SAMPLINGS 



Sink in the Sea 



When fossil fuels burn, carbon compounds, 

 once sequestered deep in the ground, turn 

 into carbon dioxide and ascend into the at- 

 mosphere. There, they contribute to global 

 warming. Scientists want to know more 

 about any natural systems — called "carbon 

 sinks" — that reverse the process and pull 

 carbon out of circulation. Two recent studies 

 highlight one of Earth's most important car- 

 bon sinks: the entrapment of organic materi- 

 al deep in the oceans. 



Krill are small invertebrates that feed 

 nightly in the oceans' rich surface waters, then 

 descend to deep water. Investi- 

 gators assumed that the krill 

 descend slowly once every 

 twenty-four hours and are thus 

 still close to the surface when 

 they release their waste (and 

 the carbon it includes). Surface 

 currents would then keep the 

 carbon in circulation. Now, 

 however, oceanographers 

 Geraint A. Tarling of the Nat- 

 ural Environment Research Council in Cam- 

 bridge, England, and Magnus L. Johnson of 

 the University of Hull in Scarborough, Eng- 

 land, have observed that Antarctic krill de- 

 scend rapidly whenever they are full, perhaps 

 as many as three times a night. Thus krill are 

 probably very deep (well below 1 30 feet) 

 when they release their waste, which eventu- 

 ally settles to the bottom. If so, the amount of 



Two carbon sinks: mangroves in Australia (above); Antarctic krill (photograph below) 



carbon trapped in the Southern Ocean could 

 be 8 percent greater than previously thought. 



Some of the oceans' carbon comes from 

 land. Mangroves grow in tidal flats and, like all 

 plants, build their tissue from carbon dioxide 

 they extract from air. When mangrove debris 

 rots, it leaches dissolved or- 

 ganic matter into the sea. 

 Thorsten Dittmar, an oceanog- 

 rapher at Florida State Univer- 

 sity in Tallahassee, and three 

 colleagues estimate that 

 though mangroves cover less 

 than 0.1 percent of the Earth's 

 land, they contribute more 

 than 10 percent of all the or- 

 ganic carbon transferred from 

 land to sea. They reached their conclusion by 

 analyzing carbon isotopes to determine the 

 origin of dissolved organic matter in water off 

 the mangrove-rich coast of Brazil. Alas, man- 

 groves are declining worldwide; they're trap- 

 ping less carbon in the oceans just when we 

 need them to trap more. (Current Biology 

 16:R83-4, 2006; Global Biogeochemical Cy- 

 cles 20:GB1012, 2006) —Stephan Reebs 



Follow My Eyes 



Following someone else's gaze is irresistible, 

 isn't it? And people aren't the only ones 

 who do it. Many monkey species are known 

 to follow other monkeys' glances. It's no 

 mystery why this behavior evolved; after all, 

 a fellow monkey's gaze could disclose im- 



t 1 ft? 



Rhesus macaques: leading by looking 



portant information — say, the location of a 

 tasty fruit or a lurking rival. 



But each group has many sets of watchful 

 eyes, and deciding whose eyes to follow isn't 

 always easy. The key is social status, accord- 

 ing to Stephen V. Shepherd and Michael L 

 Piatt, neurobiologists at Duke University in 

 Durham, North Carolina, and a colleague. 

 The team tested gaze-following behavior in 

 rhesus macaques and observed that low- 

 status members of the group almost immedi- 

 ately followed the gaze of any other monkey. 

 High-status macaques, however, followed 

 only the gazes of other high-ranking individ- 

 uals, and took longer to do it. The 

 difference in response times suggests that 

 gaze-following is both reflexive and volun- 

 tary. Low-status monkeys behave reflexively 

 because they face possible threats from 

 other members of the group as well as 

 predators. They need to monitor every 



Ear to the Ground 



When elephants bellow warnings of danger, 

 their low-frequency calls resound in the air 

 and rumble through the ground. Of course, 

 no one has ever doubted that elephants 

 hear and respond to the airborne sounds — 

 their ears are not exactly hidden. But 

 whether they could detect underground vi- 

 brations has, until now, been unclear. A 

 team led by Caitlin O'Connell-Rodwell, 

 a biologist at Stanford University, recently 

 demonstrated that African elephants not 

 only perceive each other's seismic warn- 

 ings, but respond to them, too. 



O'Connell-Rodwell's team recorded seis- 

 mic components of vocal alarms. They 

 played the alarms back to elephants at a 

 popular watering hole in Namibia through 

 transmitters buried three feet underground 

 and a hundred feet away. The elephants re- 

 sponded by grouping more closely together 

 with their bodies perpendicular to the alarm 

 source (perhaps to better perceive the warn- 

 ing), and by leaving the watering hole 

 sooner than unperturbed elephants. 

 Acoustic alarms prompted more dramatic 

 responses than underfoot rumbles alone did, 

 suggesting that the elephants interpreted 

 the seismic alarms as coming from too far 

 away to signal any imminent danger. 



How elephants detect seismic signals is 

 unknown; perhaps their trunks, pressed to 

 the ground, or feet pick up the rumbles. 

 Elephants thus join certain insects, amphib- 

 ians, reptiles, and small mammals in taking 

 advantage of underground communica- 

 tions. (Behavioral Ecology and Sociobiology 

 59:842-50, 2006) —Samantha Harvey 



nearby individual. But high-status monkeys 

 are relatively safe from most other members 

 of their group; within the group, only their 

 social peers present a big threat. That's why 

 they take a little time to make sure they 

 know the social status of an individual before 

 choosing to follow its eyes. (Current Biology 

 1 6:R1 19-20, 2006) — Nick W. Atkinson 



12 NATURAL HISTORY May 2006 



