Their natural urges proved deadly for two dogs that went on a mating frenzy in the middle of Kamo's Three Mile Bush Rd.
Oblivious to passing traffic, the dogs were bowled by a car outside Kamo Primary School about 6am yesterday.
People who stopped, including those in the vehicle that hit the dogs, called the SPCA to say one animal was dead and the other injured.
They then delivered the dogs to the SPCA but inspector Gill Loney said both animals had died by the time they arrived.
She said they appeared to be well fed and cared for, and not strays.
However, neither had a microchip so the SPCA has been unable to trace their owners.
"It's a very good example of why people should have their animals desexed and microchipped," Loney said.
As required by law, the SPCA will keep the bodies for seven days in case the owners search for or want to claim them.
Whangarei District Council health and bylaws team leader Reiner Mussle said dog control staff were not called.
"Car accidents, stock attacks, dog bites are all risks when dogs wander.
"Every dog owner must make sure they know where their dog is at all times, and take proper care of it - including being aware that dogs often try to escape and precautions must be taken against that."
Unneutered male dogs can smell a bitch on heat from several kilometres away and are usually very determined to escape any containment to follow the hormonal drive.