Small Mammals Flashcards
What are some general anatomical characteristics of small mammals?
- Endothermic
- Tactile vibrissae
- Possess hair – can be modified into fur, wool, spines, scales
- 4 chambered heart
- Diaphragm
- Mammary glands
- Separate urinary and excretory passages
- Atlas-axis complex
- Ribs restricted to thoracic vertebrae
What are the anatomical features of the rodent oral cavity?
- Cleft upper lip = philtrum
- Diastema
- 1 pair upper and 1 pair lower incisors
- Distance between cheek teeth greater in lower than upper jaw
- No bony symphysis between mandibles
- Very large masticatory muscles
- Incisors are aligned for gnawing, cheek teeth are not
- Lip folds and bulges inwards through diastema to meet behind incisors, divide oral cavity into gnawing and grinding compartments
How are the rodent types distinguished?
On the basis of jaw/skull anatomy and masticatory
Name and describe the rodent types.
Myomorphs – rat, mouse, gerbil, hamsters
Hystricomorphs/caviomorphs – porcupine like rodents. Giant capybara, guinea pigs and chinchillas
Sciuromorphs – squirrel-like rodents. Beavers and chipmunks
Distinguish the dental formulae of myomorphs and hystricomorphs.
Myomorphs: 16 teeth: 4 incisors, no premolars, 12 brachydont/closed rooted molars
Hystricomorphs: 20 teeth: 4 incisors, 4 premolars, 12 aradicular/open rooted molars
Which rodent type has more dental issues and why?
Have more dental issues in myomorphs, because if they are fed inappropriately and there is malocclusion, can end up with dribbling guinea pigs/chinchillas or anorexia.
Distinguish the anogenitals of myomorphs and hystrocorphs.
Myomorphs: separate anal, genital and urinary openings in female. Anogenital distance smaller in females.
Hystricomorphs: separate anal, genital and urinary openings in female. Anogenital distance not always useful in sexing.
Distinguish the gestation and young of myomorphs and hystricomorphs.
Myomorphs: short gestation and altricial young
Hystricomorphs: long gestation and precocious young
List the external and internal features of rodents.
External: head, digits, mammary tissue, genitalia
Internal: skull, musculature, teeth and oral cavity, skeleton, thorax, abdomen, reproductive tract
Use the rat as an example to describe the myomorph head.
- Pointed triangular head, short neck
- Yellow-orange pigmented teeth as they store iron in their teeth
- Harderian gland – porphyria common, produce red-white pigment, if stressed
- Cleft upper lip – philtrum
- Tactile vibrissae – touch sensation is highly developed
Use the guinea pig as an example to describe the hystricomorph head.
- Short thick neck
- White teeth
- Tactile vibrissae
Use the rat to describe myomorph digits.
- 4/5 digits on forefoot
- Digit I is very reduced
- 5 digits on hindfoot
- No hair on pads
Use the guinea pig to describe hystricomorph digits.
- 4 digits on forefoot
- 3 on hindfoot – I and V missing
- No hair on pads
Use the rat to describe myomorph mammary tissue.
- Extensive mammary tissue from neck to anus, apart from costal arch region
- 6 pairs of nipples
- Mammary tumours are very common and are usually benign fibroadenoma
Describe the hystricomorph mammary tissue of guinea pigs and chinchillas.
Guinea pigs – 1 inguinal pair/2 teats and reduced mammary tissue
Chinchilla – 2 thoracic pairs and 1 inguinal pair teats
Describe the rodent skull.
- Very large tympanic bullae for acute sense of hearing
- Hystricomorphs commonly have dental overgrowth because aradicular and constantly growing
Describe the rodent skeleton.
- Rats, guinea pigs and chinchillas have clavicles (if they pick up food with forelimbs)
- Hind limbs are plantigrade, forelimbs vary, mostly digitigrade
- Pelvic bones separate in mouse and guinea pig at parturition so guinea pigs will have to be bred before the age of 4 before their pelvic bones fuse, or it will result in dystocia.
What does the rat/myomorph skull have?
Cartilaginous mandibular symphysis
Describe the hystricomorph skull.
- Large tympanic bullae
- Prominent zygomatic arches
- Mandible united at cartilaginous symphysis
How are rodents adapted for gnawing?
- Large masseter muscles and strong mandible for insertion
- Loose jaw articulation to allow
- Cheek folds separate compartments
- Mandibles tend to be wider than maxilla
What does loose jaw articulation allow?
- Rotary movement
- Forward and backward movement
- Incisors in apposition while molars not and vice versa
Describe the musculature of the rat/myomorph head.
Large lateral masseter – much larger than in carnivores who have relatively larger temporalis.
Medial masseter asses through infraorbital foramen to rostral muzzle – greater level arm for superior gnawing.
Describe the musculature of the hystricomorph head.
Larger lateral masseter
Medial masseter is deep and less powerful in myomorphs. Inserts on rostral muscle but at a straighter angle than in myomorphs.
Describe the teeth and oral cavity of myomorphs.
