When the sun goes down in the subtropical forests of Puerto Rico, hundreds of thousands of bats emerge from the caves that stud the island’s northern end. After a day of sleeping, the animals are ready for a hard day’s night of hunting insects. For some of them, though, there will be no feast of beetles and mosquitos, and they’ll instead wind up a meal themselves for the snakes that have set up an ambush at the cave’s entrance.
After providing a warm, safe place to sleep all day, the caves become death traps once darkness falls. Puerto Rican boas slither in from all over the forest to turn the bats’ exodus into their own hunting ground. At the mouth of the caves, they lay on the ground, cling to roots and vines on the walls or hang down from the ceiling, their tails finding footing in the tiniest cracks and crevices. As the bats flit past them on all sides, the snakes wait, swinging like clock pendulums. When a bat brushes by or collides with a snake’s head, the snake grab it in its jaws, squeeze it in its coils and eats it whole.
The nightly ritual is played out on its largest scale at La Cueva de Los Culebrones, or the Cave of the Long Snakes. The cave’s estimated 300,000 bats can empty out in as little as three hours, providing the boas with an all-you-can-eat bat buffet. Biologists observers have seen as many as 20 boas hanging around the cave entrance, grabbing all the bats as they care to and chowing down.
Even with the steady stream of food flying right into their mouths, the snakes can be picky, and competitive. Several species of bats can inhabit the same cave, and the smaller ones tend to exit first. Although the snakes are in position and waiting as soon as the first bats, they often wait half an hour to an hour to start hunting, when the larger bat species take flight. This might be a strategic choice to save their energy for prey that gives them more calories in reward for their effort.
Sometimes, a snake that’s not having any luck with its own hunting will attempt to steal a bat from another snake. On one occasion, a biologist watched three snakes fight over a large bat carcass for over an hour and a half. By the end of it, the snakes were exhausted and the bat went uneaten.
Last summer, Neil Losin and Nate Dappen, a pair of biologists and filmmakers who run Day’s Edge Productions, were in Puerto Rico and heard stories about the snakes, and had to see and film it for themselves. The resulting video, Snakes in a Cave, is awesome, and captures some closeup, slow motion bat snatching action.
Reference: Rodriguez-Duran, A. (1996). Foraging Ecology of the Puerto Rican Boa (Epicrates inornatus): Bat Predation, Carrion Feeding, and Piracy Journal of Herpetology, 30 (4) DOI: 10.2307/1565698