SAMPLINGS 



Green Repellent 



Frogs' skins are veritable laboratories for 

 synthesizing biologically active com- 

 pounds, including antibiotics, glues, hallu- 

 cinogens, lubricants, painkillers, and a 

 stunning array of poisons. Now, yet anoth- 

 er active — and potentially useful — compound 

 has been identified in the frog's chemistry 

 set: an insect repellent. 



Health concerns have prompted a push 

 to replace widely used synthetic insect re- 

 pellents, such as diethyl-m-toluamide 

 (DEET), which is toxic at high doses. To bi- 

 ologists Craig R. Williams of James Cook 

 University in Cairns, Australia, and Michael 

 J. Tyler of the University of Adelaide and 

 their colleagues, frogs — with their remark- 

 able chemical repertoires and suscepti- 



Hot Rocks 



As our planet warms, rising air and ocean 

 temperatures make frequent headlines. 

 Of course, the land is warming, too — yet 

 remarkably, until recently, no one had 

 quantified just how much the continental 

 landmasses have warmed. Now Shaopeng 

 Huang, a geologist at the University of 

 Michigan in Ann Arbor, has made the first 

 analysis of the annual change in the heat 

 content of all the Earth's continental land 

 masses except Antarctica. 



Huang made his estimates from global 

 meteorological records, as well as from 

 the thermal properties of the uppermost 

 layers of the Earth's crust. Between 1851 

 and 2004, he discovered, a total of 1 1 .7 

 zettajoules (10 21 joules) of thermal energy 

 has been collectively trapped by Africa, 

 Asia, Australia, Europe, North America, 

 and South America. That amount is equal 

 to the world's total energy production 

 from 1970 until 2003, and is enough to 

 raise the temperature of the top hundred 

 feet of the world's landmasses by two 

 degrees Fahrenheit (1.1 degree Celsius). 

 Even more disturbing, 10 percent of that 

 heat — 1.34 zettajoules — was absorbed in 

 the most recent 2.6 percent of the time 

 interval: the four years from 2001 until 

 2004. The effects of the warming land on 

 biological, chemical, and physical 

 processes are still unknown- — but, says 

 Huang, they are likely to be profound. 

 {Geophysical Research Letters 

 33:L04707, 2006 — G.F. 



bility to insect bites — 

 seemed a promising 

 source of anti-insect 

 compounds. 



The team collected 

 skin secretions from five 

 species of Australian 

 frog. They swabbed the 

 tails of mice with secre- 

 tions from one species, 

 the Australian green 

 tree frog (Litoria Australian green 



caerulea), and confined 

 the mice in chambers with mosquitoes. 

 The mice, they found, remained bite-free 

 for about fifty minutes. By comparison, un- 

 treated mice had only twelve minutes of 

 peace before the mosquitoes attacked; 

 DEET-swabbed mice lasted two hours. The 



Long Dig 



Unless you're a Scrabble player, you may 

 never have heard of qanats. They are gently 

 inclined underground channels that bear wa- 

 ter from an upslope aquifer to a village in 

 the valley below. The first qanats were built 

 in Iran as early as 3,000 years ago; they also 

 occur in arid parts of China, the Arabian 

 Peninsula, and Mediterranean lands. 



To avoid flooding, qanat construction 

 proceeded upslope from the exit to a well 

 at the source, which could be as far as fifty 

 miles away. The builders dug a series of 

 vertical shafts about 200 feet apart, and 

 linked the shafts' bottoms to form the un- 

 derground channel [see illustration at bot- 



tree frog repels the mozzies. 



investigators also discovered that the se- 

 cretions of two other frog species exude a 

 mosquito-repellent odor. Williams sus- 

 pects that terpenes, insect-repellent com- 

 pounds that the Australian green tree frog 

 is known to accumulate from its food, may 

 help keep the bugs away. {Biology Letters, 

 in press, 2006) — Sion E. Rogers 



torn right]. But how did the ancient qanat 

 diggers, who had only imprecise surveying 

 instruments, manage to accurately aim their 

 digging and keep the tunnels' slopes even 

 over great distances? 



Redundant measurements were the key 

 to success, says Stathis C. Stiros, a civil engi- 

 neer at Patras University in Greece. Stiros ar- 

 gues that the dig planners measured the 

 ground repeatedly, with various instruments 

 at numerous times of day and throughout 

 the year (light refraction, for instance, varies 

 with outside temperature, and presents a 

 common source of surveying error). The 



process randomized the engineers' 

 inevitable errors, which then can- 

 celled one another out. The dig plan- 

 ners paid for their precision with 

 time. A Roman-era qanat in Algeria 

 took fifteen years to build, but ten of 

 them were devoted to planning 

 alone. {Journal of Archaeological Sci- 

 ence, in press, 2006) — S.R. 



Qanat-shaft openings recede across the 

 Iranian countryside (above); a schematic 

 diagram of a qanat is shown at right. 



Bedrock 



14 



NATURAL HISTORY May 2006 



