Foxes breed only once a year and can mate when they are only 19 months old. Their mating season is around January and it's around this time you'll tend to hear those bloodcurdling mating calls and barks.
When the vixen is on heat, the dog fox stays very close to the vixen rarely straying more than a few meters from her at any time. At this time males have a six-fold increase in testes size as sperm production increases. The female is receptive for only around three or four days and when mating takes place the dog may sometimes 'tie' or 'lock' with the female.
In this deft manoeuvre the dog will bring one hind leg over the vixen's back onto the same side as his other hind leg, allowing him to face away from the vixen still coupled. They may remain this way for over an hour. This behaviour is only found in canids and it's not known exactly why this happens. One theory is that it may improve the chances of the vixen becoming pregnant.
The male red fox also has a penile bone (known as the baculum) found in many placental mammals such as dogs, cats, monkeys, bears and many more.
The vixen's pregnancy lasts 53 days, which is 10 days fewer than the domestic dog. A litter can be between four to six cubs.