Camera Traps


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.

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.


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.

In legend, vampires cruise the cities at night hunting victims, preferably someone sexy, whom they can bite on the neck for a bloody meal.

In reality, vampire bats do drink blood, but prefer a fat ungulate over a slim sexy human.  Cattle are lazy fat ungulates that can be quite abundant, and are a favorite food of the common vampire bat (Desmodus rotundus) throughout their range in the new world tropics.  Vampire bats are fairly easy to see and study around cattle fields, here are two pictures I took of vampire bats, the one on the left is in the process of feeding on a cow in Costa Rica.

However, feeding on an active animal is more difficult than a sleeping cow.  Vampire bats are one of the most agile bats, amazingly good at moving around on the ground.  One of our camera traps under a fruiting tree recently captured a tapir coming in for dinner, followed by a vampire bat, coming in for dinner.  You can see the bat darting in and out to drink from a cut on the tapir’s foot and then jumping back out of the way to avoid getting stepped on. The tapir seems oblivious to the bat.

This bat puts on a good show, and this is a pretty special video.  Indeed, Dr. Bill Schutt, vampire bat expert and author of Dark Banquet said “Most videos of vampire bats feeding show the bats attacking stationary prey (like sleeping cows). This segment is quite unique since the imperiled tapir is clearly a moving target. Great stuff!”

Animals facing changes in food abundance over the year can either defend an area large enough to always have enough food, or fight to expand their territory when times get tough.

By the start of the dry season in Panama there is little new food for agoutis, and they are largely living off the seeds they have hidden in caches earlier in the year.  Then Dipteryx starts fruiting, offering a fresh fruity exterior with a nutritious nutty inside.

This is the time of year where you can see hoards of agoutis hanging out under Dipteryx trees, and some have speculated that they give up on their territoriality in this time of year and all fatten up on Dipteryx like grizzly bears at an Alaskan salmon stream.

Annemarie found this wasn’t exactly the case.  Agoutis were 5 times more common underneath Dipteryx trees than other locations, but radio-collared agoutis didn’t abandon their territories to camp out under these trees.  Some animals would boogie out of their normal range for a quick breakfast at a nearby tree, but returned to spend most of their day back home.  The degree to which they changed home range depended on how many Dipteryx there were nearby, and also the local density of Astrocaryum trees, their primary cache fruit.  One agouti seemed to do just fine without any Dipteryx in his home range at all , presumably living off the numerous seeds he cached earlier in the year.

Congratulations to Annemarie on a very successful thesis!

Animal density can be accurately estimated from camera trap photos. That is what our intern Lennart Suselbeek argues in his MSc thesis. Lennart took all 862 paca visits that our camera traps recorded in ten different 1-ha plots on BCI ; over 6000 camera traps nights in total. Lennart went through all photos to try individually recognize these animals by the unique spot patterns on their sides (see picture). He found 54 different individuals.

Then, Lennart estimated paca densities for each plot in two ways. First, he used a generally accepted Capture-Recapture model (SECR) that uses the individual identities. Then, he used a new “Random Encounter Model” (REM) that uses visitation rates, movement speeds and activity levels but not individual identities. The two estimates nicely correlated across the ten sites, which suggests that the REM, developed by our collaborators Marcus Rowcliffe and Chris Carbone, does a good job. This means that we can use camera traps to estimate densities of mammals that we cannot recognize.

And so did Lennart. He just received his MSc degree from Wageningen University and immediately started a PhD on seed dispersal by rodents with Patrick Jansen. Congratulations Lennart!

Paca in the lab clearing

Paca in the lab clearing

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