Plants


To some, the continuous green canopy of BCI’s rainforests looks the same across the island,

Rainforest Canopy

Rainforest Canopy

even though the forest is made up of 100’s of different tree species.  To an animal trying to make a living off seeds dropped out of these trees, however, there are the good and the bad areas.  The good neighborhoods have lots of food and the bad neighborhoods have little food.  From an agouti’s perspective, this comes down to how many palm trees are around, since palm nuts are their favorite food.

Our tree mapping already showed that there is huge variation in the number of palms in different agouti ‘neighborhoods’ across the island. In this new paper just published in the journal Biotropica, we added radio-tracking data collected both by following animals around in the forest, and by using our Automated Radio Tracking System.  We show that “rich” agoutis living in areas with palm (Astrocaryum) density had much smaller home ranges than their poorer island-mates. The reason behind this pattern is straightforward: if you have a high-quality all-you-can-eat restaurant just around the corner, why would you bother to waste your time and energy and face the risk of getting run over by a truck while going to the exact same restaurant eight blocks farther away? Although there are not too many trucks driving around on the BCI-trails, there are ocelots hunting agoutis, and the more an agouti has to run around looking for food the higher risk it has of running into an ocelot-truck.

But, agoutis live in holes in the ground or in hollow logs, not expensive houses.  These do provide refuge from ocelots, as dramatically shown in the below video.  So, if you are an agouti stuck in a bad neighborhood, why not just dig a few extra holes around your territory to give yourself more places to hide from the ocelot-trucks?  This seems like such a good idea the theory even has an official name ‘multiple-central place foraging’.  Do agoutis ‘multiple-central place forage’ to reduce ocelot predation risk in crappy neighborhoods?

Surprisingly, no, agoutis do not increase their ‘multiple-central place foraging’ in bad neighborhoods.  We tracked them down at night to see where they were sleeping, using our radio-tracking antenna to push through the thick vegetation and find their hide-outs.  Although most animals had more than one hidey-hole, there was no relationship with range size – big territories did not have more refuges.

And so we end with the classic scientific conundrum, answer one question, get a bunch of new ones.  WHY don’t agoutis make more holes in large territories?  Are refuges a limiting resource?  Do they need to import more armadillo construction workers to dig more holes?  Or maybe running away from ocelots isn’t that big of a concern for agoutis? We just don’t know, yet….

by Willem-Jan Emsens and Roland Kays

Agouti RefugeTypes

Agouti RefugeTypes

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Imagine you’re a tree.  You’re rooted to the ground and can’t move.  You have a crown full of seeds ready to usher in the next generation of your species, but all you can do is drop them straight down, where they are doomed by the shade of your own crown, not to mention the predators and parasites attracted to this bounty of seeds. Meanwhile, all these animals are scurrying and flying around you, flaunting their mobility.  You’ve got a serious case of movement envy.  You need to move your seeds as far away as possible to give them a chance to survive and germinate. You need mobility assistance.  Maybe the animals can help.

Animals do help.  Many trees have co-evolved relationships with animals and offer them some reward for moving their seeds around – a bit of nutritious fruit-bribe surrounding the seeds.  But some animals are not helpful, eating the fruit without moving the seed away from the mother tree, or even consuming the seed along with the fruit.  Trees don’t appreciate these cheaters, but evolution’s rule book allows it.  The trees can fight back by co-evolving with certain species of animals that are the most beneficial to them, and putting up defenses against the others.  When evaluating these relationships biologists consider the ‘disperser effectiveness’ as the value of an animal species as a disperser for a given tree species.

Palm trees have been bribing animals and fighting cheaters for millennia, evolving a variety of sweet fruit lures surrounding hard nuts.  The trees “want” a cooperative animal to eat the fruit and move the seed away from the mother tree.  Our earlier work using cameras to monitor animals feeding under fruiting Astrocaryum palms trees found Agoutis, Spiny Rats, Red-tailed Squirrels, and Collard Peccaries to make up 99% of the visitors. The next question was to evaluate the disperser effectiveness for this group.  Lieneke Bakker took this challenge on as her MS thesis project at Wageningen University.

