>OVERVIEW<

Degus are medium-sized32 diurnal crepuscular rodents21, despite being one of the smallest caviomorphs to inhabit South America51. They have five clawed toes on each of their and five on their 30, 52. Each digit is well developed, apart from the fifth digit which is reduced52. The nails on the hind feet are curved51 and covered with long, comb-like stiff bristles51 , 52. They have a 'moderately long'52 (shorter than the head and body) with a small black brush at the tip51, 52. The is short and soft52, and is lighter towards the belly (see below). Degus have 20 teeth52, 57; two pairs of incisor teeth 52, 57, two pairs of premolars 52 and six pairs of molars 57, 52. Degu are high-crowned52 and ever-growing, consistent with eating fibrous plant matter in the wild 51. The molars have only moderate indentation52 (whereas other species have deep molar indentation52). The is orange52 (see below). The are well developed and of a moderate size52. Ears are darkly pigmented with little fur52, but the ear canal is protected by long, slightly coarser . The ears are shaped to capture sound and to dissipate body heat. Degus have large, dark surrounded by a ring of light fur 52, positioned on the side of the head to improve the range of vision. There is sometimes a of light fur around the sides of the neck52. The is hairless, small and usually dark in appearance, although there are . Just behind the nose are a series of , which vary in length depending on the size of the degu. Whiskers are a sensory mechanism and are very sensitive to touch stimuli. are also present to allow the degu to determine both the width and height of a space (for example a burrow). Note that female degus have a better sense of smell than males as it has been found that the hormone testosterone, present in males only, can de-sensitise a male degu's olfactory organs to certain scents7. Females also have four pairs of teats30, 52, three pairs of which are located on the side of the body in a line between the front and hind legs52.

>DIGGERS?<

Degus are semifossorial2 which means they are adapted for living partly (semi) underground (fossorial). This is in comparison with a fossorial mammal (for example a mole), which is almost completely adapted for life underground. Degu anatomy not consistent with burrowing includes large eyes, long toes, a long tail, smaller clavicle, smaller deltoid crest and nails instead of claws2, 52.

>TEETH<

Degus, like their relatives the chinchillas, have orange enamel on the front surface of their incisors with a multiserial microstructure52 , 129. This colouration occurs during amelogenesis58, when the enamel is formed in ameloblasts (tooth enamel forming cells)58. Amelogenins of developing dental enamel are specific proteins rich in proline, leucine, histidine and glutamyl residues that are synthesised by the ameloblast cells of the inner enamel epithelium59. These proteins become mineralised to form mature enamel, thought to be regulated by an acidic enamel protein called tufletin59. The exact process by which the enamel is coloured orange is still debated, but it has been suggested that the forming enamel is stained by oral enzymes in the degus saliva57, although this is speculation only. It is hypothesised that such chromogenic enzymes are taken up by the pellicle60 and are derived from polyphenolic compounds which provide the colour in foods60, such as carotenoid pigments. There may also be a link between enamel colouration and iron intake187. Therefore, it is thought nutrition plays a major role in the enamel colour of degus' teeth (see illness section).

Below are two pictures showing the orange colour of healthy adult degu incisors. Note that it is only the front portion of the tooth that is covered with enamel. 


The orange colouration of degu enamel develops slowly from birth; pups are born with pale cream teeth which slowly darken over the first six months to the adult deep orange:

>OSTEOLOGY<

There is limited research on the of degus52. However, from reports, we know the following:
The has a large, well-developed infraorbital foramen52 with a ventromedial grove 52 and an open pterygoid fossa 52. The tip of the hamulus pterygoideus is in contact with the bulla52. The bulla are of moderate size52, but the auditory bulla is well developed52 and the paroccipital process hugs the posterior edge and surface52. Interestingly, the malleus and incus ossicles are not fused129. In the mandible, the two rami are closely joined52, with the coronoid process appearing delicate and sharply pointed52. The clavicle and deltoid crest are well developed52. The scapular spine extends from near the vertebral border of the scapula to past the height of the tip of the coracoid process52. The scapular spine is a thin, unsupported, fingerlike projection from the midscapular region to the ends of the large acromion and metacromion processes52. There is an entepicondyloid foramen in the humerus52, and the proximal ends of the tibia and fibula are fused52. 

>MUSCULATURE<

There are several important myological characteristics that distinguish the degu. There is a complex, four parted M. cutaneus maximus in which the anterior part extends to the lateral surface of the shoulder52. There is a M. scapuloclavicularis52, a M. scalenus anterior ventral to the brachial plexus52 and the lack of a M. stylohyoideus despite having a well developed hyoid aparatus52. There is also a M. mandibulolabialis consisting of a complex multiparted group of masticatory muscles, with the medial masseter passing through the infraorbital foramen and pars reflexa of the superficial masseter, passing into the medial side of the mandible via a grove52. It is interesting to note that the musculature of the degu is consistent with all other hystricognaths, but is not found in any other rodent group52.