- Incisors open rooted, molars closed rooted (brachydont)
- Lower incisors normally long – do not mistake for overgrowth
- Chisel like, lingual surface is softer with no nerve
- Incisors yellow pigment in rats
- Long tongue and fleshy with developed root (torus linguae)
Describe the hamster oral cavity.
- Yellow/orange incisors
- Extensive buccal pouches for food storage and transport
- Can become impacted
What is dribbling in guinea pigs and chinchillas?
Dental disease more common is hystricomorphs due to aradicular teeth. An example is dribbling: dribbling, wet fur on chin or forelimbs, anorexia, weight loss due to dental disease.
Describe the teeth and oral cavity of hystricomorph.
- Molars difficult to visualise as in many rodents
- All teeth are open rooted aradicular and constantly growing, so overgrowth and malocclusion is common
Describe the guinea pig oral cavity.
- Caudal tongue continuous with soft palate except for palatal ostium
- Care with intubation so often in practice gas or injection anaesthesia used
What are complications of malocclusion?
If malocclusion is quite severe and due to nature of the teeth, may have to be done every month. Weight up costs and also fairness on animal with the owner.
Malocclusion can cause a chain reaction in apical cells where teeth tissue grows in the wrong direction and ends up with lumps on the bone of the mandible.
Describe the cardiorespiratory system of rodents.
- Obligate nose breathers, larynx placed high in the oropharynx
- High oxygen demand because of small body weight
- High chest wall compliance, low residual lung capacity so can expand their lungs well
- High respiratory rates
- Nasal cavity also has a major role in olfaction so if the animal gets a respiratory infection it can affect the sense of smell.
Use the rat to describe the myomorph cardiorespiratory system.
- Not normal mammalian heart
- Only right lung is divided into 4 lobes
- Rats are very prone to respiratory disease
Use the guinea pig to describe the hystricomorph cardiorespiratory system.
- Normal mammalian heart
- Both lungs lobated – 3 left and 4 right
- Respiratory disease less common in guinea pigs than in rats
Describe the rodent digestive system.
Myomorphs are omnivores. Hystricomorphs are strict herbivores.
- Large caecum
- Caecotrophy – fibre and protein digestion
- Very efficient digesting fibre due to caecotrophy
Describe the myomorph digestive system.
- Frequent feeders
- While rats are omnivores, they do have large caeca
- Caecum in left caudal abdomen
- 4 lobed liver
- No gall bladder
Describe the myomorph monogastric stomach.
- Stomach Limiting ridge
- Non-glandular forestomach
- Distal glandular region
- Heavily muscles pyloric sphincter
Very marked limiting ridge in hamsters to allow separate environment.
Describe the guinea pig hystricomorph digestive system.
- Crepuscular
- Monogastric
- Stomach entirely glandular
- Liver has 6 lobes and a well developed gall bladder
- Caecum very large and fills most of the left side abdominal cavity
- 3 taeniae
- Proximal and distal colon
- Proximal ascending colon separates digesta knot faecal pellets and caecotrophs. It is a flattened spiral with no taeniae or haustra
Describe the chinchilla hystricomorph digestive system.
- Very long colon that is 2 times the size of the small intestine, possibly for water conservation as chinchilla are desert mammals
- Proximal colon taenia and haustra
Describe the myomorph urogenital system.
- Separate urinary, genital and anal orifices in female
- Anogenital distance used for sexing
- Genital orifice not always patent, may have a small membrane covering it
- Genital papilla in both sexes but larger in male
- Scrotal bulge. Hamsters have seasonal variation in testicular size, with sudden appearance in summer
Describe the hystricomorph urogenital system.
- Separate urinary, genital and anal orifice in female
- Anogenital distance not always helpful for sexing
- Genital orifice not always present
Describe the guinea pig hystricomorph urogenital system.
- Anogenital distance not helpful as they look very similar
- Perineum of female inverted Y
- Vaginal membrane
- Developed penis in male that can be extruded for sexing
- Scrotal swelling in older males
Describe the chinchilla hystricomorph urogenital system.
- Anogenital distance is helpful
- No scrotal sac in males so testes mainly sit in the inguinal canal
- Vagina closed except during copulation and parturition
Describe the internal genitalia of female myomorphs.
- Short gestation and altricial young reproduce rapidly with large litters
- Duplex uterus – 2 spearate uterine horns, each with their own cervix entering the vagina
- Copulatory plugs
What are the gestation periods of myomorphs?
Rats and mice gestation = 21-23 days
Hamsters gestation = 15-18 days
Gerbils gestation = 22-26 days
Describe the internal genitalia of male myomorphs.
- Cranial fat bodies on testes, project through inguinal canals
- Large paired vesicular glands
- Paired coagulating glands close to vesicular glands
- Ampullae, prostate, bulbourethral glands
- Os penis/baculum
Describe the internal genitalia of female hystricomorphs.
- Uterus bicornuate
- Long gestation and precocious/mature young
What are the gestation periods of hystricomorphs?