Lieneke used small radio-transmitters to follow the movement and fate of Astrocaryum seeds, and used camera traps to determine which species of animal was doing the moving.  She considered a number of factors to evaluate the effectiveness of each species as a disperser including: how often they moved seeds, how far they moved them, rather or not they buried them and, if so, how long the seeds survived.

Agoutis moved by far the most seeds in Lieneke’s experiments, followed by squirrels and then rats.  Peccaries did not move any seeds, probably because Lieneke cleaned the fruit off the seeds for her experiments, and peccaries only eat the fruit part, being unable to crack the hard seed.    All three rodents moved seeds about the same distance (average 9-15m), but they treated them much different, with agoutis and rats burying most of their seeds in underground caches, and squirrels taking about half of them up into the trees to eat or store them.  Finally, the rat-buried seeds were typically dug back up after just 1-2 days, while the squirrel or agouti buried seeds remained underground for three weeks, or more.

Lieneke defended her Masters Thesis this month, concluding that agoutis are by far the most effective initial disperser of Astrocaryum seeds, moving more of them, burying more of them, and leaving them in their underground caches longer than the other rodent species.  Add to this Veronica’s earlier results showing that once a seed is dug up from an agouti burrow it might get re-buried into another underground cache (i.e. secondary dispersal), and Agoutis come out as the main mobility assistant for Astrocaryum trees.

Congratulations to Lieneke on an outstanding Masters thesis.

Different rodent species seem to possess different techniques to handle the seed that they retrieve. For an example, we have observed that a red squirrel prefers to stick to its arboreal nature by dragging the seed towards the canopy away from the ground, a spiny rat prefers the deep dark world below the ground and an agouti prefers to bury the seed just a few centimeters below the surface. Even within agoutis, there are differences in the sites they choose to cache their seeds. We have seeds cached in the stream beds, in the gaps and in the understory of saplings, trees, lianas and vines. In order to know if there are differences in cache sites of agoutis, red squirrels and spiny rats in terms of environmental conditions such as light intensity, soil characteristics and leaf litter depth, Lieneke Bakker, a Master’s student recently completed measurements of microsite characteristics of cache sites. The seeds and fruits of black palm (Astrocaryum standleyanum) are not only preferred by agoutis, spiny rats, red squirrels, peccaries, howler monkeys, spider monkeys, but are also predated upon by insects such as Bruchid beetles and Scotylid beetles. While the fruit is in its early developmental stage, female Bruchid beetle oviposits eggs on the flesh and the newly hatched larvae emerge out of the seed through a perfectly circular hole in the seeds.

                           

We are in the midst of the wet season. Almost every afternoon, dark clouds arrive rolling towards the island and announce rainfall through its thunder and lightning. Rain seeps through tree trunks and between gaps in the palm fronds, the streams swell up with rain water and the soil remains moist under the shadow of leaf litters. We suspect that many transmitters are washed off the magnets after heavy rainfall. But surprisingly, very few transmitters are washed off the magnets. On one hand, the rain makes the surface slippery to walk on; on the other hand it serves as a perfect platform for animal species to leave their footprints. While tracking seeds, we encounter beautiful tracks of ocelots, agoutis, peccaries and deer that decorate the muddy trails. In addition to radio tracking seeds, we are also doing mammal monitoring by using motion sensitive cameras to study the diversity and abundance of terrestrial bird and mammal communities and we recently started the seed excavation project to record the diversity and abundance of palm seeds and seedlings.

