[You must be registered and logged in to see this image.]When Roland Kays wanted to investigate the cycle of life and death in the rainforest, he decided to track an expert in the subject – the ocelot. And he discovered that this little cat’s killing ways help to keep the forest healthy.
Death is inescapable. Evolution puts a premium on avoiding death longer than the competition, leading to fascinating adaptations for escaping death, such as developing antibodies, camouflage or swift movement. While the death of an animal is a fleeting, unpredictable event that is almost impossible to study, some animals experience death on a daily basis: predators.
Predators must kill for every meal. On Barro Colorado Island (BCI) in Panama, the ocelot is one of the most successful predators, eating most small and medium-sized animals. This little cat thrives in almost every habitat type: thorny scrub, swampy grassland and ever-wet forests. Its adaptability means it can deal with human changes to the landscape, and it’s often the only predator in marginal forests or agricultural areas.
The life of an ocelot combines nocturnal hunting with long catnaps in the daytime – though it remains always alert for pickings that come its way.
BCI’s ocelot population has grown in recent years, which is a nightmare for local rats, and other mammals. Ecologists estimate that an ocelot needs nearly 1kg of food per day, and that BCI’s ocelot population consumes 10 per cent of the available mammalian biomass every year.
The question of what exactly an ocelot eats is easy to answer – if you’re happy to pick through dung. Ricardo Moreno from the Smithsonian Tropical Research Institute has collected 190 ocelot scats on BCI over three years, identifying 23 different prey species in the samples, including mice, agoutis, monkeys, opossums, armadillos, anteaters, snakes and turtles. A calculation of the relative volume of different species’ remains in the scats revealed that the agouti was nearly twice as plentiful as the next species, and therefore a good candidate for a detailed study of predator-prey interactions with ocelots.
Variety of prey
Agoutis are large rodents (2-4kg), making them a prized meal for ocelots. Both ocelots and agoutis are predators, though at different levels. Agoutis are seed predators, favouring larger fruits. The agouti-seed relationship is more complicated than simple predator-prey.
Agoutis bury some seeds in holes throughout their 1-2 hectare territory. This transport of seeds away from the shade and parasites of the parent tree is great for the tree… until the agouti returns to claim its prize. Like squirrels, agoutis have good spatial memory and harvest their cached seeds later in the year, when there are fewer fresh fruits and seeds. But if agoutis ate all the seeds they stored, trees would not regenerate and forests would decline. Clearly, this hasn’t happened, so what is stopping the seeds being eaten?
We think ocelots could be the seeds’ saviours. If there is a high turnover in agouti territories due to predation, new agoutis entering a recently vacated territory wouldn’t know where to look for the previous owner’s seed caches, thus leaving them to germinate.
Gathering the evidence
So do agoutis die often enough to affect seed survival? To answer this question, we trapped agoutis and fitted them with radio-collars that, using automated radio telemetry (ARTS), allowed us to monitor where they were and if they moved or not. Combining off-trail tracking with remote real-time tracking, we could detect the death of a radio-collared agouti within hours of the event.
Finding dead animals quickly is critical to rainforest ‘crime scene investigation’, as corpses rot quickly in these humid, insect-rich forests. We found six of our study animals dead during our research. In five instances, the evidence indicated early-morning predation by ocelots, something we confirmed by catching the perpetrator returning to the scene on a motion-sensitive camera.
Once we even recorded a radio-collared ocelot killing a radio-collared agouti. In total, we estimated that 70 per cent of agoutis die from predation every year.
Next, we needed to track the seeds as the agoutis moved them from hole to hole. Using ARTS again, we tied a tiny, motion-sensitive radio transmitter to each seed. This transmitter conserves its battery by turning off for the weeks or months that the seed is buried, only triggering when it is dug up and eaten or moved and reburied. When this happens, the ARTS sends an email to alert us, and we go out to find the seed and record its fate.
By tracking the predation of seeds by agoutis, and of agoutis by ocelots, we hope to highlight another reason why predators are important for maintaining healthy forests. One animal being eaten by may horrify us, but it is a natural event. Avoiding predators is the most important thing an animal must do. Miss the odd meal and you can make it up, but fail to avoid a predator once and the story is over.
Fri Nov 09, 2012 8:42 am
