Being in heat is a colloquial term for the estrus cycle in female dogs, so male dogs do not come into heat but instead are always fertile. Male dogs who are exposed to female dogs in heat might attempt to escape from their homes, become distracted or act aggressively.
Male dogs can smell a female dog in heat from long distances, and the scent can be distracting to a male dog that has not been neutered. Male dogs in this situation might become restless and anxious. Owners might notice a decrease in attentiveness and a lack of response to commands. Male dogs usually try to reach the female dog by any means, including digging or jumping over fences. If they are tied up, they might attempt to chew through the tether. They also often increase scent-marking behavior in an attempt to ward off potential rivals.
Intact male dogs might also become aggressive when exposed to a female dog in heat. This behavior is usually directed at other male dogs but can transfer to anyone they see as a threat. They are especially aggressive toward other unneutered males, so owners of intact dogs should prevent them from interacting with each other during this time. Male dogs might also increase sexual humping, even if they cannot access the females they smell.