Fox Sex

Fox Sex

Lateral (side-on) view of a Red fox skull.
- Credit: Marc Baldwin

Dorsal (top-down) view of a Red fox skull.
- Credit: Marc Baldwin

Ventral (bottom-up) view of a Red fox skull.
- Credit: Marc Baldwin

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The question of how to tell the sex of a fox in the field is one frequently asked, but to which there isn't a straight answer. Over the years, many authors have professed sure-fire methods for telling vixens (females) from dogs (males), but in practice there is very little sexual dimorphism in Red foxes (i.e. the males and females look very similar). Some proposed methods of separation, the origins of which are lost in antiquity, have since been proven untrue. A white tip on the tail isn't, for example, indicative of either sex—this “brush tag” can be present in both dogs and vixen, or may be absent altogether. Similarly, a proportionally shorter coat and squatting while urinating are often, but not exclusively, associated with vixens. Indeed, in his 2000 book, My Life With Foxes , New Forest naturalist Eric Ashby noted how his captive dog fox would squat like a vixen, depositing only a little scent, for most of the year and only cocked his leg to urinate during the breeding season.
In his 1906 Foxes at Home , Colonel Talbot described how vixens carry their heads higher than dogs, have thinner necks and hold their brushes differently. Subsequent authors, Roger Burrows for example, have failed to notice any significant differences in those features. Burrows also mentioned how some hunters claim to be able to sex a fox based on close study of its paw print (that of a vixen being smaller than that of a dog), but pointed out that this was not something he could ever do.
Brian Vezey-Fitzgerald, in his 1968 Town Fox, Country Fox , provided several intimations given by others on how best to sex a fox in the field, adding to the aforementioned suggestions that there is apparently a belief among country folk that golden cubs are always vixens. Fur colour is not sexually dimorphic in foxes and, as such, golden cubs can be of either sex. Size is sometimes used to infer sex, insomuch as dogs tend to be larger and (approx. 17%) heavier than vixens, as are the type of calls each sex make . With the possible exception of the wow-bark call, however,these are generally unreliable indicators of sex. Vezey-Fitzgerald came to the conclusion that many naturalists still hold today:
“ I remain firmly of the opinion that you cannot tell dog-fox from vixen at sight in the field with any certainty. ”
Similarly, in 2008, Norfolk-based naturalist John Hampshire described watching two foxes in a post-copulatory lock being pestered by a third male and wrote:
“ As an aside, it was interesting to see how close in size the three animals were, I don't know whether the vixen was particularly large for her sex but she appeared only marginally smaller than the two dogs. It made me realise that those people who confidently sex every fox they see are probably making a lot of mistakes. ”
There are, nonetheless, a couple of well-cited methods that are apparently fairly accurate. The first, and by far the most straightforward and obvious, is to look for the cream-coloured fur of the scrotum (which identifies a male) or the teats of a lactating female; these features are, however, not always apparent, being prominent only during the breeding and cub-rearing seasons, respectively.
The second method involves looking at the face of the animal; more specifically, the breadth of the head and the position of the ears relative to the nose. In 1962, the eminent naturalist and fox farmer Henry George Hurrell noted that males generally have broader heads and thicker muzzles than females and, in 1968, Burrows described how this broadness causes the ears of a dog fox to form a W-shape when viewed head-on, whereas they form a V in vixens whose heads are generally more triangular in shape. Several authors have apparently used these features to successfully sex foxes in the wild and Stephen Harris included it in a brief article to the May 2007 issue of BBC Wildlife Magazine , although in The Red Fox , Lloyd cautioned:
“ There certainly is such a difference, but it is not often easy to distinguish the sexes in isolation.”
Even when the sexes are together, head broadness may not be a reliable guide as the photo comparison below shows. Photographer and fox-watcher Alannah Hawker, who kindly provided me with one of the photos, told me:
“ I've found the head sizing as not much use, we had a female a few years ago with a very broad head, but she was definitely a vixen. ”.
The thickness of the neck can sometimes be used in conjunction with head broadness to identify a male. In the example below, the male has a substantially bulkier neck than his mate, which is a combination of fur and hypertrophied musculature. This does, however, vary on an individual basis and, presumably, in accordance with dominance.
If you happen to be examining the skull of a fox and have a pair of calipers to hand, taking some measurements and applying them to a simple calculation allows for the separation of vixens and dogs in 70% to 90% of cases. Early work on farmed and wild foxes by Canadian researcher Charles Churcher revealed that taking three measurements of the skull (the total length, zygomatic width and mastoid width—click link below) allowed the correct sexing of 101 of 113 specimens (i.e. was about 90% accurate). Churcher looked at various measurements of the skull and found that he couldn't accurately separate males from females based on a ridge of bone that runs along the top of the skull called the sagittal crest.
Subsequent authors have since found skull measurements to be of considerable use and, in their contribution to the fourth (2008) edition of Mammals of the British Isles , Stephen Harris and Phil Baker note that, in animals 10 months and older, males can be separated from females by the sagittal crest with an accuracy of 70%. Harris and Baker do note, however, that the crest is poorly developed or absent in males younger than 10 months, causing their skulls to look like those of vixens. This is a finding echoed by many naturalists, who note that young males have faces more similar to vixens; the skull broadening with increasing age.
