Hungry Rainforest Rodents Most at Risk of Becoming a Meal Themselves

On an island in the middle of the Panama Canal, the battle between predators and their prey is playing out over the airwaves, as predatory ocelots and the tiny rainforest rodent agoutis are tagged with radio transmitters and researchers document the cycle of their life and death.

For the little agouti, death comes before dawn and after dusk, according to the results of a study by the Smithsonian Tropical Research Institute in Panama.

The researchers found that when the agoutis leaves its burrow earlier in the day to forage for the tree seeds it eats, it is more likely to become the next meal of an ocelot.

"Agoutis eat tree seeds. Ocelots eat agoutis," said Patrick Jansen, research associate at the Smithsonian Tropical Research Institute and assistant professor at Wageningen University. "Where food is hard to find, agoutis spend more time foraging and are more likely to be eaten by an ocelot."

Jansen and his colleagues studied the daily movements of ocelots and agoutis on Barro Colorado Island by tracking them 24/7 with radio collars and viewing their movements with camera traps step up around the rainforest island.

They determined that during that during the day, there were few active ocelots, but many active agoutis. Around sunset, the agoutis would retreat back to their burrows and the activity of the nocturnal ocelot would spike. These levels would reverse again around sunrise.

The radio transmitters affixed to the animals broadcast a signal to radio towers, which was then relayed to a server and made viewable online so that the researchers could monitor the creatures' movements whenever they wanted. When an animal stopped moving, researchers saw a flat line similar to those seen in an electrocardiogram of a heart-attack victim. When this happened, the team would go to the location of the animal and attempt to determine the cause of death.

Of the 19 dead agouti found during the study period, 17 of those were killed by ocelot. Most of these kills happened before sunrise or after sunset, when the ocelot is most active.

By placing cameras near agouti burrows and documenting the rodents' exit and reentry times, as well as observing the amount of available food resources in the vicinity of their burrow, the researchers were able to piece together a clearer picture of exactly what was happening to the agoutis.

They found that agoutis in areas with less food left their burrows earlier in the day and returned later as well, which placed them at greater risk of becoming the meal of an ocelot.

"We knew that hungry animals tend to take more risks" Jansen said. "But this is the first study to so thoroughly document the behavior of both predator and prey."

The research is published in the journal Animal Behavior.