The first thing that will let you know that the babies have arrived is likely to be a lot of little squeaks emanating from inside the nest. The babies start out as tiny little pink things no bigger than the last joint on your little finger. They are blind, deaf and hairless. The only real sense they have that is fully developed is determining temperature so they can find their littermates. That is why if you watch them after the mum has moved away they will burrow under each other to be the warmest on the bottom.

Left: newborn pinkies less than 24 hours old, Right: two day old pinkies

They start getting pigmentation very quickly (about three to five days or sooner), and you will be able to tell if they are going to be marked, self, tan, etc. The darker pigments are easy to tell from the normal pink and show a lot earlier. At around seven days they suddenly start getting fur. It is very fine at first, and mostly on the back. They are considered fully furred when their stomachs are furred at about 10-12 days, and this is the best time to sex them.

'Velvet' baby at around 8/9 days

The babies open their eyes at around 14 days (although runts may be a day or so later than their brothers and sisters). From the time that they are furred until about three weeks, or 21 days, they are in what is known as the 'flea stage'. You must be very careful when handling them at this stage because they have no fear of heights and will jump right out of your hands. Babies tend to jump around all over the place and it may be hard to pick them up.

Puberty
The onset of puberty - when ovulation first occurs in a female, and when males have achieved full spermatogenic activity - is variable even among different animals within the same inbred strain. Although it is possible for some outbred females to reach puberty by the age of four weeks, the majority of females from most inbred strains first ovulate naturally between six and eight weeks after birth. Numerous environmental factors appear to have an effect on the timing of this event (Whittingham and Wood, 1983). Exposure to adult males or their urine can bring it on sooner, whereas adult females or their urine may retard its onset. Furthermore, three to six-week-old females can be induced to ovulate with a specific regimen of hormone treatment. The onset of male puberty in most laboratory strains usually occurs between 34 and 38 days, however, it is sometimes possible for non-inbred males to reach sexual maturity by 30 to 32 days after birth. Thus, if you do not want littermates to mate with each other, they should be separated according to sex before the appropriate age is reached.

Young cream mouse ready to be separated from its mother

I tend to separate at four to four and a half weeks (leaving girls with mum), depending on the development rate of the litter. At this point you will notice that the babies are not always burrowed under the mother and you will see them eating solid food. The boys and the girls should really be separated by five weeks unless you want a LOT of babies!

 

How the Experts Explain It: Postnatal Development
A mouse is born naked with closed ears and eyes, and if a female with a closed vagina. Hair begins to appear at two to four days, ears open at three to five days, and eyes open at about 14 days. Typically, the vagina opens at 24-28 days of age, but it can be delayed in some mice until they are 35-40 days old. As soon as the eyes are fully functional, at about 16 days, pups will begin to eat solid food. However, nursing can continue to at least the end of the third week and sometimes a week or more longer. By the end of the third week of life, a young mouse resembles the adult in every aspect other than size and sexual differentiation.

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