ARBOREAL RICE RATS — ^HERSHKOVITZ 



527 



Figure 6. — Hair-scale relationships on basal portion of tail in Oryzomys. Hairs of one 

 scale only are shown; the same number and disposition of hairs occurs on every scale. 

 (X40.) 



aquatic cricetines, the tail has become longer, at least on an average. 



The degree of specialization of hind foot and tail of cricetines may 

 be measured by the amount each organ has diverged from the critical 

 proportions given above. Thus, in oryzomyines the longer the hind 

 foot than 21.5 percent of combined head and body length, or shorter 

 than 21 percent, the more specialized it is. In other cricetines, the 

 critical proportions may be a little more or less. Likewise, the longer 

 or shorter a tail than combined head and body length, the greater 

 the specialization. This, in effect, implies that a specialized short- 

 footed species could not have evolved from a specialized long-footed 

 form, or vice versa; and that a specialized long-tailed species cannot 

 claim a specialized short-taUed species as its ancestor, or vice versa. 



On the basis of the above criteria, Oryzomys (Oecomys) concolor 

 is more highly speciahzed for arboreal life than 0. (Oecomys) bicolor. 

 Its hind foot is comparatively shorter, its tail proportionately longer. 

 The more developed temporal ridges also indicate greater special- 

 ization of the masseter muscle. 



Habits and Habitats 



The two known species of subgenus Oecomys nest in tree hoUows, 

 tangled masses of epiphytes or vines, palm fronds, abandoned bird 

 nests and in thatched roofs of houses. They normally feed above 

 ground, but famine or drought may drive them to the ground in search 



