Thousands of years ago massive elephantlike creatures wandered the landscape, gobbling up then defecating fruit. In the process, they may have planted the seeds for primordial forests. But with these creatures long extinct, ecologists have been left with a puzzle: If these trees are still with us, what—if anything—disperses these seeds to create today's woodlands?
The answer—at least for one type of forest—may lie in the criminal antics of a cunning rodent. A group of scientists working with the Smithsonian Tropical Research Institute in Panama and Wageningen University in the Netherlands report their hypothesis in today's Proceedings of the National Academy of Sciences.
The rodent in question is the agouti—a house cat–size critter that resembles a leggy, tailless squirrel. Although an agouti cannot devour large tropical fruits the way an ancient mammoth might have, it consumes the seeds, collecting and burying some to store as snacks for later.
Traditionally, the agouti has been described as a seed predator. Studies monitoring their caches suggest that it doesn't take long for an agouti to come and collect. However, the research published today suggests that caching a fallen seed is just the beginning of a seed-stealing saga that carries each over long distances.
The research group studied agouti caching on a Panamanian island, Barro Colorado, where they marked 16 agoutis for individual identification and set up video cameras to monitor activity at cache sites. To follow the fate of individual seeds, they attached a long thread with a transmitter unit to 589 seeds, then radio-tracked locations over the course of a year.
More than half of the seeds cached were then stolen by another agouti and re-cached elsewhere. The thieving rodents would steal and cache repeatedly, carrying the prized seed ever farther from its original location to evade other interested agoutis. For example, one seed tracked was cached by agoutis more than 30 times, journeying 280 meters from its starting point and more than 750 meters total. As a video by biologist Ben Hirsch at The Ohio State University reveals, these rodents aggressively snatch seeds from one another, sometimes moving in on a fellow agouti that has just retrieved a seed.
According to Hirsch, one of the study's co-authors, the rates of re-caching they have observed are unprecedented in agouti research. "This led to seed-dispersal patterns much greater than would have been predicted previously, and we documented that this phenomenon was indeed the result of massive cache thievery."
Dispersal is a key factor in ensuring the survival of a species. For tree species such as the black palm, spreading individuals over a broader range can mitigate the effects of pests, move organisms into new climatic ranges, and increase the flow of genes between populations.
"The work clearly shows that, despite their size, small rodents play a much larger role in seed dispersal for some tree species than previously recognized," says Joshua Plotkin, a population biologist at the University of Pennsylvania who mathematically models evolutionary and ecological questions. Plotkin, who was not connected with this study, adds—as the authors noted—that this finding casts doubt on the hypothesis that megafauna were crucially responsible for seed dispersal thousands of years ago, when it seems probable that rodents played a role even then.
Ultimately, most seeds were consumed by agoutis—or by the occasional land crab, squirrel or spiny rat. An estimated 14 percent of seeds survived a full year, however. By that point, the scientists believe the agoutis will move on to fresher food, allowing these survivor seeds time to grow into seedlings. As Plotkin notes, however, more research is needed to clarify what happens next and how seed dispersal relates to the success and maturation of adult trees.