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)