Comments & suggestions  should be directed to Visit my website Special Anatomical & Physiological Features Dental formula: 2 (I1/1,  C0/0,  P0/0,  M3/3).  The incisors grow continuously throughout life, and are referred to as open-rooted incisors.  The molars are rooted.  Mice with malocclusion of the incisors require periodic trimming of the incisors so that the animal can eat.  Such animals should not be used as breeding stock as the condition is inherited. On the left side the lung consists of a single lobe, while there are 4 lobes on the right.  Mice have a simple intestine.  Mice have a gall bladder, while rats do not.  The inguinal ring remains open throughout life and the testes can be withdrawn from the scrotum into the abdomen.  Mice have no tonsils. The mouse has a large surface area per gram of body weight when compared to larger species of animals: as a result it has a high metabolic rate and at rest uses 22 times more oxygen per gram of body weight than an elephant.  To provide adequate oxygenation to body tissues, the mouse has a rapid respiratory rate (average 163 breaths/minute), short air passages, a high RBC concentration (hematocrit  40 - 50% ), and a high hemoglobin concentration.  Mice are obligate nasal breathers, meaning that they cannot breathe through their mouths.  The heart rate is also extremely rapid, at 400 - 500 beats/minute. Although mice have a wide range of temperature adaptation in the wild, sudden temperature variations in the laboratory setting can result in death.  The mouse has a low tolerance for acutely increased temperatures.  Mice do not have sweat glands and cannot pant, depending primarily on vascularization of their ears and tail for loss of heat.  Wild mice deal with environmental temperature increases by seeking shade or underground shelter.  Ambient temperature is therefore, a major consideration when mice are being shipped or transported even for short distances.  Temperatures greater than 90°F can result in heat prostration and death. The mouse produces very concentrated urine (average urine specific gravity, 1.058), and water conservation is important for survival of wild mice.  As mentioned earlier, mice dehydrate rapidly when access to water is restricted.  Mouse urine normally contains large amounts of protein. The spleen may be up to 50% larger in male than in female mice.  Mature male mice have higher granulocyte (neutrophil, eosinophil) counts in their peripheral blood than female mice of the same age.  Basophils are only rarely seen in the peripheral blood. Summary of physiological and behavioral parameters for rats and mice (Table 5). Malocclusion - A frequently occurring problem in rodents.  The condition results when the incisors are misaligned and thus do not undergo normal wear.  An animal with malocclusion cannot eat and often appears runted after weaning. Euthanasia is suggested