April 2010

From the agouti’s perspective, the whole point of burying a seed is to store it away so it can be eaten later in the year, when less food is available.  The problem is that all the local seed eating animals know this, and everyone is searching for these buried treasures.  By spreading the seeds out in scattered caches each agouti does their best to hide seeds in places unlikely to be  found by other animals.

Biologists studying this phenomenon have assumed that there is a cacher’s advantage – that the individual who buried the seed is more likely to find it again than some other animal because they remember the location.  However, this had never actually been tested, and our recent results on dramatically high cache removal made us second guess this assumption.

Michiel Veldhuis, a MSc student at the University of Groningen, tackled this problem by pretending he was an agouti.  Michiel went out to the forest and buried seeds, 700 in total.  But Michiel is not your average agouti, he’s got technology on his side (not to mention opposable thumbs and mad dance moves).  Michiel followed the fate of 200 of these seeds with tiny radio-transmitters to see how long it took for other animals to find them, and what they would do with them next.

Michiel embedded this experiment within the framework of our larger projects by creating these cache mimics in 19 known agouti territories.  In these areas we knew not only how much food was available, but also the local densities of agoutis, and other animals. This design allowed him to test for factors that affect seed pilferage.  Clearly, these other factors must be important, as the rates varied greatly across the territories (100-15% pilferage).  Michiel found that cache pilferage increased in areas with fewer fruits and more agoutis, i.e. where competition for food was higher, hidden seeds were more likely to be stolen.

Interestingly, not all stolen caches were eaten, rather, many were moved and reburied.  These aren’t hungry thieves, but individuals raiding each others food stores to prepare for the upcoming lean season.  Areas with fewer available fruits had seeds pilfered and re-cached multiple times, and carried further away.

Finally, the key test for the existence of a cacher’s advantage required comparing pilferage between his artificial caches (i.e., without owner) and natural agouti caches (i.e., with owner).  For this he teamed up with Veronica Zamora who was using the same methodology to track natural agouti caches.  Caches with an owner did have a significantly higher recovery rate than the mimicked caches without an owner. This difference reflects the a recovery advantage of the original cacher remembering the location of seeds over naïve foragers.

Michiel returned from his life mimicking agoutis in Panama to share these secrets of agoutis in his thesis defense last month in the Netherlands.  Congratulations Michiel!

In a recent poll of BCI frugivores 13 out of 15 species agreed – Attalea palm fruit restaurants are much better than Astrocaryum palms.  The two hold outs, rodents agouti and spiny-rat, both prefer the uncrowded atmosphere at Astrocaryums, but are also known to grab a bite at Attalea’s when they’re in the neighborhood.

The results of this surprising poll were recently presented by Vivian Maas as she defended her MSc degree in Forest and Nature Conservation at Wageningen University.   Vivian took the poll by using camera traps to record the frequency and behavior of animal visitors to fruiting palms of two species: Astrocaryum standleyanum and Attalea butyracea.  These have long been considered keystone restaurants for the mammal community on Barro Colorado Island.  Although researchers frequently see animals loitering around these establishments, there had been no systematic poll of their opinion of these two restaurant chains.

“I just love chomping Attalea fruits”, said one macho tapir, “their flavor puts me at ease and lets my mind wander to happy times”.  A coati matriarch agreed, “The open understory under their big fronds is a perfect place to let my kids play while I catch a bite to eat.  Under an Astrocaryum I’m constantly worried they’ll put there eye out on a spine.”

For each mammal species, Vivian calculated how often they visited the trees, how much time they spent at the trees, and was even able to tell how many fruits they took by watching the videos. She also compared the animal activity under palms with that of random sites.  In total, both restaurant chains attracted nearly 10x more animal visitors than a typical random place on BCI.  Agoutis and squirrels were the primary patrons of Astrocaryum, while Attalea was also popular with coatis, tapirs, peccaries, opossums, and other species.

However, not all species like the Palm restaurants, Brocket deer seemed to avoid the trees, and when they did wander by, didn’t eat any fruits.  “I don’t need to fight through the crowds only to break my teeth on this sugary fast-food” commented one Brocket deer, “they don’t even have a salad bar!”  Pacas, another fruit eating mammal, also visited the fruiting palms less than expected given their overall abundance on BCI.

A primary hypothesis of the study was to test if the mammal community varied with the abundance of the palms: do isolated restaurants attract a different suite of animals than those clusters? Do palm restaurants get the most business by follow the Starbuck’s strategy of one on every corner? To find out, Vivian choose palms ranging from isolated to clustered individuals and used multivariate statistics to determine how community composition varied with palm abundance. She found that agoutis were more dominant visitors as the palms were more isolated in Attalea, while in Astrocaryum, the reverse was true.

Spokespersons for both restaurant chains had no comment, because they are plants, and can not talk.