>BRAIN<

The brain of the degu is reported to follow the general pattern of rodents, but the olfactory bulb (that used to distinguish smells) is particularly well developed52. Unusually for rodents, the vomeronasal nerve connects to the accessory olfactory bulb laterally165. In addition, there is an indentation between the boundary of the rostral and caudal olfactory bulb subdivisions165, and the rostral olfactory bulb is twice the size of the caudal bulb and shows male-biased sexual dimorphism in terms of size165. These unique features are likely to be related to semiochemical communication in degus165. The surface of the brain is moderately convoluted52, which again is an unusual feature in rodents other than those belonging to the suborder Hystricognatha52. Blood is circulated to the brain via the vertebral arteries only52 , 130, because the internal carotid arteries and tympanic stapedial arteries have been lost52 during evolution.

>VASCULATURE and THYMUS<

As previously stated, some aspects of the degus circulation have been modified. The stapedial area blood supply has been replaced by the external carotid arteries52 , 130. It has been reported that (with a few exceptions) the degus cephalic arteries are identical to other hystricognaths 52. The degu has both a left and right anterior vena cava52. There is a double thymus with cervical and mediastinal components52. The mediastinal component is multilobed and amorphous52, which is morphologically similar to a typical rodent thymus52. The cervical component is bi-lobed52 and remains active throughout the entire life of the animal52, unlike the mediastinal component52. The coeliac artery and mesenteric artery originate from the abdominal aorta137. Degus appear not to have a gastroepiploic artery137. The caecum is supplied by the ileocolic artery in the direction of head to apex of the caecum137. The liver is supplied by branches of the hepatic artery137, with a middle right branch extending to the gall bladder137. The pancreas is supplied by the cranial pancreaticoduodenal artery, caudal pancreaticoduodenal artery and splenic artery137. The dorsal spleen is supplied by a right branch of the dorsal gastrosplenic artery137, while the ventral spleen is supplied by a right branch of the ventral gastrosplenic artery137. 

>WATER UTILISATION<

Since degus originate from a semi-arid climate, their anatomy is adapted to utilise water more efficiently42. There is a high degree of water reabsorption in the colon9, which helps degus to maximise the amount of water they can extract from food. Degus have about twice the colonic reabsorption capacity as that of a lab rat9, which may be due to presence of a mercurial agent known to inhibit water channels140. Degus don't have sweat glands and so minimise the amount of water lost through evaporation9 , 140. Degus are also able to condense water in their nasal passages9 in order to prevent exhalation of essential moisture. This process is achieved through aquaporins in the nasal mucosa166, first AQP-3 in non-olfactory epithelial cells and then AQP-1 in the capillary lumen166. Of course the greatest loss of water occurs through urination, therefore degus urine is highly concentrated9 as a result of a long loop of henle in the kidneys. Degus also have comparatively large kidneys138. Interestingly, degus (particularly males) produce more urine at night than during the day52. Research has shown that degus can go without water for up to 13 days42, and after seven days without drinking, the urine's concentration can reach 4604  mOsmoles l^-1 52 (see vital statistics above for normal levels)!

Although degus are adapted to survive on little water, you should ALWAYS supply them with a good supply of clean, fresh water daily.

Further study of the degus kidneys has shown that they have the unusual ability to concentrate urinary potassium to a greater degree than sodium52, and (equally unusual) can retain magnesium52. The possible reasons for this were not stated.

>EATING and DIGESTION<

Degus are herbivorous12 (eat plant matter only), so their digestive system is specifically adapted for this. Their gut contains sucrase34, an enzyme that hydrolyses plant carbohydrates34. This allows degus to digest sugars specific to plants, but due to abnormal insulin function, degus should avoid being fed diets containing sugar. Degus possess a caecum, which varies in form and position between degus136. The Caput caeci is typically on the left with cranial or caudal rotation136, the apex being on the left in front of the pelvis136. The ascending colon is arranged in varying superimposed and often spiral folds136.

Newborn pups also possess the enzyme lactase which enables them to digest their mother's milk34.

Degu are small, dry, compact and very inoffensive. They don't smell and are very easy to clean up.

>VISION<

Degus have well-developed vision and sensitive eyesight during daylight hours191. Other nocturnal species of degu, such as the moon-toothed degu, have eyesight that is specially adapted for night vision, enhancing contrast and light sensitivity191.

The degu lens selectively absorbs short-wavelength light139, with increasing optical density as the degu gets older139. Degu retinas possess one type of rod cell139 and two types of S-cones24 , 139 so can see in dichromatic colour24. The colours visible to degus are green and UV (Ultra-Violet)24 , 21 , 139. The rods have a spectral peak at 500 nm (green)139, with each respective type of cone cell having spectral peaks at 507 nm (green) and 362 nm (UV)139. The retinas consist of around 9 million photoreceptors139, one-third of which are cone cells139. The two cone types exist in a ratio of 13:1 (green:UV)139.


of what the world looks like to a degu (note- humans can't see UV!).

