Male dog chasing recently fixed female???

I recently had my female dog spayed (A day or so ago) but from the moment I brought her home, (since I brought her through the door) my unfixed male dog has been acting as though she's in heat. Popping his jaws at her, trying to sniff at her or spots she's been laying, attempting to bait her with food or play. He even attempted to mount her to no avail. (I was standing right there) I've been keeping them separated in different rooms, but it doesn't deter him. We have one smaller unfixed male as well, but he's showing no interest in her at all. I don't understand why my male is acting like this. 

1
Mapping: DefaultPageMap
Map Field: TopLeft
Ad Slot: PW1_RON_Top_Left
Size Mappings: Desktop Only
Mapping: DefaultPageMap
Map Field: TopRight
Ad Slot: PW1_RON_Top_Right
Size Mappings: Top_Right
Submit your own photos!
Dog Breeds Selector

Find your perfect match based on activity level, size, intelligence and more!

Mapping: DefaultPageMap
Map Field: BottomRight
Ad Slot: PW1_RON_Btm_Right
Size Mappings: Btm_Right