SAMPLINGS 



Germ Warfare 



In spite of their unicellular condition, bacteria 

 can be highly social creatures. Millions of the 

 soil-dwelling bacteria Myxococcus xanthus, 

 for instance, live in cooperative swarms, 

 feeding together on detritus and other mi- 

 croorganisms. When food is scarce, 100,000 

 or so of them gather together and form mul- 

 ticellular reproductive structures that gener- 

 ate stress-resistant spores. 



But social interaction does not always imply 

 cooperation. In nature, a single species of 

 Myxococcus tends to inhabit a given area. And 

 when brought together in the laboratory, M. 

 xanthus and its relative M. virescens are known 

 to be fiercely competitive. Each species forms 

 its own reproductive structures and secretes 

 compounds toxic to its rival. Eventually, M. 

 virescens invariably dominates M. xanthus. 



Now it appears that such aggression also 

 takes place between members of the same 

 species of Myxococcus. Francesca Fiegna and 

 Gregory J. Velicer, both evolutionary biologists 

 at the Max-Planck Institute for Developmental 

 Biology in Tubingen, Germany, experimentally 

 paired nine strains of M. xanthus in all possible 

 combinations. They found that in a nutrient- 

 poor environment the bacteria engage in ram- 

 pant antagonism, suggesting that the organ- 

 isms distinguish "us" from "them" even for 

 various genetic strains within their own spe- 

 cies. Most strains fared worse and produced 

 fewer fruiting bodies in pairs than they did in 

 isolation. Some even died off altogether (uni- 

 cellular victims, as it were, of "ethnic cleans- 

 ing"). A few dominant strains thrived on the 

 competition, however, producing more spores 

 than they did when they grew alone. (PLoS 

 Bio/ogy 3:1980-87, 2005) 



— Graciela Flores 



Delayed 

 Gratification 



Given the choice, many animals prefer a small, 

 immediate reward to a larger one in the fu- 

 ture. Both common marmosets and cotton- 

 top tamarins, two species of South American 

 monkey, fit that profile — though a marmoset 

 will wait quite a bit longer than a tamarin will. 

 Such behavior actually makes good sense in 

 the wild: a monkey that waits too long risks 

 losing its reward altogether. The food could 

 be snatched up by a competitor, spoiled in 

 the heat, or blown away by the wind. 



Now a team of Harvard University prima- 

 tologists, led by Jeffrey R. Stevens, has dis- 

 covered a twist to this tale. If the rewards are 

 separated by distance instead of time, cotton- 

 top tamarins switch their preference. Tamarins 

 faced with either a small food reward nearby 

 or a larger one farther away, choose to make 

 the longer trip, even though it takes more 



Cotton-top tamarin 



time. Common marmosets, however, are less 

 willing to travel for food — they will wait, but 

 they won't walk. 



Stevens and his team suggest the reason 

 for the unexpected reversal arises from differ- 

 ences between the two species' foraging 

 habits: tamarins range over great distances to 

 find insects, whereas marmosets rely on more 

 predictable, localized food sources, such as 

 tree sap. {Current Biology 1 5:1855-60, 2005) 



—N.W.A. 



The Earth Gets Clocked 



How old is the Earth's core? You might think such a fundamental question would have long 

 since been settled, yet various geological "clocks" give conflicting birthdays as far apart as 50 

 million years. That's a big discrepancy and a big puzzle for earth scientists. 



One clock relies on the rate at which hafnium-182 radioactively decays into tungsten-182. By 

 that reckoning, the core was formed 30 million years after the origin of the solar system, or about 

 4.54 billion years ago. But a second clock, based on the decay of two isotopes of uranium into 

 lead, dates the core to 80 million years after the solar system's birth. Now the geochemists 

 Bernard J. Wood of Macquarie University in Sydney, Australia, and Alex N. Halliday of the Univer- 

 sity of Oxford think they have resolved the inconsistency. 



Wood and Halliday maintain that the hafnium-tungsten clock is correct. But, they point out, 

 about 45 million years after the birth of the solar system a Mars-size object hit the young planet. 

 It added its metallic core to Earth's and spun out debris that coalesced into the Moon. 



The investigators think the collision sparked a dramatic change in Earth's chemistry, causing 

 lead in the mantle to join the core. That event reset the uranium-lead clock, and so by its esti- 

 mate the core looks much younger than it really is. (Nature 437:1345-48, 2005) — G. F. 



Too Clever by Half 



Male jumping spider 



Many harmless, tasty animals mimic others that are dangerous or poisonous — an evolutionary 

 tactic that affords protection from predators without the metabolic costs of the real threat. 

 Jumping spiders of the genus Myrmarachne, for instance, bear a striking resemblance to ants, 

 whose powerful mandibles and stings many predators avoid. More precisely, the adult female 

 and juvenile spiders look like ants; but for the adult males, things are more complicated. 



Intense sexual selection — competition between males to mate — has led male jumping spi- 

 ders to evolve greatly elongated mouthparts. Those mouthparts, which make the male spider 

 look as if it is carrying a large object such as prey, are enticing to female spiders. But such 

 "compound mimicry" has a downside, according to research done at the University of Canter- 

 bury in New Zealand by behavioral ecologists Ximena J. Nelson and Robert R. Jackson. Preda- 

 tors that avoid ants are still deterred by the ant mimicry. But predators that specialize in eating 

 ants know that the best time to attack is when the ants' pincers are clamped on something 

 else. Bye-bye, spider! {Proceedings of the Royal Society B, forthcoming) — N.W.A. 



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NAlnkAl lllsioKY February 2006 



