ARBOREAL RICE RATS — IIERSHKOVITZ 525 



table 2 of a series of the equally small but terrestrial species Oryzomys 

 alfaroi. Measurements in tables 3 and 4 are, respectively, of a series 

 of the large arboreal Oryzomys (Oecomys) concolor from eastern 

 Colombia, and those of a series of the comparably sized terrestrial 

 Oryzomys (Oryzomys) laticeps from the same region. Because of 

 distortions of the hind|foot in dry specimens, no reliable measurement 

 can be given for the width of the metatarsus relative to foot length. 



The hind foot of Oecomys appears short in comparison with terres- 

 trial types. Nevertheless, the proportions between metatarsalia and 

 digits have probably diverged little from those of the scansorial type 

 of primitive cricetines. On the other hand, the greater breadth of 

 the metatarsus of subgenus Oecomys (fig. 4a,b) does appear to be 

 a specialization for arboreal life. The broader foot provides a more 

 pow^erful and expansive grasp for climbing, perching, and hanging. 

 It also permits an effective degree of opposability between the first 

 and fifth metatarsals and a considerable amount of adduction and 

 abduction in the corresponding toes. 



Plantar tubercles: Six plantar tubercles, the primitive nimiber, 

 are present and well developed in subgenus Oecomys (lig. 4a,6). The 

 first four postdigital pads are large and with little space between them. 

 The smaller fifth postdigital pad is also well developed, and is sepa- 

 rated from the fourth by a space less than its smallest diameter. The 

 metatarsal pad is long and narrow, as usual in cricetines. The function 

 of the postdigital plantar tubercles has not been observed in subgenus 

 Oecomys. Homer (Contr. Lab. Vert. Gen. Univ. Michigan, No. 61, p. 

 13, 1954) states that in scansorial forms of Peromyscus the pads are 

 used for grasping. The method was not described but undoubtedly 

 slender branches and twigs may be clutched between apposing 

 tubercles. 



Claws: The claws are thick, comparatively short, recurved, their 

 tips sharp and raised well above the ends of the toes. These claws 

 cannot serve for digging and their position does not interfere with the 

 action of the toes as grasping organs. In contrast, claws of closely 

 related terrestrial cricetines such as Oryzomys laticeps are slender, 

 little curved, and shield the fleshy tips of the toes when used m 

 scratching or digguig. 



Tail: Modification of this organ as a specialized tool for arboreal 

 life is not pronounced in subgenus Oecomys. It is not remarkably 

 long and in many individuals is actually shorter than head and body 

 combined. The average length, however, is greater than that of head 

 and body combined. The tail of subgenus Oecomys appears to be 

 slightly thicker than those of comparable sylvan cricetines of pre- 

 dominately terrestrial habits. This character indicates greater 

 power for balancing, prehension and support in climbing. The 



