1936] MAMMALS OF OREGON 197 



ing among the leaves or against the trunks of trees or in the forks of 

 the branches. 



Breeding habits. The females have but 2 pairs of mammae, on 

 the posterior part of the belly. Sets of 1, 2, and 3 embryos have 

 been noted in those taken for specimens. Four is probably the nor- 

 mal maximum number of young for fully adult females, about half 

 the number of most microtines. On September 14 jewett took a 

 female in Curry County that contained 3 embryos, and Shelton took 

 2 young from a nest on Spencer Butte near Eugene on February 21. 

 In California, Wilder found young in the nests in January and Feb- 

 ruary, and Taylor records young found in the nests at Mendocino 

 City on July 15 and 17 and half-grown young in Humboldt County 

 in July (1915, p. 151). From these dates it would seem that breed- 

 ing may occur at any time of year, but it is probable, as believed 

 by Wilder and Clay, who have had unusual opportunity for observ- 

 ing these mice, that the main breeding season is from early spring 

 until late summer. 



Howell found the young at birth in about the same helpless and 

 primitive condition as young meadow mice, but with the difference 

 that the eyes did not open until the young were 19 days old and that 

 the}^ showed no inclination to leave the nest until 29 days old. For 

 the first 2 weeks they were generally found attached to the mother's 

 teats, and when the family was disturbed the young clung so tightly 

 that she would drag them all around the cage and could even be 

 lifted if one took hold of one of the young. This habit and the slow 

 development of the young are excellent adaptations for the protec- 

 tion of arboreal species. The slow development is almost squirrel- 

 like, and the small number of young in a litter in comparison with 

 ground-dwelling microtines is paralleled by the corresponding con- 

 ditions in the tree squirrels as compared to the large litters of ground 

 squirrels. 



Food habits. The stomach and intestines of one collected near 

 Eugene were full of the green pulp of Douglas fir leaves, and the fresh 

 pellets in numerous other nests examined in that part of Oregon were 

 all of pure green material, indicating that their food was all, or 

 mainly, derived from these leaves. None of the rolled oats placed in 

 their nests or on the feeding platforms was eaten, and no trace of 

 food other than the millions of leaf fibers containing in some cases 

 the central vascular bundles and in others the marginal resin ducts 

 were found on or around the houses. Apparently the animals are 

 active throughout the year and a permanent supply of their ever- 

 green leaves is always available. 



Howell in a careful study of their food habits in captivity could 

 not induce them to eat anything but the leaves of conifers, of which 

 Douglas fir was preferred. Grand fir (Abies grandis) was also 

 eaten, as well as the leaves of the large cone fir (Pseudotsuga 

 macrocarpa) ) of an exotic fir from the Caucasus (Abies nordman- 

 niana), and of the deodar cedar (Cedru<s deodara). 



Their usual method of feeding is to cut off the terminal twigs of 

 green branches and eat the leaves or carry them to the nest or house 

 each night to provide enough to last through the day, placing them 

 within easy reach of the nest. These are eaten at frequent intervals 

 as determined by Howell, averaging about 100 leaves an hour for 



