In the Northern Hemisphere the breeding season for Red foxes typically runs from November until early February, with the majority of conceptions happening after Christmas. Indeed, James Fairley's studies of foxes in Northern Ireland over three years in the mid-1960s found that virtually all vixens fell pregnant between 4th January and 15th February, and the majority of those were between 7th January and 8th February. Studies elsewhere paint a similar picture, although there are exceptions, with some vixens reported in season as early as mid-October.
Writing in 1863, German reproductive biologist Theodor Bischoff noted that mountain foxes bred later in the year than those in the lowlands, although Idwal Rowlands and Alan Parkes (in their 1935 paper on the reproduction of the fox) found no significant difference between the breeding dates of foxes in Scottish and English farms. Rowlands and Parkes do, however, mention a silver fox farm in Sussex that, in 1934, recorded matings as late as 19th March.
Former MAFF biologist Huw Gwyn Lloyd calculated that vixens come into season from about 94 days after the summer solstice, which means they can come into season any time from late September. In a study of 610 vixens from Wales, Lloyd found only two had conceived during early November, while a vixen shot on Boxing Day 1962 at Llandudno was near full term, suggesting she'd mated during mid-October. A few Welsh vixens were found to be pregnant during mid-April (suggesting conception during late February) and there are unconfirmed reports of pregnant vixens in July, August and September.
In a 1989 paper to the Journal of Reproduction and Fertility, Mats Forsberg and his colleagues report that long summer days followed by short winter ones are required to 'reset' the foxes' breeding season and that artificially manipulating the photoperiod (i.e. the length of the days) or manipulating melatonin levels by injection can prevent testicular regression and extend the breeding season. Work by Larissa Kolesnikova on the Russian fox domestication project produced similar results, suggesting an increase in day length triggering enhanced melatonin production and subsequently sex hormone production. Thus, it seems that it is an increased sensitivity to shortening day length that triggers breeding in the Red fox. Interestingly, the domestication project found that tame vixens often came into season earlier than controls, although it was exceptionally uncommon for them to conceive more than once per year.
The precise timing of breeding is, as we shall see later, closely tied to the condition of the vixen, and so the breeding season may be delayed where food supply is intermittent. Working in Scotland during the 1970s, for example, Hugh Kolb and Ray Hewson found that foxes bred later in the west of the country than in the north-west; they considered this was an adaptation to a more intermittent/unpredictable food supply (voles, in the west).
Generally speaking, foxes in the north of the species' range breed progressively later than those farther south—in the UK, Scottish foxes may breed up to one month later than those in the far south. Thus, at high latitudes, the breeding season may run into late March, although most of the mating activity occurs during December and January; in the UK, dogs are at the peak of their reproductive potential during January. In Australia, most mating takes place between mid-June and the end of July.