crater.

On average, the model suggests, aster-oid fragments at least 3 meters across andcapable of blasting a crater 100 m widewill strike Earth once every 300 years orso. A piece of asteroid about 220 macross—one large enough to cause a dan-gerous tsunami if it were to strike theocean—might smack Earth only onceevery 170,000 years. That’s a rate aboutone-fiftieth of that estimated by other sci-entists using other models. However,Bland notes that major impacts mightoccur even less frequently because stonyasteroids may not be as strong or as denseas he and Artemieva assumed.

The type of aerodynamic simulationthe researchers used is far better thansimulations used in previous analyses,says Douglas O. ReVelle, an atmosphericscientist at Los Alamos (N.M.) NationalLaboratory. However, he notes, observa-tions of meteors disintegrating high inthe atmosphere suggest that the objects’strength and density vary widely, soBland and Artemieva’s results may still beno more than ballpark estimates of howfrequently objects of various sizes punchthrough the atmosphere.

Nevertheless, the team’s report is “impor-tant work” that suggests that Earth isn’t asvulnerable to extraterrestrial impacts assome scientists had thought, says WilliamK. Hartmann of the Planetary Science Insti-tute in Tucson. Also, he notes, similar sim-ulations should shed light on the crateringrates on other celestial objects with atmos-pheres, such as Venus, Mars, and Saturn’smoon Titan. —S. PERKINS

City SongBirds sing higher nearurban traffic

City songbirds that stake out territories nearloud traffic tend to pitch their songs athigher frequencies than do birds in qui-eter neighborhoods, Dutch researchers

have found.Recordings of a common European

species, the great tit (Parus major),showed a higher minimum frequency inthe noisier parts of Leiden, says HansSlabbekoorn of Leiden University. In theloudest places, engine roars overlappedthe lower frequencies of the tits’ songs,Slabbekoorn and his Leiden colleagueMargriet Peet report in the July 17 Nature.

“We don’t know enough about the effectsof noise pollution,” says Slabbekoorn. “Thishints at a difference between birds thatadapt to the city and those that can’t.”

Great tits, relatives of North America’schickadees, sing several songs, includingone that Dutch bird-watchers compare tothe “tee-tah, tee-tah” of a bicycle pump.

A classic study in 1979 demonstratedthat great tits living in dense woods tendtoward songs simpler than the more orna-mented vocalizations of birds living inareas with more open ground. Research-ers have also shown that birds such asnightingales sing louder in a laboratorywhen there’s background noise.

For their study of urban birds,Slabbekoorn and Peet turned to great tits,which abound in European cities. “I’verecorded tits under the Eiffel Tower; I’verecorded tits in Buckingham Palace,” saysSlabbekoorn.

The data the team has analyzed so farcome from recordings of 32 male tits invarious parts of Leiden. The researchersalso took a series of recordings and back-

ground-sound measurements in eachlocation before, during, and after rushhour.

The average minimum frequency of themales’ songs, ranging from 2.82 to 3.77kilohertz, was lower in quieter neighbor-hoods than in noisier ones. Urban noise,mostly from engines in cars, trucks, boats,and modern conveniences such as leafblowers, encroached on birds’ lower fre-quencies in the loud neighborhoods, theresearchers say.

Slabbekoorn points out that young greattits learn their songs in large part whenthey establish a territory and have songduels with neighboring males. He specu-lates that in noisy spots, the higher-pitchedsongs may be more effective in deterringrivals, and it’s these songs that the youngmales are more likely to copy.

André Dhondt of Cornell University, whohas spent 25 years studying great tits, sayshe’s not surprised by the finding, consider-ing the previous work on the effects of nat-ural background noise. His own workshowed that male great tits with longer,more precise songs tend to live longer andfather more offspring than less-vocal malesdo. He emphasizes that males have a strongincentive to make themselves heard, nomatter what the environment.

Haven Wiley from the University ofNorth Carolina at Chapel Hill, whoseresearch team has studied the effects ofnatural background noise on animalcommunication, points out that urban-

ADAPTABILITY The great tit can fit in all over Europe, from serene forests to traffic-cloggeddowntowns, but the birds’ songs differ with their environments.

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