At the tropical island, located in the Panama Canal, the team combined transmitters and cameras to follow agoutis, cat-sized rodents that feed on large seeds of forest trees. Agoutis are active during the day and take shelter in burrows during the night, when ocelots, their felid enemies, roam the forest. The agoutis’ challenge is to gather sufficient food while minimizing the likelihood of encountering an ocelot.
To determine which times are dangerous for agoutis to be active, the scientists first recorded daily activity patterns of agoutis as well as ocelots. Camera traps were placed in numerous locations across the island and photographed and timed all animals that passed in front of the lenses.
The photographs showed that during plain day thousands of agoutis were active, simultaneously with just few ocelots. Around sunset, however, as the agouti population gradually went to sleep, the ratio of ocelots to agoutis soared to a 10,000 times higher level, and declined equally fast around sunrise. Clearly, not only was the night dangerous for agoutis to walk around in the forest, so were the beginning and end of the day.
Crime scene investigation
The scientist also equipped dozens of agoutis with radio-transmitter collars to continuously record their activity, using an automated radio telemetry system. This allowed them to determine exactly whether and when a tagged agouti was killed: the radio signal would stop varying. When such a flatliner occurred, an investigator would hurry into the forest to investigate the crime scene and determine by traces what was the cause of death. They also placed a camera at the carcass to see who would return to eat the remains.
Thay found that 17 of 19 dead agoutis had been killed by ocelots. Most kills happened just before sunrise and just after sunset, even though relatively few agoutis are normally active. Being active during twilight was clearly risky business.
Finally the scientsits compared daily activity patterns of agoutis between parts of forest with contrasting abundance of food, the large seeds of a certain palm. They did so in two ways. They determined at what times radio-tagged agoutis in these areas entered and exited a burrow, by determining when the radio signal suddenly dropped or reappeared. Second, they placed camera traps at the entrances of burrows in these areas and recorded, from the photographs taken, at what time an agouti entered and exited the burrow.
Both methods showed that agoutis in areas with less food left their burrows much earlier than agoutis in food-rich areas, and also entered their burrows much later. Poor agoutis had to work harder and longer to gather enough food. Because they were thus much more active during twilight, the scientists conclude that poor agoutis are more likely to get killed by an ocelot.
“That hungry animals take greater risk was known for a while” says senior researcher Patrick Jansen, “but never before was this phenomenon documented in such a convincing way, supported by numbers and images”.
Jansen now wishes to study what the differences in predation risk mean for seed dispersal by agoutis, which bury seeds as food reserves in numerous scattered caches. “Once an agouti gets killed by an ocelot, it can no longer eat its food reserves. The seeds may thus get the opportunity to germinate and establish a new tree. Poor agoutis plant trees of which they will never see the fruits”.
The study was sponsored by the National Science Foundation (NSF) and by the Netherlands Organisation for Scientific Research (NWO).