1. May loose some appetite.
You dog has mating on mind and may not eat all his/her food if another dog is available for mating. Somehow there belly seems to be filled with love and your dog food may end up being eaten by ants or other rodents. My recommendation is to feed sparingly until mating season is over.
2. Dog drinks more water
The thirst is real. Your dog in heat will be more dehydrated and will pant more to keep cool, hence will probably drink much more water than usual, especially if you reside in a warm climate.
3. Keeps following The Opposite Sex
If you have a bitch at home, and she is in heat, she will probably be following the bull. And that is vice versa. Dogs in heat will follow each other up and down until they get their mating satisfaction.
4. May Not Notice Burglars
Some dogs in heat may forget that they are on guard duties as well. All it takes is for the burglar to burglarize your home is to bring a dog also in heat of the opposite sex and they have full access to your home.