Agouti Horror Show

It’s the classic horror scene – a defenseless young victim sleeps as a bloodthirsty predator stalks just outside the bedroom.  This plays out nightly in the rainforest as ocelots stalk past agoutis sleeping in hollow logs or tree holes.  Our camera traps on BCI occasionally record the end result of this drama, for example, this ocelot toying with a baby agouti in the middle of the night.

From previous work we knew that ocelots eat a lot of agoutis, often catching them at night near their burrows.  However, we didn’t know if this was just random encounters, or if the ocelots were seeking out the sleeping rodents.  This question is more than simple curiosity given the importance that refuges are thought to play in how animals move around when they aren’t sleeping too.

The ‘Central Place Forager’ hypothesis suggests that prey should stick close to their refuges so they can quickly run to safety if they detect a predator sneaking up on them.  However, if predators cue in on sleeping sites, prey should avoid these areas when they aren’t actually sleeping.  Two opposite predictions  – so which is it?

To answer this question Willem-Jan Emsens led an effort to radio-track agoutis to find where they slept, then ran camera traps to monitor the agoutis as they come and go.  These cameras also recorded ocelots.  Not only did the ocelots walk by, but our videos show them actively trying to get into the agouti hide-outs.

Our camera traps recorded ocelots at agouti refuges more than 2x as often as at non-refuge sites, and showed that they hung out at agouti holes  5x longer than other sites.   Ocelots apparently could tell if the refuge was occupied or not, as they spent about a minute trying to get at agoutis in holes, but took just a few seconds to figure out that no one was home, and move on.  No agoutis were harmed by ocelots while our cameras were running, but they must have been well terrified as the cats tried to claw their way in.

So the answer is YES – ocelots do target agouti refuges, but agoutis seem safe as long as they stay tucked away out of reach.  Their bed is safe, but their bedroom (the area around the refuge) is risky.  I just hope they don’t have to get up in the middle of the night to go to the bathroom!

Based on our new paper “Prey refuges as predator hotspots: ocelot (Leopardus pardalis) attraction to agouti (Dasyprocta punctata) dens” in Acta Theriologica 2013.

 

All the fancy agouti neighborhoods are lined with palm trees (our new publication in Biotropica)

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

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

 

New adaptive GPS provides freeze-frame quality picture of animal movement.

If you have ever tried to take a ‘freeze-frame’ photograph your pet leaping in a trick move, a bird flicking past your feeder, or your kid at a sporting event, you have probably looked at more than a few blurry photographs.  Getting a crisp photograph of a body in motion requires a faster shutter speed to provide a sharp image of what is happening at the instant the photo is snapped.

A blurry photograph of a mother tamandua carrying its baby.

Using GPS tags to study animal movement can be compared to taking snapshots of behaviors we can’t observe with our own eys – scientists connect a series of locations taken by an animal’s GPS collar to create a picture of how the animal uses its environment.  The schedule of the GPS unit sets the resolution of the picture we get about the animal movement.  More frequent fixes (e.g. every few minutes) give a high resolution image of where the animal goes while less frequent fixes (e.g. every few hours) are analogous to a blurry photograph. But as anyone who has ever used a handheld GPS unit knows, they chew through batteries like nobody’s business if they collect fixes constantly.  Unfortunately, wild animals won’t change their collar’s batteries when they run out, so scientists have to make the most out of one battery, and therefore face a dilemma in how to program the collars they will use on animals: infrequent GPS sampling will make the collar last longer, but give a fuzzy picture of movement paths; more frequent GPS sampling gives sharp paths but dramatically shortens battery life.  This is an especially difficult problem when animals spend long stretches of down time in tree cavities, burrows, or thick vegetation where a GPS collar has little hope of successfully connecting with satellites and wastes even more battery power in futile attempts to record locations on schedule.

What is needed is a flexible GPS schedule tied to the behavior of the animal wearing the collar.  A more active animal would trigger the GPS unit to record locations more often, while a resting animal would signal the GPS unit to record locations less often.  What kind of sensor monitors the moment-by-moment behavior of wild free-living animals in any type of habitat?  An accelerometer—a matchbook-sized wireless device that measures the change in speed of an animal’s body over time as it moves through its environment.  Accelerometers are in everything from vehicle airbags to Nintendo Wii handsets and over the past few years they have exploded onto the scene in the world of animal movement research.  They are an ideal sensor for linking animal behavior to the location recording schedule of a GPS collar because they are low-cost, can easily be incorporated into a standard GPS collar and they sample movement behavior every few seconds without using much battery power.  So in theory a researcher can have the best of both worlds: a long-lasting GPS collar that gives sharply focused pictures of the paths animals take as they move around their habitats.