Guinea pig gestation = 65 days average
Chinchilla gestation = 111 days average
Describe the internal genitalia of male hystricomorphs.
- Cranial fat bodies on testes, project through large open inguinal canals. Closed castrations are required
- Paired coagulating glands close to vesicular glands
- Prostate, bulbourethral glands but no ampullae
- Large paired vesicular glands
- Developed penis, os penis/baculum
Distinguish the vesicular glands of guinea pigs and chinchillas.
Guinea pigs have twisted cylinders
Chinchilla have long tubular
What are mustelids?
Ferrets
Weasels
Skunks
Badgers
Otters
Describe the special senses of mustelids.
- Acute hearing and smell
- Elaborate turbinate system
- Eyesight relatively poor but reasonable in low light levels – making them nocturnal hunters
Describe the cardiorespiratory system of mustelids.
- Thorax is large for body size and has increased lung capacity
- Thoracic inlet is narrow
- Anatomically like human lungs
Describe the digestive system of mustelids.
- Mostly typical carnivore
- No caecum or appendix
- No ileocolonic valve
Describe the urogenital system of mustelids.
- Mammary tissue, 4 teats
- External genitalia is similar to dog and bitch – have prepuce and os penis. Scrotum in a very similar position to cats
- Jills induced ovulators
- Prostate is the only accessory gland
What is the consequence of jills being induced ovulators and what is done about this?
- If they do not mate, persistent oestrogen stimulation causes bone marrow suppression and anaemia
- Requires ovariohysterectomy
- Bicornuate uterus
- Castration and vasectomy common
How does the colour of the incisors differ between the rat, guinea pig and chinchilla?
Rats have orange yellow teeth due to iron pigmentation. Guinea pigs have white teeth. Chinchilla have yellow teeth.
Identify the major bones of the skull. Note the size of the tympanic bullae in rodents. Why are they so large?
All rodents have an acute sense of hearing. Of the domestic species, chinchillas have the largest tympanic bullae.
In which species is there a clavicle?
Some rodents have clavicles. The rat, guinea pig, chinchilla and hamster. This provides a greater rom for grasping food.
Why must guinea pigs be bred before 7-10 months of age?
Fusion of pelvic symphysis occurs over a year old resulting in dystocia.
Observe the cheek folds. What function do they serve? In which species are cheek pouches present?
Cheek folds. Separates the rostral and caudal regions of the oral cavity. Gnawing and grinding occur simultaneously.
What is the name of the aperture between the tongue and soft palate? Why is it remarkable, and what clinical significance does this have?
Palatal ostium. the narrow oral cavity and presence of the palatal ostium make intubation in guinea pigs difficult. Injectable or masked gaseous anaesthesia is typically performed.
How does the guinea pig stomach differ to that of the rat and hamster?
Entirely glandular epithelium
Locate the inguinal ring. Note that it is permanently open throughout life. What is the functional and clinical significance of this?
The inguinal canal is open. A closed or modified closed castration must be performed to prevent post castration herniation.
What foramen does the medial masseter pass through?
Infraorbital foramen
Why do myomorphs have superior gnawing capabilities compared to hystricomorphs?
The medial masseter has a greater lever arm as it passes through the infraorbital foramen at a more acute angle.
What causes the red pigment seen around this rat’s nares, is this normal?
Porphyria is a red pigment which looks like red tears or blood. Occasional low levels of porphyrin staining are normal but regular large amounts indicate stress, sickness or poor diet. It occurs when the rat’s Harderian gland becomes inflamed and secretes porphyrin pigmented tears.
How do the feeding habits of rats and hamsters differ?
Rats are omnivores and frequent feeders. Hamsters are also omnivores but feed in a 5 minute bursts followed by a 2 hour fast. The hamster has evolved certain patterns of behaviour consistent with a burrowing and hoarding desert animal.
How do guinea pigs breathe?
Guinea pigs are obligate nasal breathers as their larynx sits high in the oropharynx. High oxygen demand because small body weight. High chest wall compliance, low residual lung capacity so can expand their lungs well. High respiratory rates.
What role does the proximal colon have in digestion?
Separates digesta into faecal pellets and caecotrophs. It has no taenia and is a flattered spiral.
What is the largest accessory sex gland in the guinea pig?
The vesicular glands. These can be confused with the uterus. They can be identified by their sacculated ends.
What is significant about the length of the colon in the chinchilla?
Natural habitat of the chinchilla is arid. Long colon to reabsorb water.
How many teats do ferrets have?
4
How does the dentition differ from that of rodents? What does this tell you about the ferrets method of feeding?
Carnassial teeth – upper PM4 and lower M1. Carnivorous diet.
Why is the thoracic cavity large in a ferret for an animal of this size?
It is a slim active hunter and requires a large lung capacity. Any enlargement of organs in this region, such as lymph nodes, will compromise chest function.
Why is catheterisation difficult in this ferrets?
The os penis is curled distally.
How many accessory sex glands are there?
Prostate only
