We saw a few of these little bats in Tortuguero
at the beginning of our trip. After that we journeyed to the Arenal region and signed up for another boat tour in Cáno Negro during which we saw many more of them.
The guide on the second trip told us these little bats lined up to resemble a poisonous snake. They remain motionless until threatened, then wiggle their bodies and that motion re-inforces their masquerade act. As our boat drifted close to observe them that was exactly what they did! The boat, of course, backed off right away because we didn't want to upset these little guys any further. 