Testing the performance of just such a collar is the subject of a paper we recently published in Wildlife Society Bulletin (Brown et al 2012).  We worked both with fisher in upstate New York and Tamandua anteaters on Barro Colorado Island in Panama, testing two types of GPS: one that recorded locations on a fixed schedule every 15 minutes and the other with a flexible schedule that recorded locations every 2, 15 or 60 minutes based on accelerometer-measured movement behavior.  The accelerometer-informed collars performed considerably better than the traditional GPS collars: they attempted 74% more locations per day and had 62% higher location success rates, which means that on days animals were more active, GPS collars recorded more locations thus providing more detailed movement paths.  At the same time they spent 28% less time searching for satellites and recorded 67% fewer locations when animals were at rest, reducing the overall amount of battery power used for each unique location and lengthening the lifespan of the collar.  Ultimately the accelerometer-informed GPS collars produced more information about animal movement for a given battery size and study period when compared to traditional fixed-schedule collars.  This technological development is a boon for researchers and potential study animals alike: ecologists get higher quality data with little additional cost per collar and can instrument fewer study animals for shorter periods of time than they would using traditional collars because the snapshots of daily movements are so much clearer.  Currently only two companies (e-obs (used by our study) and Telemetry Solutions) produce accelerometer-informed GPS collars, but as word gets around, those scientists studying animal movement ecology are sure to appreciate the value of this novel tool.

Map showing high resolution 'picture' of the life of Phineas the Fisher as recorded by an adaptive GPS collar. Click the map to zoom in on the map at Movebank.org

Reference:  Brown, D.D., LaPoint, S., Kays, R., Heidrich, W., Kuemmeth, F. and M. Wikelski.  2012.  “Accelerometer-Informed GPS Telemetry: Reducing the Trade-Off Between Resolution and Longevity” Wildlife Society Bulletin 36(1):139-146.

 

Written by Danielle Brown

 

Agouti Kleptoparasites

In our first field season we noticed that some seeds traveled so far that they must have moved out of the home range of an agouti. This led us to wonder whether the massive seed re-caching behaviors we observed were the result of one agouti caching its seeds many times or agouti thieves moving seeds from one territory to the next.

To solve this question we needed to be able to identify individual agoutis, which otherwise all look alike. After substantial live-trapping effort we were able to catch >20 agoutis in one portion of the island (in and around the 25 ha plot). We individually marked each agouti we caught with a distinctive radio-collar, ear-tag and/or freeze brand. Then, we put a motion-sensitive camera next to a buried seed where we knew the “owner” and recorded every agouti (or other species) that walked by. This allowed us to determine whether the cached seeds were being dug up and moved by their owners, or by thieves. We are still crunching the numbers from these experiments but in the process discovered a cool video that shows just how crafty agouti thieves can be.

Many rodents bury seeds in times of plenty to save them for lean times, and these hidden food caches are critical to their survival. Rodents will go to great lengths to protect their seeds from potential thieves. For example, Michael Steele and colleagues found that if a squirrel wanted to bury a seed but was being watched, it would often behave “deceptively” by making fake caches and making caches behind trees so the observer could not see the cache being made. While I would have loved to do some similar experiments with the agoutis, it would have been hard to observe the agoutis without scaring them away. However, if you run enough camera traps for enough time you eventually record some surprising clips.

In this case, I didn’t notice what was going on the first time I saw this video, and only recently realized how it shows the subtle dynamics between different agoutis sharing an area. The seed that this camera was monitoring was previously cached by an agouti named Tracy, who can be identified by a small diagonal white freezebrand mark on her body. In the video you can see Tracy come to uncover her seed four months after she originally placed it there (we had a camera there the whole time). Given the high rates of cache movement, Tracy was lucky her seed lasted that long and her strategy of saving food underground for the low food season seems to be a good one. Unfortunately for her, as soon as she begins digging up her cached seed she gets chased away by another more dominant agouti who steals her seed. Even sadder is the fact that Tracy came back to the old cache location a few seconds later to see if it was still there (it wasn’t). While this isn’t fair to Tracy, the other dominant agouti got a free meal on the cheap. The degree to which our agoutis used this kleptoparastic strategy is unclear, but given the behaviors seen in this video, we think it might be a good idea if agoutis used the same sorts of deceptive behavior found in squirrels.

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Rodent Mobility Assistants

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.

 

Team Agouti in BCI says “Farewell, Dr. Hirsch!”

October 3, 2010. Ben Hirsch, the post-doc on the major seed dispersal project bid his farewell to the island residents and flew to Washington D.C. where he lives currently.

Although his love for coatis knows no bounds, for he studied this mammal species in Argentina for his PhD, when it comes to agoutis, he cannot hide his emotions. In fact, he is ready to give one a jolly ride on his shoulders!

Apart from his work talents, his immense knowledge on a wide variety of wines and good restaurants in Panama make him a popular individual. He is a go to person for information after the weekly Tupper seminars in Panama City.

In order to keep his spirit and talents around after his leave, we embarked on a scientific journey to clone Dr. Hirsch. But the results were not as we had expected. We didn’t acquire his charisma and talents. But we were happy to have acquired his facial features!

 

Caching strategies, insect seed predators, wet season and additional projects

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.

                                                              

 

An agouti caught in action and many more

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.

 

Seed Movement Variation with Astrocaryum Tree Density

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

 

Astrocaryum standleyanum black palm fruit census in 25 ha plot in BCI

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.