Lecture 1 -- Small furries (Rodents)

What is the order of rodents?

Rodentia

What is the smallest rodent species?

Pygmy mouse or Pygmy gerboa

What is the largest rodent species?

Capybara

What are the suborders in rodents? Give examples for each order.

• Myomorpha

  • Rat, Mouse, Gerbil, Golden hamster, Russian hamster and Chinese hamster

• Hystricomorpha

  • Guinea pig, Chinchilla, Degu

• Sucriomorpha

  • Chipmunk

• Anomarluromorpha

  • Springhares, scaly tailed squirrels

• Castorimorpha

  • Beavers, Kangaroo mic, Kangaroo rats, Pocket mice

What is the significant dental formula for Myomorpha?

1/1, 0/0, 0/0, 3/3 ×2 

Total: 16 teeth 

What is the significant dental formula for Hystricomorpha?

1/1, 0/0, 1/1, 3/3 ×2 

Total: 20 teeth

What shape are rodent incisors and why?

Chisel-shaped 鑿

• Due to hard enamel on one side and softer dentine wearing away faster

What is the key feature of rodent incisors?

• Open rooted (Aradacular) = lack fully formed root → Continue to grow throughout the rodent's life

• Continually growing (Elodont) → “Elo” = Continuous; “Dont” = Tooth

• Hard enamel on rostral surface, with softer dentine behind = Chisel shaped 鑿子

• Length of lower incisor is 3 times the length of upper incisor

Do myomorphs and hystricomorphs commonly suffer from molar overgrowth? 

Myomorphys: 

• NO because their molars are anelodont = Grow for a short period then stop erupting 

Histricomorphs: 

• YES because their premolars and molars are elodont = Continuously grow → Make them prone to overgrowth if not worn down properly

Compare the features of myomorph and hystricomorph premolars and molars.

Myomorphs

• Absent of premolars 

• Molars are anelodont = Grow for a short period then cease erupting 

Hystricomorphs

• Presence of both premolars (1/1)and molars (3/3)

• Premolars and molars are elodont = Continue to erupt throughout the life of the animal

How do rodent chew food? 

By pulling the lower jaw back → (Lower incisors behind uppers) + Molars align and can grind food

How do rodents gnaw 啃?

By moving the lower jaw forward → Incisors opposed to each other (but molars are not) + Cheeks drann in to diastema

What special features are found in rodent mouth anatomy?

• Large diastema

• Ability to gnaw without involving molar 

  • They can move the lower jaw forward so the incisors oppose, while the molars do not touch 

• Ability to chew

  • They can bring the lower jaw back → Lower incisors behind uppers incisors → Molar oppose for grinding 

Why can rodent incisors be safely shortened with a dental burr?

Incisor pulp cavity is small and beneath gumline → Allow crown trimming without hitting the pulp, which contains the blood vessels and nerve 

Are rodents monogastric or polygastric?

Monogastric (Except hamsters, which have two compartment stomach)

What are the main gastrointestinal (GI) system adaptations in rodents?

• Fore-stomach is non-glandular and tough

• Glandular part of the stomach is separated from the fore-stomach by ridge

• Large caecum

• Elongated colon

Why do rodents have a large caecum?

Rodents often consume high-fiber, plant-based diets → Caecum acts as a fermentation chamber

Why do rodents need an elongated colon?

• Prolong hindgut fermentation → Longer fermentation produces more vitamins e.g. vitamins B and K and amino acids → More uptake of vitamins and amino acids through caecotrophy 

Do rodents vomit? Why? 

No 

• Strong oesophageal sphincter  

• Weak diaphragmatic muscle = Unable to have a big contraction of diaphragm to cause vomit 

• Limited ridge between oesophagus and cardia of stomach (J shaped cardia)

Should rodents be fasted before surgery? Why or why not?

NO

• Rodents have fast gut transit and high metabolic rate → Fasting can cause hypoglycemia

What species of rodent is most prone to GI issues and why?

Hamsters 

• Since they have two compartments of stomach, they are more rely on healthy gut flora → When hamsters’ gut bacteria are disrupted, they are more likely to have GI problems

What type of nasal breathers are rodents? What anatomical features make rodents this type of nasal breather?

• Rodents are obligate nasal breathers

• Anatomical features

  • Elongated soft palate

  • High larynx

What types of signs indicate that a rodent is in respiratory distress?

