Hot time in the city

Natural History, May, 2007 by Graciela Flores

As the Earth warms, life in big cities is getting tougher. Abundant dark, sun-absorbent surfaces and heat emitted by cars and buildings, among other factors, push temperatures as much as twenty-two Fahrenheit degrees higher than those in the surrounding countryside. One way or another, people manage to avoid the excessive heat. But what about the rest of the urban fauna?

Michael J. Angilletta Jr., a thermal biologist at Indiana State University in Terre Haute, and a team of investigators argue that so-called urban heat islands are excellent natural laboratories for testing the possible effects of climate change on organisms. In many species, populations from warm habitats tolerate heat better and cold worse than populations from cooler climes. Angilletta and his colleagues predicted that the same would hold true for urban and rural populations of the leaf-cutter ant Atta sexdens.

To test the prediction, the investigators collected A. sexdens in the megacity of Sao Paulo, Brazil, and in rural areas nearby. Then they exposed the insects to a stressful temperature of 108 degrees F. and compared the time it took the two groups to lose mobility. Finally, they chilled members of both groups for twenty minutes, then timed the ants' recovery. As predicted, the urban "Paulistanos" survived the heat 20 percent longer than their rural counterparts. But their greater heat tolerance came at no obvious expense of cold tolerance: both groups of ants recovered from "chill coma" in nearly identica times.

Angilletta and his colleagues can't tell whether the different responses of urban and rural ants come from genetic adaptations or are simply the result of physiological acclimatization. In either case, their study hints that ants. at least, might be able to beat the heat of a warming Earth. (PLoS ONE)