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Rodent robbers fill role of mega mammals, help spread tropical trees

An agouti in Panama. Photo by: Rhett A. Butler.
An agouti in Panama. Photo by: Rhett A. Butler.

In order to disperse their seeds, large-fruited tropical trees probably relied on massive mammals that roamed the earth over 10,000 years ago. But with giants such as the mastodon now extinct, thieving rodents—who continually excavate and rebury others’ seeds—may be filling their role, according to a new study in the Proceedings of the National Academy of Sciences (PNAS).

Historically, tropical trees such as the black palm (Astrocaryum standleyanum) relied on large fauna to eat their oversized fruits and spread the large seeds within. With major mammals now absent from new world landscapes, “the question is how this tree managed to survive for 10,000 years if its seed dispersers are extinct,” said study co-author Roland Kays in a press release.

The answer may lie in the thievish activity of small scatter-hoarding rodents. The agouti, a common rodent found in Central and South America, has long been known to bury seeds, but what surprised the researchers was “this constant digging up of the seed, moving it and burying it, over and over again,” Kays stated.

The long-distance spread of the seeds was driven by an unusual behavior: retaliation. Individuals whose seeds had been robbed tended to steal seeds from others, and this helped spread the seeds beyond the range of a single Agouti. According to the study, “the stepwise dispersal of seeds across home range boundaries was driven by [this] reciprocal theft.”

To capture the thefts, the researchers relied on a combination of video surveillance cameras and radio transmitters to track and tag both seeds and Agouti. This approach led them to find that just 16 percent of dug up seeds were recovered by their owner. Furthermore, instead of being eaten, 88 percent of the excavated seeds were moved to a new site and reburied. This movement helped disperse the seeds up to 280 meters from their starting point.

Beyond helping prevent certain tree species from becoming extinct today, the robbing rodents may also play a beneficial role in the future.

“When you think about global climate change and habitats shifting, for a forest to move into new areas, trees need to have their seeds move into new areas,” Kays elaborates in the press release. Even in the absence of large mammals, thieving rodents may continue to provide a pathway for that to happen.

CITATION: Jansen, Patrick A., Ben T. Hirsch, Willem-Jan Emsens, Veronica Zamora-Gutierrez, Martin Wikelski, and Roland Kays. 2012. “Thieving Rodents as Substitute Dispersers of Megafaunal Seeds.” Proceedings of the National Academy of Sciences (July 16). doi:10.1073/pnas.1205184109.

Tyler Lark is an Environment & Resources graduate student researching agricultural land use change at the Center for Sustainability and the Global Environment (SAGE) at the University of Wisconsin-Madison.

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