The humerus of South American caviomorph rodents: shape, function and size in a phylogenetic context C. C. Morgan 1,2 & A. Álvarez 1 1 Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, La Plata, Buenos Aires, Argentina 2 CONICET, Buenos Aires, Argentina Keywords functional morphology; geometric morphometrics; humerus; phylogenetic comparative methods; South American hystricomorphs. Correspondence Cecilia C. Morgan. Sección Mastozoología, Div. Zoología Vertebrados, Museo de La Plata. Paseo del Bosque s/n°, B1900FWA, La Plata, Argentina. Email: cmorgan@fcnym.unlp.edu.ar Received 3 August 2012; revised 8 January 2013; accepted 14 January 2013 doi:10.1111/jzo.12017 Abstract South American caviomorph rodents comprise four major lineages encompassing wide taxonomic and ecological diversity, but the morphological diversity of their postcranial skeleton has not been thoroughly explored using phylogenetic com- parative methods. The main goal of this work is to analyze their humerus using geometric morphometrics in a phylogenetic context and attempt to tease apart the influence of locomotory preferences and shared evolutionary history on morpho- logical variation. We examined 28 genera in 9 families representing all major clades. Humeral shape was captured by 13 landmarks and four semilandmarks in 2D. In the morphospace of the first two principal components, most taxa were located near the origin along both axes. Fossorial octodontoids were apart from this central group. Most caviids were separated in one extreme of the morphos- pace; the specialized digging ctenomyid Ctenomys, the fossorial chinchillid Lagos- tomus and the generalized cavioid Cuniculus were located at the opposite end. Phylogenetic signal was significant. Regressions of shape on size were not signifi- cant; regression of shape on habit was significant for raw data and not significant after phylogenetic control. Humeral shape variation was primarily associated with the phylogenetic structure of the group; additionally, some morphological traits were associated with particular habits and interpreted as functional specializa- tions. This association between humeral shape and both phylogeny and habit at different hierarchical levels suggests early ecomorphological diversification of caviomorphs. Introduction As a major component of the forelimb skeleton, the humerus provides much functional information. The strong link between humeral morphology and locomotory and substrate preferences has been widely recognized; for example, runners tend to have longer, gracile humeri with narrow distal epiphy- ses, while diggers have shorter, more robust humeri with well- developed deltopectoral crest and broad distal epiphyses (see Hildebrand, 1988; Polly, 2007 and literature cited therein). Morphological variation is also expected to reflect shared evo- lutionary history (Felsenstein, 1985; Losos & Miles, 1994). Thus, humeral shape results from a complex interaction of factors, and phylogenetic context is essential to analyze and understand putative morphological adaptations. South American hystricomorph rodents (caviomorphs) are an excellent model to analyze such interactions, as the clade encompasses wide taxonomic and ecological diversity, distrib- uted in four major lineages traditionally considered as super- families: Erethizontoidea, Chinchilloidea, Octodontoidea and Cavioidea (Woods & Kilpatrick, 2005). They occupy different habitats and present varied habits: climbers such as the ereth- izontoid Coendou (prehensile-tailed porcupine) and the octo- dontoids Phyllomys and Dactylomys (spiny rats); cursors, as the cavioids Dasyprocta (agouti) and Dolichotis (mara); diggers including the chinchilloid Lagostomus (viscacha) and the specialized subterranean octodontoids Ctenomys (tuco- tuco) and Spalacopus (coruro); as well as generalized epigean forms that climb, dig and swim to some extent, such as the cavioids Cuniculus (paca) or Microcavia (mountain cavies). Their size also ranges widely from the small coruros, tuco tucos and some spiny rats (with a body mass as low as 80 g) medium-sized forms such as Myocastor (coypu; 7 kg), Doli- chotis (12 kg) or Lagostomus (6.5 kg), to the largest living rodent, the capybara Hydrochoerus (53 kg; Nowak, 1991). The postcranial skeleton of caviomorphs has been analyzed following different approaches (Lehmann, 1963; Biknevicius, 1993; Vassallo, 1998; Rocha-Barbosa et al., 2002, 2007; Weisbecker & Schmid, 2007; Samuels & Van Valkenburgh, 2008; Seckel & Janis, 2008; Morgan, 2009; Morgan & Verzi, 2011; Rocha-Barbosa & Casinos, 2011). In particular, Elissamburu & Vizcaíno (2004) and Candela & Picasso (2008) studied the humerus of a wide taxonomical sample, while Morgan & Verzi (2006), Steiner-Souza, de Freitas & Journal of Zoology Journal of Zoology. Print ISSN 0952-8369 Journal of Zoology •• (2013) ••–•• © 2013 The Zoological Society of London 1