<oological Journal of the Linnean Sociely (1993), 107: 107-115. With 6 figures Allometry and adaptation in the long bones of a digging group of rodents (Ctenomyinae) A. CASINOS, F.L.S., C. QUINTANA* AND C. VILADIU Departament de Biologia Animal ( Vertebrats) , Universitat de Barcelona, 08028 Barcelona (Spain) and *Facultad de Ciencias Exactas y Naturales, Departamento de Biologia, GIBE, Ciudad Universitaria Nuiiez, Pabelldn II, 4O piso, 1428 Buenos Aires (Argentina) Heceiued September 1991, revised and accepted for publication March 1992 Previous studies of rodent appendicular morphology suggest that digging activity induces changes in long bones, producing shorter and thicker structures. Subsequent hypotheses have been tested in Ctenomyinae, a group of octodontid rodents globally adapted to subterranean life. Slopes of the equations calrulated for extant animals and their corresponding confidence intervals agree with expectations in almost all cases. Results on fossil taxa are less clear, but suggest a morphocline from a plesiomorphic condition of the appendicular skeleton, present in the fossil genera, departing little from that of the current epigeous rodents, to a more derived long bone design in the species of the living genus Ctenomys, in accordance with their digging activity. ADDITIONAL KEY WORDS:-Mammalia - Rodentia - fossil - morphametrics. CONTENTS Introduction . . . . . . . . . . . . . . . . . . . 107 Material and methods. . . . . . . . . . . . . . . . . 108 Results and discussion. . . . . . . . . . . . . . . . . 109 Conclusions . . . . . . . . . . . . . . . . . . . I1 I Acknowledgements . . . . . . . . . . . . . . . . . 114 References . . . . . . . . . . . . . . . . . . . 114 INI'RODUC'I'ION Two approaches are possible to explain the relationships between limb allometry and locomotion. First, hypotheses in which natural selection is not considered (McMahon, 1975), establishing predictions in a purely biomechanical way. In its most radical version, this approach denies the possibility that selective pressure can modify allometric proportions ( L ~ v t r u p & Hild, 1979). Judgement about the inclusion of this approach within a non- adaptationist tradition in comparative biology has been published elsewhere (Casinos, 1990). The second approach assumes that particular locomotor patterns exert selective pressures, changing the normal quadrupedal morphology (Bou, Casinos & Ocaiia, 1987; Casinos, 1989; Castiella & Casinos, 1990). According to the Castiella & Casinos (1990) predictions of the geometrical similarity (L a D, L or D a IM0.33, L, D and M, being length, diameter and body mass respectively) are sufficient to explain allometry in the limbs of most 0024 -4082/93/020107+09 SOS.OO/O 107 0 1993 The Linnean Society of London