Mouth breathing since rodents are obligate nasal breathers

How many lung lobes do most rodents have? Which species are exceptions?

• Left lung: 1 lobe

• Right lung: 4 lobes (Cranial, middle, caudal, intermediate)

P.S. Hamsters - Have 5 lobes in right lung (Caudal accessory) 

Rodents have high oxygen demands, so what adaptations help them optimise respiration?

• Short airways → Increase respiratory rates

• More alveoli of thinner diameter → Increase surface area 

• High chest wall compliance → Increase vital capacity → Reduce residual lung capacity → Optimise lung expansion 

• O2 dissociation curve is shifted to right = Hemoglobin releases oxygen more readily to tissues 

Why rodents require high metabolic rate to maintain body temperature in cold climates?

High surface area to volume ratio → Loss heat easily → Require high metabolic rate to maintain body temperature in cold climates 

What are the mechanisms of thermogenesis in rodents in cold environemnt?

• Behavioural

  • Huddling, nest building, tunnelling

• Shivering

• Peripheral vasoconstriction

• Non-shivering thermogenesis

  • Brown adipose tissue behind scapula and around neck

What are the mechanisms for heat loss in rodents?

• Sweat glands (Minimal and on feet)

• Evaporation of saliva groomed over body 

• Arteriovenous shunts in ears, tails and feet 

What is the reproductive strategy of myomorph rodents, in terms of litter size, gestation period and youngs?

• Large litter size

• Short gestation period

• Altricial young (Undeveloped at birth)

What is the reproductive strategy of hystricomorph rodents, in terms of little size, gestation period and youngs?

• Small litter size

• Long gestation period

• Precocial young (Well-developed at birth)

What type of uterus do rodents have, and which species have this type?

Bicornuate uterus

• Found in guinea pig

What type of uterus do rodents have, and which species have this type?

Duplex uterus

• Found in rat, mouse hamster, gerbil and chincilla

Where are the testes located in most rodents, and how does the chinchilla differ?

• Most rodents: Testes descend into the scrotal sac after puberty

• Chinchilla: Testes often stay inguinal

Do rodents have an os penis?

Yes

What accessory sex glands do rodents have?

• Seminal vesicles

• Coagulating gland

• Prostate gland

• Preputial gland

• Bulbourethral gland

List out the anatomical feature in this picture

What is the most reliable method for sexing rodents?

Ano-genital distance

• Males have longer distance between anus and genitals than females

What are the other method of sexing rodents?

• Presence or absence of nipples 

  • In most rodents, only female mice have nipples

• Presence or absence of testes penis 

  • Cannot rely on that because rodents have open inguinal canal and often retract testicles back into the abdomen

How is the oestrus cycle assessed in rodents?

Cytology of vaginal secretion 

What are the 4 stages of the rodent oestrus cycle?

• Proestrus – Mostly round nucleated epithelial cells, few leucocytes

• Oestrus – Some nucleated cells, few leucocytes

• Metoestrus – +++ leukocytes, non-nucleated epithelial cells

• Dioestrus – Some leukocytes, non-nucleated epithelial cells

Why does the number of leukocytes increase in metestrus and diestrus?

Defensive mechanism after mating

What is copulatory plug? What is its function?

• Congealed凝結的 sperm, mucus and accessory ejaculatory fluids, which is harden within the cervix

• Seal vagina → Make it more likely for mating to be successful

What are the special reproductive features of mice? 

• Males:

  • Longer anogenital distance than females

  • Large testicles relative to body size in adult mice (Not reliable) 

• Females:

  • Only female mice have nipples

Why are rat incisors yellow?

Due to iron pigment

Are rats nocturnal or diurnal? 

Nocturnal 

What is the special gland in rats located behind their eyes? What happens when stressed?

Harderian gland

• Red brown gland fills large part of the orbit and lies behind the globe

• Increased secretion - porphyrin when stressed = Red tears

What are the common venipuncture sites in rats?

• Lateral tail vein

• Lateral saphenous vein

• Ventral tail artery

Do rats have gall bladder?

No

How many mammary glands do female rats have?

6 pairs

Extensive mammary tissue from axilla to groin

What tumors are common in rats’ mammary tissue? Are these tumours more likely to be benign or malignant?

• Fibroadenomas (Majority); Adenocarcinomas (10%) 

• Benign → Fast growing and prone to ulceration and secondary infection

What are the special reproductive features of rats? 

