 By the time they enter their oestrus period, their uterine horns double in size, and their ovaries grow 1.5–2 times larger.Sperm formation in males begins in August–September, with the testicles attaining their greatest weight in December–February.The vixen's oestrus period lasts three weeks, during which the dog-foxes mate with the vixens for several days, often in burrows.