We finally came across this Myotis bat during a bat research trip to San Pedro, Misiones.
This is a large bat for the Argentine genus that weighed 10 grams and had a forearm length of 41.3mm, which is large even for this species. All of the other distinguishing features were typical Myotis such as the semi-furred uropatagium, dentition, tragus.
The main distinguishing feature is the brick red coloured fur, unicolour on the dorsal side and bicoloured on the ventral side.
It can be confused with Myotis simus, which also has bright coloured more yellowish fur, but in that species the wing membrane is attached to the ankle whereas in ruber it is attached to the toes.
On release we recorded its echolocation and it came out at around 58kHz, which is similar to Myotis riparius.
We have since come across several examples of this species in Misiones, including in the Provincial Park of Urugua-i.