In a 1980 paper to the Journal of Zoology , M.A.F.F. biologists L.W. Huson and Robert Page noted similar age-related variation; they found that male skulls changed more with age than those of vixens. More recently, a team of Czech researchers found that, in both sexes, the skulls grow rapidly in length during the first six months of life, reaching full length at around this age; the skull then continues to grow in width. The researchers also noted some differences between the sexes: in males the jugular breadth, for example, increases after six months old, while in the female it does not. Consequently, sexual dimorphism was evident in skulls at the age six-and-a-half months to one year old.
In an earlier paper to the same journal, Huson and Page were able to correctly sex 80% of the 192 skulls they collected from south-east England using only two measurements (total length and palatal length) and, by adding two more measurements (zygomatic width and condylo-basal length) they were able to correctly sex 88% of 379 Welsh fox skulls. Similar success was achieved by John Lynch working in north-east Ireland during 1996.
In his study of 204 adult Red fox skulls held at the Ulster Museum in Belfast, Lynch found that dog skulls were larger, longer and had a narrower post-orbital constriction than vixen skulls; based on these characters, he was able to correctly sex 87% of his skulls. Lynch considered that the narrower inter-orbital constriction could permit the dog fox a greater bite force and therefore allow it to handle relatively larger prey. If you're interested in the calculation Lynch presented click the link below.
In a 1996 paper to the journal Biology and Environment, University College Dublin zoologist John Lynch studied the skulls of 210 foxes in the collections of Ulster Museum in Belfast. Lynch came up with a mathematical discriminant function to predict the sex of a fox based on its cranial morphometrics. In other words, he devised an equation that can give you a good idea of whether the fox skull in front of you came from a male or female animal.
We don't need to worry about the maths of it (which is just as well, as I'm no mathematician!), all we need are two measurements to get us our D value. We need the total length of the skull and the constriction in the top of the skull just behind the orbital processes; both in centimetres. Here's what I mean:
So, if we take these two measurements and plough them into the equation, we get the following:
According to Lynch's data, a D (sex indicator) value of greater than zero (i.e. a positive number) indicates a male, while less than zero (a negative number) indicates a female. Therefore, in our example, we most likely have the skull of a vixen. This method isn't foolproof, but Lynch was able to correctly sex 86% of vixens and 87% of dogs.
The Czech research mentioned earlier found similar results to those reported by Lynch and, in a 2010 paper to Folia Zoologica ; they noted that male skulls were significantly larger than those of females in all measurements they took except one: the postorbital breadth, which was wider in females than males. The anatomists wrote:
“ The narrower postorbital constriction and a longer and higher sagittal crest enlarge the area for muscle insertion and the male jaw becomes stronger. ”
In other words, having this narrower area of bone just behind the eye sockets leaves more room to anchor a large masseter (chewing) muscle, thereby potentially allowing the fox to take relatively larger prey. The suggestion is that this may allow for some niche separation between the sexes, i.e. males taking larger prey items than females, and more recent data suggest that, in some populations at least, males certainly have a wider diet breadth than females. Working in northeastern Poland, for example, Dorota Kidawa and Rafal Kowalczyk found that the diet of both adult and juvenile males was significantly broader than that of females, with females taking mostly voles while males took a wider range of mammals and birds. In their 2011 paper to Acta Theriologica , Kidawa and Kowalczyk wrote:
“ We noted that the diets of adult males and young foxes converged; both had large food niche breadths and hunted bigger and potentially more difficult to obtain prey such as birds and other mammals, whereas females concentrated mainly on small mammals .”
A similar observation was made by Jesse Forbes-Harper at Murdoch University and her colleagues recently. In a 2017 paper to the Journal of Zoology , Forbes-Harper and her co-workers present their analysis of skull morphometry and diet from 540 foxes shot as part of a government-run control programme in Australia. The researchers noticed a dietary separation between males and females, with dog foxes taking more sheep carrion (47-65% of the diet by volume) while adult vixens took more mice and invertebrates.
Interestingly, Forbes-Harper and her team also observed that males had generally larger skulls, with higher sagittal crests that allowed for greater attachment of the temporalis muscles that open and close the jaws. The result was that male foxes had a greater estimated bite force than females. Very approximately, based on the box-plot they included in their paper, males had a bite force between about 218 and 290 Newtons, while the female range was roughly 195-250. When considering only adults (2 years or older), there was a clear separation: most vixens had a bite force between 230 and 250 Newtons, while most males fell in the range 260-290. Forbes-Harper and her colleagues note that fox skulls change as the fox grows up to handle a more carnivorous diet, with relatively larger jaws and a more robust skull.
Finally, it is not just skull measurements that can be used to separate the sexes. In a 2008 paper Elwira Szuma looked at the teeth of just under 3,000 Red foxes collected from across the Northern Hemisphere and found that those of males and females differed from one another, most notably the canines and carnassials; overall, male canines were 3.6% larger than those of females. Szuma also found that there was more of a difference between the sexes in the Palaearctic (Europe and Asia) than in the Nearctic (North American) foxes.