• Males: 

  • Longer anogenital distance compared to females

  • Larger testicles relative to body size, especially in mature males

• Females:

  • Only female rats have nipples

What special structures do hamsters have in their cheeks? What is its function? 

• Bilateral cheek pouches (Extend caudally as far as the shoulder blades) 

• Functions: 

  • Food storage

  • Transport of food

  • Can inflate → Helps to keep hamster afloat in water 

  • Immunologically privileged site = Don’t tend to get infection in there 

How is the hamster stomach structured?

Two compartments:

• Forestomch (Like ruminants) 

• Glandular stomach

List out the anatomical structure of hamster’s stomach

A: oesophagus

B: Forestomch

C: Glandular stomach

D: Pylorus

What are the special reproductive features of hamsters? 

• Males:

  • Longer ano-genital distance compared to females

  • Large testes in males  

  • Flank gland is more obvious in male

• Females

  • Separate vaginal and urethral opening 

Same as gerbils, both sexes have nipples 

How long is the gestation of the Syrian hamster?

Shortest gestation length of all eutherian 胎生 mammals 

~16days 

What are flank glands in hamsters?

• Androgen responsive glands

• More obvious in males than females 

What are the common venipuncture sites in hamsters?

• Jugular vein 

• Cephalic vein 

• Cranial vena cava (Must be anaesthetise) 

Why can’t we use the tail vein to take blood from a hamster?

Because hamster has no tail

Do hamsters hibernate? If yes, below what temperature do hamsters enter torpor/hibernation?

<18°C

Are hamsters nocturnal or diurnal? 

Nocturnal but very active during hours of darkness

How much water does a hamster consume?

High water consumption → Up to 20ml a day in some large adults

What is unique about gerbil social behavior?

Form monogamous pairs = Male contributes to raise the young and they usually form a single lifelong pair bond with a mate → Aggressive to any newcomers 

What are the special reproductive features of gerbils? 

• Males:

  • Longer ano-genital distance compared to females

  • Large testes in males 

  • Pigmented scrotum 

• Both sexes:

  • Ventral scent gland (Hairless patch) (More obvious in males)

  • Have nipples (More obvious in female)

What is “degloving” in gerbils?

Tail skin can shed if grabbed

How much water do gerbils drink?

Gerbils are desert species = Excellent urine concentrating abilities → Drink very little (4ml/100g/day)

What are the special reproductive features of guinea pigs? 

• Males:

  • i shaped genital opening

  • Large obvious testicles

  • Penis can be everted from prepuce 

• Females:

  • y shaped genital opening

Both species have inguinal nipples

Which vitamin is an absolute dietary requirement for guinea pigs? Why? 

Vitamin C because they cannot synthesize it themselves → Scurvy can develop within 4 days of decreased intake

How much vitamin C is required daily in adult guinea pigs and pregnany females?

• Adults: 20–25 mg/kg/day

• Pregnant females: 30–40 mg/kg/day.

What is the ideal age for the first breeding of guinea pigs? Why?

• Before the age of 6-8months

• Since public symphysis starts to fuse after that → It cannot dilate to allow parturition → High risk of dystocia

What is the gland shown in the picture? Are males or females more prominent with this type of gland?

Caudal sebaceous gland 

Males have more prominent caudal sebaceous gland than females

What are the blood sampling sites of guinea pig?

• Cranial vena cava

• Jugular vein

• Femoral vein

• Lateral saphenous vein

What is the gestation period of guinea pig?

59–72 days (average 63 days)

Are young guinea pigs precocial or altricial?

Precocial 

What is the gestation period of chinchillas?

111days

What are the special reproductive features of chinchillas? 

• Males:

  • Longer ano-genital distance compared to females

  • No true scrotum

  • Testes often stay inguinal

External genitalia look similar to female (Mistake the penis for clitoris)

Are young chinchillas precocial or altricial?

Precocial 

Chinchillas have dense fur, which allows them to tolerate what type of weather?

Cold weather (But not wet)

What happens when chinchillas are stressed?

Fur slip = Sudden loss of furs that lets them escape from predators

What are the blood sampling sites of chinchillas?

• Cranial vena cava

• Lateral saphenous vein

Are chinchillas nocturnal or diurnal? 

Nocturnal but can adapt to diurnal lifestyle 

• Need 12hr :12hr light dark periods 

Are young chinchillas precocial or altricial?

Precocial