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Rodent poop gauges ancient rains

Size of chinchilla pellets reveals past desert environment

12:33pm, August 10, 2010
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IGUASSU FALLS, Brazil — The size of fecal pellets in ancient rodent middens can provide clues about the abundance of rainfall in times past, new analyses suggest.

Middens are, in essence, rodent latrines shared by a family or social group. Abandoned middens are composed of large numbers of fecal pellets cemented together by crystallized urine, reported Claudio Latorre Hidalgo, a paleoecologist at Pontificia Universidad Católica de Chile in Santiago, on August 9 at the Meeting of the Americas. Besides containing the DNA of the creatures that deposited them, the pellets contain pollen and undigested bits of plants, a trove of ecological data for the region surrounding the midden.

But new analyses by Latorre Hidalgo and his colleagues focus on nothing more than the size of the pellets. The larger the fecal pellets, the larger the creatures that deposited them, the researchers suggest, and the lusher the climate conditions at the time.

For their study, the team scrutinized middens left by chinchillas at nine sites along the edge of South America’s Atacama Desert, one of the world’s driest regions. The researchers measured the size of all pellets, then excluded the smallest 80 percent to ensure they considered only the ones deposited by the largest adult members of the group, not ones left by juveniles or adolescents. Carbon dating of the organic material in a midden provided its age.

At the sites studied, the average size of the largest fecal pellets in a midden was correlated with rainfall for that site at the time the midden was formed, as determined by previous studies by other teams using other gauges of ancient rainfall, said Latorre Hidalgo.

The new proxy for rainfall may prove useful when other sources of climate clues, such as sediment cores drilled from lakebeds, aren’t readily available, he noted.

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