The stomach has three types of contractions: There are rhythmic, 3 per minute, synchronized contractions in the lower part of the stomach, which create waves of food particles and juice which splash against a closed sphincter muscle (the pyloric sphincter) to grind the food into small particles.
7 What is the gestation period, how many babies do they have, and when are they weaned? 1  The gestation period for rats is 19â22 days, for mice it is 17â21 days.  Rats and mice average 8â12 babies in a litter, but can have as few as 1 or as many as 20.
updated 2/4/16. Note about wild rats: Please see the info at the bottom of the page. Baby rats are about 1 Â¼ inches (3.2 cm) long at birth, and are pink and hairless. House mice are only about Â¼-inch (0.64 cm) long at birth. Deer mice, which is a common wild species found as orphans, are about one inch (2.54 cm), long at birth. All of these species start getting fur at about one week of age.