Haven't seen 'leur' before.. How was I suppose to know? You stole a heart from me duolingo! :'(
I made it plural and feminine "Leurs chiennes mangent ... " . Would that not be correct.
If it were feminine, you would hear the "n" at the end of "chiennes" more clearly, so it would have to be either masculine singular or masculine plural.
L'eau sounds exactly like "low." Leur you hear the "r" Rhymes with "burr" (like you are cold).
You can hear a difference between chien and chienne. Chien is shorter and more nasal. Also, French speakers rarely use chienne unless it is somehow important that the dog is female.