EIGHTH WEEK: All individuals | Growth completed in some growing; total lengths of 156-170 | individuals; those in larger millimeters; weight 17-22 grams. | litters have total lengths of | 128-144 millimeters; weight | 14-17 grams. ----------------------------------+------------------------------------- | NINTH WEEK: Testes partly scrotal | Scrotum in season usually large, in one male on 59th day. | vaginae open, evidence of coitus | common. (McCabe and Blanchard, | 1950:39). | New brown pelage encroaching on | Postjuvenal molt completed in some saddle and on hind legs; | individuals by end of week. New postjuvenal molt completed in | pelage tends to be concealed under some individuals by eleventh | juvenal pelage longer than in _P. week. | truei_. ----------------------------------+------------------------------------- CHANGES OWING TO INCREASE IN AGE Increase in length of limb bones, changes in proportion of bones in the skull, eruption and degree of wear of teeth, and changes in pelage can be used to ascertain relative age. Different investigators might choose different limits for the three categories young, subadult, and adult. Museum specimens were assigned to one of five age groups listed below mostly on the basis of tooth wear, essentially as described by Hoffmeister (1951:1). Juvenile: M3 just breaking through bony covering of jaw or showing no wear whatsoever. Young: M3 worn smooth except for labial cusps, and M1 and M2 showing little or no wear. Subadult: M3 worn smooth; labial cusp may persist, but is well worn; M1 and M2 having lingual cusps worn, but not smooth; labial cusps showing little wear. Adult: Lingual cusps worn smooth and labial cusps showing considerable wear; labial cusp of M3 may persist. Old: Cusps worn smooth; not more than one re-entrant angle per tooth discernible, frequently none. For live animals examined in the field, criteria based on pelage and breeding condition were used, as follows: Juvenile: Only gray, juvenal pelage present. Young: Subadult pelage apparent on lateral line or on sides; body usually smaller than in adults. Subadults: Subadult pelage having mostly replaced juvenal pelage; mice often as large as adults; testes of males often abdominal in breeding season; gray juvenal pelage may persist on head of some individuals. Adult: Adult pelage present; body usually largest of all animals in population; females may have enlarged mammae from nursing previous litters; testes of males usually scrotal in breeding season; gray pelage may be present on head of some individuals.