Having UV sensitive S-cones is relatively uncommon in mammals24. Degus have been shown to be able to distinguish between UV and visible light in behavioural tests139.

>DAY-GLO DEGUS?!<

Degus fur is camouflaged, however their ventral (belly) fur is lighter52 and highly UV reflective; it reflects up to 20% of UV light21 suggesting a signal for communication during alarm calls or vigilance, where degus expose their bellies by standing on their hind limbs21.

Degus urine also reflects UV and up to 40% of incident light21 , acting as a visual territorial scent mark21.

>COLOUR VARIATION<

Did you know that degus come in more than one colour? Of course degus are normally brown51 , 52 in colour, each hair being gray below and brown at the tip (also known as 'agouti'), but there are slight in pigmentation. Some degus are lighter than others52 , whilst some can appear a very dark brown. Coat colour can also vary slightly depending on on season, age and diet.

There are also new captive bred colour varieties under development around Europe. Details and pictures of these varieties can be found on the 'colour varieties' page.

>SEXING<

Although degus have different genital anatomy for males and females, it can be quite hard to tell them apart just by glancing. Their urogenital anatomy is similar to that of other caviomorphs45. have a pair of testes that are always inside the abdomen45 , 133. The penis points posteriorly133 from the perianal circle45, beneath which (internally) is the cremasteric sac45. The penis is stored internally in an S-shape133 and the tip is covered with tiny spines133. Inside the body, the vas deferens and seminal vesicles open independently into the urethra45. The testicular artery is short, with few loops and a wide diameter45. A sacculus urethralis is present129.  have a bicornuate uterus52, which branches into two sections. The vaginal opening is located immediately (posteriorly) behind the urethral projection. Externally, females also have four pairs of nipples, three of which are positioned high on the sides between the front and hind limbs52. One pair is positioned on the belly between the hind limbs to enable suckling to occur whilst the female is vigilant.

, there is a larger distance between the anus and urethra in the male, whereas the female has virtually no space at all between the urethra and anus.

Complete instructions for determining the sex of a degu can be found on the sexing guide.

>SPERMATOZOA<

Degus' spermatozoa are different from other species. The head measures 7.7 μm long by 5.9 μm wide47 while the tail is 41 μm long47. The head is flattened dorso-ventrally and oval in outline47.  The gametic DNA content is about 2.7 pg48. An interesting feature is that there is a recombination index of 7352, which is high for rodents52.

The acrosome is the most distinctive feature in the sperm of the degu47. Viewed head-on, the rim of the acrosome is the shape of an inverted 'U'47. The acrosomal region has dome-shaped protrusions that are regular (not random)47. The acrosome makes up about 60% of the head length48. There are also subtle differences in the morphology of sperm between degu species. The sperm head of the Bridge's degu is more elyptical than rounded48, whereas that of the moon tooth degu is paddle shaped and basally broader48. On average, the common degu has the smallest sperm head length48, Bridge's degu has the smallest sperm head width48 and the moon tooth degu has the shortest sperm tail length48.

>PLACENTA and OVARIES<

During pregnancy, the placenta has a hemomonochorial placental barrier with continuous, non-fenestrated capillaries112. At day 27 of gestation the subplacenta emerges under the wall of the central excavation114 and the outermost trophoblast of the ectoplacental cone begins differentiation114. These secondary trophoblast giant cells lie on the outside of the placenta forming an interface with the maternal cells of the decidua114. These cells contain cytokeratin and placental lactogen until term114. During gestation the extrasubplacental trophoblast merges from the subplacenta to the decidua114. The vascular mesenchyme of the central excavation invades the chorioallantoic placenta during gestation to form two foetal lobules which is the zone of the placental barrier114. The activity of sodium and potassium ion ATPase in the placental barriers constant throughout gestation114. The residual syncytium at the edge of the placental disc/between the lobules is not invaded by foetal mesenchyme and forms the marginal and interlobular labyrinthine syncytium114. These cells are likely to have a secretory process between maternal blood vessels114. Placentas passed out of the body during birth show a large, single lobe, have no subplacenta and a reduced interlobular labyrinthine syncytium114. The inverted visceral yolk sac can be seen at day 27 of gestation and has features to suggest it functions as an early secretory organ114. The epithelium of the parietal yolk sac covers the entire placenta114, and comes directly into contact with the uterine lumen135. The full-term placental weight is roughly 8 g and has a diameter of around 13 mm135.

Even from an early age, females posses an antral follicle in the ovaries134 which does not ovulate, but becomes atretic134. Interstitial tissue is abundant134, and interstitial cells are vacuolated and possibly involved with synthesis of steroids134. During pregnancy, the corpus luteum persists throughout134, but may regress towards parturition134. Several small accessory luteal bodies are formed toward the end (days 75-80) of pregnancy134, persisting up to 20 days after parturition134. Other structures present in the ovaries include epoophoron tubules134 and rete ovarii134.