The ferret has an elongated, cylindrical body with short limbs and a short tail. This shape allows the animal to slip through very small openings; furthermore, the body can be twisted 180° with ease. The sexual dimorphism is considerable: the adult male has a much larger body than the female and usually weighs about double: 1-2 kg compared to the 0.5-1 kg of the female. This difference is less marked between sterilised subjects. The ferret’s body temperature is 38.8°C (range 37.8-40°C).
COAT AND SKIN
The skin is covered by fur (Fig. 1) formed of a topcoat of longer guard hairs and a shorter undercoat. The length, thickness and even the colour of the fur change with the seasons. There is one variety of ferret, called the angora ferret, which has a particularly long topcoat without an undercoat; this animal also has a double nasal fold and fur on its truffle. Ferrets moult twice a year, in spring and in autumn. The tail often loses some of its hair between autumn and spring; this ‘tail alopecia’ is normal and should not be confused with the hair loss due to adrenal gland tumours.
The skin is very thick and smooth and should not show desquamation. It is rich in sebaceous glands that produce a greasy secretion, which is particularly abundant in sexually intact animals during the reproductive season. These glands, which are under the influence of sex hormones, are responsible for the strong musky smell emanated by both hobs and jills. Sterilisation almost completely eliminates the smell and greasiness of the skin. Without this intervention it would be quite awkward to keep ferrets in a house.
There are large anal glands lateral to the anus; these glands produce a yellowish, liquid secretion with a very pungent and persistent aromatic scent. The anal gland secretions are released only in particular situations, such as when the ferret is frightened or very excited, and is not responsible for the body odour of the sexually intact ferret. Removal of the anal scent glands will not change that smell and is contraindicated unless performed for medical reasons (abscesses, tumours).
The inferior surfaces of the limbs have well-developed, hairless footpads. Each toe has a non-retractable nail.
SKELETAL SYSTEM
The skull (Fig. 2) has an elongated shape, but the maxilla and mandible are relatively short. The dentition is that typical of a carnivore, with teeth suitable for tearing food rather than masticating. The deciduous teeth erupt at 20-28 days, while the permanent teeth erupt between 50 and 74 days. The adult dental formula is I 3/3, C 1/1, P 3/3, M 1/2.
The vertebral column is composed of 7 cervical, 14 thoracic, 6 lumbar, 3 sacral and 18 caudal vertebrae. There are 14 or 15 pairs of ribs (sometimes there are 14 ribs on one side and 15 on the other). The first ten pairs are joined to the sternum, the other four or five pairs are free and form the costal arch. Each limb has five digits. The ferret has a plantigrade stance.
DIGESTIVE SYSTEM
The ferret has five pairs of salivary glands: parotid, zygomatic, molar, sublingual and mandibular. The gastrointestinal tract is relative short and simple, being typical of pure carnivores. The stomach is simple and very expandable. The small bowel (Fig. 3) is about 182-200 cm and does not have a caecum. The large bowel, about 10 cm long and not easily distinguishable from the small bowel, is divided into the colon, rectum and anus. The intestinal flora is simple and is not involved in digestion.
The pancreas has two lobes joined to the body of the gland which lies next to the pylorus. The right lobe, which is larger, flanks the descending part of the duodenum. The left lobe is situated between the stomach and the spleen.
The liver is relatively large and has six lobes: left lateral, left medial, quadrate, right medial, right lateral and caudate. The gallbladder is pear-shaped with a volume of 0.5-1 ml and is situated between the quadrate and right medial lobes of the liver.
The spleen is fairly large, long, and flattened and lies on the greater curvature of the stomach; it is attached to the stomach and liver by the gastrosplenic ligament. The spleen may reach a considerable volume in elderly ferrets, even in normal conditions.
CARDIOVASCULAR AND RESPIRATORY SYSTEMS
The thoracic cavity is elongated, being narrow cranially and wider caudally (Figs. 4a and b). The heart is located between the sixth and ninth/tenth ribs, while the lungs are situated between the first and tenth/eleventh ribs. Instead of two bilateral carotid arteries, the ferret has a single central artery, called the innominate or brachiocephalic artery. The heart rate is 180-250 beats per minute and the volume of blood in the adult is, on average, 5-7% of the body weight.
The ferret does not have blood groups. The haematocrit is relatively high.
The left lung has two lobes (cranial and caudal), while the right lung has four (cranial, middle, caudal and accessory). The respiratory rate is 25-40 breaths per minute and the tidal volume is 10-11 ml/kg.
