Sometimes in science, the answer you end up with is not exactly the question you started with. The path to discovery is not always predictable. Researchers have to constantly evaluate what they are finding, and be ready to adjust their course when the data leads down a different path. This is especially true in tropical ecology, where there is so much basic information yet to be learned.
Such is the case with our new paper published this week in the Proceedings of the National Academy of Sciences (PNAS).
We started tracking the fate of tropical seeds with small radio-transmitters because we thought that the predation of agoutis (the main mover of palm seeds) by ocelots (the main predator of agoutis) would leave a bunch of “orphan seeds” buried in the forest where no other agoutis would discover them. These orphaned seeds would thus be free to germinate and grow into new palm trees. It was a cool idea, and would show how predators affect prey, ultimately trickling down through the tropic levels to affect seed survival, forest regeneration, etc… We had all the hypotheses, sub-hypotheses, and sub-sub-hypotheses worked out. Now we just had to go into the jungle and prove ourselves right.
We set out to map all the palm trees, radio-collar a bunch of agoutis, have them disperse our special radio-tagged seeds, and then wait for the ocelots to pick them off one-by-one. Earlier research suggested that only about 1/3 of these rodents survive one year, with most falling to the island’s ocelots. If we did our part we knew we could count on the ocelots to do theirs.
This was actually a huge amount of work, we needed “our agouti” to move “our seed”, and bury it in a little hole for safe-keeping. Camera traps told us whether one of “our agoutis” moved a particular seed, and more often than not it was an un-marked agouti, or a rat or squirrel. Initially animals just ate most of the seeds, but once they recovered from the recently-ended hungry season, they started storing seeds in scattered underground caches for later, when little fresh fruit will be available.
Finally our radio-tagged seeds were moving. Only, and here’s where the change in the-path-to-discovery comes in, the seeds didn’t stop moving. Once a seed was buried we figured we’d just sit and wait till it was dug up and eaten, sometime in the next few months or year. Instead, the seeds were quickly dug-up, moved, and buried again, and again, and again. During our first season of field-work this high rate of movement caught us off guard and the additional work of tracking down these crazy seed movements completely wore down everyone on the project. Given the super-high rates of seed movement, we realized we needed to look for (actually, listen for radio-signals) moving seeds every single day. Even daily checks didn’t catch all the movements because we observed some seeds actually move twice in one day.
What the heck was going on? Why were agoutis moving seeds so often? Some seeds were going 100’s of meters. Were agoutis shifting home-ranges and taking their seeds with them? Or, were there thieves amongst us?
For our second field season we decided to switch tactics a bit, and investigate this new research path illuminated by the crazy seed movements. We mounted a major trapping effort to try and capture and mark as many agoutis as we could. By being able to recognize lots of animals in one area, we hoped to determine who was taking the seeds. We hid motion-sensitive cameras next to the buried seeds to see which animal’s dug the seeds up. Our videos (example above) showed that most (84%) of seeds were being stolen by robber-agoutis. These unscrupulous rodents weren’t just eating the buried treasure, but often moved it over to the center of their territory, where they could more easily find it during the upcoming hungry-season. This repeated thievery resulted in seeds moving much further than you would expect from a single agouti. Slightly more than 1/3 of seeds moved more than 100m, which is typically considered far enough to escape the competition of sibling-seeds that just drop underneath the mom-tree. One seed was cached 36 different times, traveling over 749 m back-and-forth between territories until it was 280 m from its starting point. We made a movie illustrating this amazing amount of movement (shown below with a fun soundtrack).
Although our test of the predator-mediated seed dispersal hypothesis didn’t go off exactly as planned, our results incidentally disproved it. Even if seeds do become “orphaned” by predated agoutis, we now know that the rates of seed theft are so high that these orphaned seeds still have a good probability of being discovered. While this particular route of influence between predators-prey-trees is probably not important to forest dynamics, our other work shows how other behavior of these agoutis is heavily influenced by the threat of predation (recent biotropica paper, and another one in the works).
This discovery of robber-rodents helping trees by moving their seeds long distances was made even more interesting by the fact that the dispersal of this particular type of tree has been a tropical enigma since Janzen and Martin published “Neotropical anachronisms: The fruits the gomphotheres ate.” In 1982. This paper, and dozens since it, suggested that the very largest fruits and seeds found in the Neotropics must have co-evolved to be dispersed by the now-extinct Pleistocene Megafauna. How these trees have survived the >10,000 years since megafaunal extinction has puzzled tropical ecologists for decades. These results are also important when applied to current mammalian extinctions. If tree species are able to survive due to “disperser substitution” maybe this holds a glimmer of hope for trees that are dispersed by mammals that are currently being hunting to extinction or local extirpation. Alternately, our results also show how important of a role these little agoutis can play in their ecosystems. When poaching gets so bad that they also deplete these smaller-sized mammals, the trees seeds may have no chance to survive.
Our accidental discovery of robbing rodents offers a new potential answer to this mystery, and highlights the potential rewards of following thieves down the dark and mysterious scientific path to discovery.
By Roland Kays