Inferring adaptation within shape diversity of the humerus of subterranean rodent Ctenomys

In subterranean rodents of the genus Ctenomys , excavation activity can be carried out with the claws and forelimbs (scratch‐digging) as well as with the skull and incisor teeth (skull‐tooth digging). Within the forelimb myoskeletal system, the humerus is a main bone concentrating a large number of muscles and bearing tensions during excavation. The genus Ctenomys is considered primarily a scratch‐digger and secondarily a skull‐tooth digger. We analysed the humerus ( N  = 165) of four species of Ctenomys from southern Brazil, in areas ranging from the soft soils of the first lines of coastal dunes ( Ctenomys flamarioni , Ctenomys minutus ), through the sandy fields of the coastal plains ( Ctenomys minutus , Ctenomys lami ), on to the hard soils of the southern pampas ‘gaúchos’ fields ( Ctenomys torquatus ). The differences in the form (size + shape) were quantified using geometric morphometrics methods and interpreted in the light of myological descriptions. As expected from a phylogenetic and ecological point of view, C. flamarioni had the most divergent shape and larger size among the species analysed, showing a more slender humerus, especially in the head region, than C. lami , C. minutus , and C. torquatus . Crossing the osteology data with the qualitative observations of the musculature, it was possible to detect large differences in the proximal portion of the humerus that could be related to the insertion of important extension muscles of the pectoral–shoulder joints, which could increase force. The comparison of shape differences between the three closely‐related species ( C. lami , C. minutus , and C. torquatus ) revealed unexpected patterns because C. lami was the species phenetically more distant from C. flamarioni and not C. torquatus as expected from ecological data and phylogenetic relationships. A two‐step adaptive path to humeral shapes better fit to digging is postulated where the deltoid crest and epicondylar crest increases precede an articular surface area increase. The absence of sexual dimorphism in C. torquatus is discussed with regard to the optimal size required to dig in hard soils. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society , 2010, 100 , 353–367.