URINARY SYSTEM AND ADRENAL GLANDS
The kidneys have the typical bean shape and a smooth surface (Fig. 5). They are situated in the cranial part of the abdomen (the right kidney is slightly more cranial than the left and is in contact with the caudate lobe of the liver), surrounded by retroperitoneal fat. The left adrenal gland is located medially to the cranial pole of the left kidney, immersed in adipose tissue; it is supplied by one or two branches of the left adrenolumbar artery. The right adrenal gland is located more cranially, in close contact with the vena cava and is covered by the caudate lobe of the liver. The bladder is quite expandable; the daily production of urine is 26-28 ml.
REPRODUCTIVE SYSTEM
The testicles descend into the scrotum at about 6 weeks of age, after which the inguinal ring closes. The size of the testicles varies greatly depending on the season: they are largest during the reproductive period (Fig. 6) and shrink to just a few millimetres with chemical castration (implantation of deslorelin). The os penis is “J”-shaped and palpable under the skin of the abdomen. The opening of the prepuce, which is not very obvious, is caudal to the umbilicus.
The prostate, which is small, is the only accessory gland and is found at the base of the bladder surrounding the urethra.
The ovaries, which are paired, are located caudal to the kidneys. The uterus (Fig. 7) has two long, thin horns, a short body which communicates with a single cervix and short vagina. The vulva, located ventrally to the anus, is not very apparent (Fig. 8) but enlarges considerably during the reproductive season under the effect of oestrogens (Fig. 9). Both the males and females have three to five pairs of nipples.
REPRODUCTION
Ferrets reach puberty in the spring following their birth, at 5-9 months of age. Reproductive activity is influenced by the photoperiod: ferrets kept in artificial light can reach sexual maturity by the age of 4 months. The male is sexually active from January to August and in this period the testicles double in size and become clearly evident. From September to December the testicles become smaller.
The females show seasonal polyoestrus and are fertile from March to September. The jill can be recognized to be in heat by the increased size of the vulva, which is largest 2-4 weeks after starting oestrus and becomes smaller within a few days of mating. While the female is in heat, a serous fluid is discharged from the vulva, which can soil the genital area, abdomen and inner parts of the posterior limbs.
The ferret belongs to the group of animals with induced ovulation, which means that oestrus, once started, continues until the jill mates, which induces ovulation. If the jill does not mate, oestrus can last 6 months (the whole reproductive season); this can have severe consequences due to the presence of oestrogens in the circulation which cause myelosuppression. Mating is rather crude and sometimes violent: the male seizes the female by the scruff of her neck and drags her around. In order to protect the jill from lesions, the male and the female must not be left together for more than 2 days for mating. A few days after ovulation has been induced by the mating, the hormonal variations cause a reduction in the size of the vulva.
If the male is not completely sexually mature (and does not, therefore, produce vital sperm) or the female is mated with a vasectomised male, the mating is followed by a pseudopregnancy of 40-42 days, which leads to an increase in the volume of the uterus and mammary glands. Pseudopregnant jills can show maternal behaviour towards their owners, other ferrets, or toys, which they take into their nest and look after as if they were kits.
A diagnosis of pregnancy can be made by palpation at 14-21 days. The gestation lasts about 42 days (range 38-44). During the last 2 weeks of gestation, if the jill is accustomed to living free in the house, it is worthwhile getting her used to staying in a cage in order that she does not deliver in an unusual place or carry the kits around the house. The cage must be in a peaceful, warm place and contain a box-nest and rags to pad it. The gravid female should have an abundant supply of excellent quality food available. The hob or other ferrets should not be able to approach the kits, because they could kill them.
There are an average of 8 (range 5-13) kits in a litter; the kits are born very immature, and without hair (apart from a very fine down) and with eyes and ears closed. The interval between the expulsion of one kit and another is very variable, ranging from a few minutes to 1 hour. The mother rarely has problems with delivery. The kits should not be touched for the first week and the mother should not be disturbed.
It is very difficult to raise kits orphaned within the first week of life; however, lactating jills will accept foreign kits. The kits receive maternal antibodies for 30 days.
The kits open their eyes at 3-4 weeks. At 3 weeks of age they can start to eat semisolid food, such as tinned cat food and artificial milk for cats. Weaning can be started at 6 weeks, the age at which the kits can already be given high quality pellets.
The jill enters oestrus again shortly after weaning her litter, but it is unwise to let her give birth more than twice a year.