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Taxonomy and reproductive biology of rats

• General characteristics of a rodent – Most rodents are small, weighing 150g (5.0oz) or less (see Figure 1). There are only a few large species of which the largest, the capybara, may weigh up to 66kg (146lb). All rodents have characteristic teeth, including a single pair of razor-sharp incisors. With these teeth the rodent can gnaw through the toughest of husks, pods and shells. The name “rodent” comes from the Latin rodere, which means to gnaw. Gnawing is facilitated by a sizable gap, called the diastema, immediately behind the incisors, into which the lips can be drawn, so sealing off the mouth from inedible fragments dislodged by the incisors. Rodents have no canine teeth, but they do possess a substantial battery of molar teeth by which all food is finely ground. Convoluted layers of enamel traverse these often massive and complexly structured teeth. The pattern made by these layers is often of taxonomic significance. Most rodents have no more than 22 teeth, though one exception is the Silvery mole-rat from Central and East Africa which has 28. (see Singleton and Dickman 2001)

• Introduction of key species for different regions of Asia – see summary Table 3

• Use of taxonomic key and link to resources – the CSIRO Rodent Group has produced a rodent key specific for rodents in agricultural landscapes in Lao PDR. A CD ROM using LUCID software is currently under development. Course participants will be shown key characteristics for distinguishing rodent species and then work through a dichotomous key. (Practical)

• How to sex a rat - The sex of adult mice is normally easy to determine, as scrotal sacs, vaginas and teats are all quite obvious, especially during the breeding season. The distance between the genital papilla and the anus (a---b in the diagram) is always greater in males. It is more difficult to determine the sex of a juvenile as the sexual organs are much less obvious. The best way to differentiate is to compare the distance between the anus and the genital papilla. In addition, there is usually a distinct furless line between the vagina and the anus on female rodents.

• Determination of breeding condition of females

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Vagina  

The vagina will be open if the animal has recently mated (the opening may be small or large) or closed if the animal has never mated or has mated, but not recently.

Occasionally, there will be a vaginal plug which looks like a wax stopper in the vaginal opening. This indicates that the mouse has mated in the last 24 hours. Teats

Teats

Teats should be visible on females in breeding condition. To check, blow the fur away from the body or gently rub the fur back with a finger.

If the teats cannot be found the animal is either too young or not in breeding condition.

If the teats are visible but small and surrounded by fur the animal is not lactating.

If the teats are obvious and there is no fur at the base the animal is lactating. Milk can sometimes be expressed at this stage, or dried milk can be seen on or around the teat.

Pregnancy by palpation

When palpating a mouse, hold the mouse in one hand and let the intestine and uterus move between the thumb and index finger. Embryos are smooth to the touch and faeces rough.

 

 

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