Call it the Robin Hood of rodents. When the cat-sized agouti comes across seeds buried by its comrades, it digs them up and hides them in a new place. The robberies are selfish, to be sure, but a new study reveals that they may be saving a tropical tree from extinction.
The seeds of Panama's black palm tree (Astrocaryum standleyanum) are a mouthful. They are about the size of a cherry and are located in fleshy fruit at the top of the plant, where only an animal the size of an elephant should be able to reach them. In the past, that wasn't a problem. More than 10,000 years ago, elephant-like animals known as gomphotheres roamed the region, swallowing the fruit whole and then pooping out the seeds, which sprouted into new palms. When hunting killed off the creatures thousands of years ago, the black palm should have gone with them. And yet the plant thrives.
Scientists had suspected that the agouti (Dasyprocta punctata) might be playing a role. When the black palm's fruit falls to the ground, these large rodents grab the seeds and bury them as a backup food source. If the animals eventually eat the seeds, they don't grow into new trees. But if they forget about them, a black palm may sprout. Still, it was unclear whether the agoutis were distributing the seeds widely enough to keep the trees flourishing.
To find out, biologists attached radio trackers to 589 seeds and placed them at 52 different sites on Panama's Barro Colorado Island. The agoutis were quick to take the bait. The furry bandits removed almost all of the seeds, hiding them in new locations right away. "The first thing that is really unique about this study is that we were actually able to track the seeds at all," says team member Ben Hirsch of Ohio State University, Columbus. When it comes to following seed dispersal, he says, researchers "always lose some of their seeds."
The trackers sent a signal every time a seed was dug up and reburied (see video). One seed was relocated 36 times, traveling 280 meters from its original spot before it was finally eaten by an agouti 209 days later, setting a new record in seed movements by the rodents. The team also found that after just 1 week, most of the seeds had been repositioned at least once, and some were moved twice in 1 day.
Intent on catching agoutis in the act, the researchers set up cameras and tagged some of the agoutis so that they could tell who was stealing from whom. By robbing each other and hiding their loot, agoutis took seeds farther and farther away from their starting point, each time replanting the seed in a new but equally nutrient-rich spot for tree growth, the researchers report online today in the Proceedings of the National Academy of Sciences.
"I think this is the best example of how scatter-hoarding rodents like the agouti move seeds to places where they can become trees," says Stephen Vander Wall, an ecologist at the University of Nevada, Reno. "A few other studies have done this, but not so dramatically."
The findings may provide hope for other plants that rely on large animals to distribute their seeds, says Hirsch. "In some areas where the big mammals are being poached or somehow getting removed from the system, there may be other species that can come and fill their role."