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The mating season for foxes occurs in December through February. In January foxes from previous litters disperse and go off on their own. This is when the parents will start to mate again. When foxes breed they have mating calls that sound like loud cries. The babies are born in the spring.
Not all species of foxes breed at the same time of the year, but most of them breed during the winter months and have their babies in the spring. In the wild, during the winter months, you can hear foxes making loud screeching noises, this is their love calls/mating calls.
Foxes have been known to be monogamous, meaning they mate for life. Foxes live in pairs, the vixen (female) and the tod fox (male.) Usually, a dominant male fox will choose the dominant female fox to mate with. The two will spend their lives together, having multiple litters throughout their lives.
It is thought by many that if the vixen dies, the male will stay single for the rest of his life. However, if the male dies, the female will most likely find another mate, and keep producing litters. Sometimes females from previous litters will stick around to help raise the new litter, like nannies.
Around October, foxes begin to get a case of the crazies. They start to become much more vocal, their behavior starts to get more aggressive, and their scent glands start to produce a more thick and heavy smell. This can last until they are done mating.
When two foxes get locked together and begin mating, they start to scream unpleasant sounds. Many believe this process is not something they enjoy, because of the amount of stress in their voices. They usually stay locked together for an hour or more.
After they have mated, the female vixen begins to prepare her natal den. Most female foxes will choose a special den to raise their young. These are called natal or maternity dens. Most foxes do not use their dens for sleeping, they use them primarily for raising their kits (baby foxes.) Read more here .
The mating season for most foxes starts in December and can last through February. The foxes who mate in winter, have their babies in the spring. They have a short gestation period of around 45-50 days. Read more about fox pregnancy here .
Foxes that mate in December have their kits around February or March, while those who mate later in the season have their kits in April or even May. Arctic foxes mate much later, usually in March or early April. They have their kits around May or June.
Foxes mate very similar to the way dogs mate. They get locked together or “stuck.” They mate in the tail-to-tail position. This is called a tie, or a copulatory lock. It usually lasts for about an hour, and sometimes can go longer. They are very noisy when mating and will make some terrifying screams.
Being stuck together can be a little bit stressful for animals. Once they are confident that the deed has been done, they will unlock. Male foxes only produce sperm during their mating season. Female foxes only get their estrous cycle for about one week, this is when their hormones make them ready to breed.
Arctic foxes breed later in the year because foxes that live in northern climates get their estrous cycle a few months later.
The red fox is the most common fox. Red foxes are bigger than most other fox species. Mating season for red foxes usually starts in January. They have their kits sometime in March or April. Red fox females are sexually mature at just 10 months, and most foxes start breeding at one year old.
Red foxes give birth to 2-12 kits per litter. The baby foxes are born a brown or dark color, then later they molt and start to get the color of their parents. Read more about red fox babies here .
Arctic foxes live mostly in the northern hemisphere where it is extremely cold. Some arctic foxes migrate into Canada, where they give birth inside of their natal dens, prepared in areas that are slightly less cold. For arctic foxes, mating starts around the end of March and into April .
Their gestation period is 49-57 days. The babies are often born a brown or charcoal color. Later they either take on the white morph or the blue color morph, which is a charcoal gray.
Fennec foxes live in desert regions and are the smallest of all of the fox species. The mating season arrives in January and February. During this time the male fennec foxes start to get aggressive. After mating, they have their kits around March and April.
Their gestation is around 50 days . The litters consist of about 2-5 kits. Fennec foxes can sometimes combine families and have large family units. Their dens can be complex with many entrances.
Gray foxes pair up and find their mates in the fall, and start breeding in the winter months. In the later winter months, males become aggressive, looking for a mate to breed with, with some competition. Most foxes are monogamous, but the gray fox has been known to form polygamy relationships and even polyandry (although this is rare.)
The gestation period for gray foxes is around 53 days and they have litters of 4-6 kits. Gray foxes mate in January and up to May. Most litters are born in March and April, but those who breed later, give birth in the later months.
Foxes can not mate with domestic dogs or any other wild canids. Foxes do not share the same number of chromosomes as other canids, making it impossible for them to produce offspring.
Foxes mate once per year, although in rare cases, such as with fennec foxes if the litter does not make it, they may mate a second time. Animals in nature do what they have to, to survive.
In some cases, hybrid foxes do exist. Arctic foxes can mate with red foxes, however, the offspring will be sterile and will not be able to produce their own offspring.
Mating season can cause some wild behavior in foxes. The males start to see other male foxes as competition and will get aggressive. After mating season their behavior starts to return to normal.
Chad Fox is an author and researcher dedicated to bringing reliable information about foxes to the public. He supports animal sanctuary awareness.
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