Spoilers

“Fearful Symmetry”

Invisible elephants? Invisible tiger?

Mulder and Scully investigate a zoo where some weird things are going on. Mulder believes that the animals of the zoo are being abducted by aliens and artificially inseminated, having the babies taken for some reason.

The action centers around a gorilla named Sophie, who could communicate via sign language and was scared of the light.

This was quite a tragic episode with a lot of animal death and discussion of mistreatment. 

Not sure why the animals were invisible. Overall the episode was below average.