                                                              

With the metal antenna held above the head and the receiver close to one ear, we the “seed-trackers” meander through the rainforest in the Barro Colorado Island, encountering numerous impressive webs of golden orb weavers, hopping over lianas and vines that braid each other making it impossible to trace their start and end, stooping under palm fronds that curve as if to touch the ground beneath, walking down fast the ravines sometimes slipping due to the wetness of the fallen leaf litter and slippery mud, and then gradually walking up the slope one step after another with a mission to trace the transmitter and recover the seed whose fate remains a mystery until it is found.

Every week brings with it numerous surprise, seeds in our experiment are either taken by red squirrels to the safety of the tree canopy, or cached in the stream bed. Some are eaten and some are detached from the fishing line. This past week, things were even more interesting.

1)      While carving her way through the lianas and vines in the process of checking the Lake route, one of the seed-trackers caught a freeze-banded agouti in action. At a distance of about 10 meters, the freeze-banded agouti passed with a seed in its mouth and was  dragging a pink flagging attached to the seed! The agouti stopped, lifted its front feet, grabbed the seed and turned the seed in different directions against its mouth. After about 20 seconds, it held the seed firmly by its mouth, put its front feet on the ground and started marching forward and soon vanished among the green vegetation.

2)      One of the streams cuts a narrow ravine that is about a meter high. We have been following a seed that was cached at the bottom of the ravine with a camera. Yesterday the seed was moved close to a hole in which lived a terrestrial crab (we can say this because we have a picture of the crab exiting the hole). Today the seed along with the transmitter is inside the crab’s burrow. The camera revealed the identity of the seed thief. Only this time, it was neither the agouti, nor the spiny rat and red squirrel.

3)      About 5 meters off the Balboa trail; a seed has been taken under the ground through a hole on the ground’s surface. We narrowed the seed’s position with a 900 MHz receiver. “Thud” “Thud” the shovel went into the red clay. A hole about a foot deep and a foot wide was dug. Then suddenly, a bullet ant appeared from about 50 cm away from the hole. Then another one and then another one. As more bullet ants began to appear, we aborted the seed excavation.

While we wait to excavate the seed out of the hole of the mysterious animal, the forest awaits for us with more surprises in the coming weeks.

There are three distinct routes/areas in the 25 hectare plot that the agouti team monitors from day to day. The two areas that contrast each other the most are the Lake-Miller route (circled in black in the picture below) and the Balboa route (circled in pink in the picture below). Walking on the Lake-Miller route (L-01 – L-05 on map), the differences between the seed movement in the northwestern area of study versus the southeastern area (25 hectare plot). Whichever agouti team member takes the Lake route for the day will see that most of the work done here is checking seed experiments that may not have any stolen seeds for up to 8 days. Of course, after a week the seeds will be replaced and monitored again. Keeping the seeds fresh is important in this study. In the area off of Balboa (B-01 – B-05 on map), experiments will usually have 50% to 100% of the seeds moved or cached in just 24 hours after the experiment has been placed. In this area, there is guaranteed to be more seed tracking from day to day. For instance, there have been a few seeds that have been moved and cached for a total of 25 times. The highest cache number along the Lake route is around 8 or 9; the seeds just don’t move as much. Why is that?

As you can see from the map, the southeastern area of the 25 hectare plot has a much lower Astrocaryum density while the northwest route is littered with astrocaryum thus giving it a higher density. The thought that arises from this is that with a greater astrocaryum tree density in an area, there will be astrocaryum seeds on the forest floor. With a high abundance of food for the agoutis in the area, there would be less seed stealing from our experiments for a multitude of reasons but the most compelling is as follows: the likelihood of the agouti passing by our experiment before happening upon many other seeds is lower. In areas with a lower astrocaryum seed density (pink in picture) the food source is much more limited thus, the agoutis – and anything else eating the seeds – are more likely to take the seeds from the experiments at a quicker rate.

Now that we see how often the seeds are being moved in these two areas, the next step would be to monitor the agouti abundance in each area as well. In the high astrocaryum density areas, there seems to be a greater variety of animals passing by the camera traps (ocelot, coati, agouti, etc). However, in the low astrocaryum density areas, the main animals that are caught on the cameras are the agoutis and the spiny rats.

