Excerpts from Jim Conrad's
Naturalist Newsletter

from the May 2, 2010 Newsletter issued from Hacienda Chichen Resort beside Chichén Itzá Ruins, central Yucatán, MÉXICO; limestone bedrock, elevation ~39m (~128ft), ~N20.676°, ~W88.569°
A BROWN BAT
A brown bat hung from a rafter in a room adjoining the old church, so I took the flash-assisted picture shown below:

probably the Argentine Brown Bat, EPTESICUS FURINALIS

Best I can tell, that bat really is sticking its tongue out at me. Also, best I can tell, it's the Argentine Brown Bat, EPTESICUS FURINALIS. There's no field guide to Yucatec bats, however, and there are similar species. Internet pictures of Argentine Brown Bats look like this one, plus that species is known to be common from Mexico to Argentina, and often roosts in buildings, so that's a good guess.

Also, the tragus -- that little triangular flap of flesh visible inside the base of each ear -- is the right size and shape for that species, and that's an important feature.

I think that Argentine Brown Bats must be this area's "standard bat" -- average size and shape, coming out at dusk, catching insects, and doing all the other normal batty things. I'll need to see a few more, though, before figuring out if sticking the tongue out is normal for them.