Black: High Density, Pink: Low Density

Every friday, along with the usual tracking equipments and seed preparation kit (every agouti team member has his/her own personalized one), I add in my binoculars and census data sheet. I head out earlier than usual in order to finish the census before the pendulous infructescences shine under the bright sunlight like wet seals under the sun and make it hard to count the fruits that are hanging onto them. I walk on the trails that wind through the 25 ha grid and look for black palm trees that are participating in the census. One will not miss the palm with its needle-like spines covering its entire trunk. I then view through my binoculars and count the infructescences with fruits and the number of fruits in each infructescence.

The census started during the first week of April when the entire island bid farewell to the dry season and welcomed the wet season. 57 black palm trees participated in this census, some with all of their infructescence full of fruits, some with few empty infructescence, some with ripe fruits (the fruits are super orange when ripe) and some with a mixture of ripe and unripe fruits. They contributed to a total of 16993 fruits. Now after 8 weeks of weekly census, only 27 trees remain with about 3539 fruits.

When it is very windy, the pendulous infructescences swing with the wind and release some fruits on the forest floor for common agoutis and spiny rats. Arboreal mammals like the howler monkeys jump onto the palm trees from the neighboring trees and hang onto the infructescence with the aid of their prehensile tails and grab onto the fruits and munch on their juicy flesh. They then throw the seeds covered in some flesh that sometimes land on the heads or footsteps of curious human beings and are then carried by agoutis and turned into invaluable caches!

Karin Steijven took this video of howler monkeys feasting on Astrocaryum fruits while she was collecting Astrocaryum seeds for her master’s project.

You would think a hungry agouti would be bad news for a seed – munch munch.  However, agoutis living in ares with few food trees may also move further, and therefore disperse seeds further than those living the easy life in a grove of palm trees.  Seed dispersal is critical for tree regeneration, so this question of tree ‘density dependence’ on seed survival has important implications.

Enter STRI intern Veronica Zamora-Gutierrez.  “Vero” spent five months with our agouti project on BCI to help track seeds that were scatter-hoarded by agoutis. Her study question was how local food abundance affects seed dispersal by agoutis. Unlike previous studies, Veronica tracked seeds continuously, even after the seeds were removed from their initial caches, with the help of new technology

Vero and the team placed tagged Astrocaryum palm seeds in the home ranges of several radio-tagged agoutis that varied in how many palm trees they had. Some agoutis had home ranges full of fruits and seeds, others had almost nothing. The seed tags have small radio-transmitters that allowed us to find them when they moved.  They also were motion sensitive, so they start transmitting a signal when the seed gets excavated and moved.  These signals show up on the Automated Radio Tracking System on BCI, allowing Vero to know each day which seeds had been moved and then go out into the field to see exactly where it had been moved, and if it had been eaten or buried again in a new cache.

What followed was a logistical tour de force. The seeds were being moved all the time! Many seeds were recovered and re-cached many times in a row, producing multi-step dispersal over impressive distances (some >250m). This was much more movement than we expected, Vero and the team worked overtime for many weeks to keep track of them.

In a nice MSc thesis, Veronica showed that seeds moved most often and furthest in areas that had least fruits – so hungry animals moved seeds further. Dispersal, however, came at a high cost: every time a seed was dug up it increased the risk that the seed was eaten rather than re-cached.

Veronica obtained her MSc degree from the University of Leiden in the Netherlands last week: congratulations!

Zamora-Gutierrez, V. 2010. Effects of food availability on seed dispersal by the Central American agouti (Dasyprocta punctata). Leiden University, Leiden, The Netherlands.


The agouti team 2009

Literature Cited

1.     Gutierrez, V.Z. 2010. Effects of food availability on seed dispersal by the Central American agouti (Dasyprocta punctata). ***UNMAPPED REFMGR FIELD #30***, Leiden University, Leiden, The Netherlands.

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