Oecologia Australis 19(1): 143-157, 2015 10.4257/oeco.2015.1901.09 CRANIAL MORPHOMETRIC ANALYSES OF THE CRYPTIC RODENT SPECIES AKODON CURSOR AND AKODON MONTENSIS (RODENTIA, SIGMODONTINAE) Diego Astúa1*, Isabella Bandeira1 and Lena Geise2 1 Universidade Federal de Pernambuco, Departamento de Zoologia, Laboratório de Mastozoologia. Av. Prof. Moraes Rego s/n, Cidade Universitária. Recife, PE, Brasil. CEP: 50670-901. 2 Universidade do Estado do Rio de Janeiro, Departamento de Zoologia, Laboratório de Mastozoologia. Rua São Francisco Xavier 524, Maracanã. Rio de Janeiro, RJ, Brasil. CEP: 20550-900. E-mails: [email protected], [email protected], [email protected] ABSTRACT Akodon cursor and A. montensis are two morphologically cryptic species of Akodontini rodents, undistinguishable by external, skin or qualitative cranial characters, and they can only be separated based on the presence or absence of a gall bladder or by genetic data. The purpose of our study was to assess whether geometric morphometrics of the skull and mandible could identify and separate specimens of A. cursor from those of A. montensis. We used 74 adult specimens (48 - A. cursor e 25 - A. montensis) previously identified by their karyotypes, from Southeastern Brazil. We digitized 24, 22, 25, 13 landmarks on images of their skulls (in dorsal, ventral, and lateral views) and mandible, respectively. We assessed size variation (sexual dimorphism and species differences) through t-tests and ANOVAs of centroid sizes of each structure and view, and shape variation through Goodall tests, MANOVAs and Canonical Variates Analyses on Procrustes coordinates. We found significant sexual size dimorphism (p<0.001) as well as size differences between species for all four views (p<0.001), with males larger and A. cursor larger than A. montensis, with an overlap between female A. cursor and male A. montensis specimens. We found significant sexual shape dimorphism in A. cursor (all views) and A. montensis (lateral view of the skull only), while the two species were completely separated in shape in all four views. Part of the observed variation in shape of the skull and mandible of the two species are likely related to size (allometry), while the lack of adequate ecological information hinders any evaluation of whether skull differences are related to feeding habits. Keywords: geometric morphometrics; Rodentia; sexual dimorphism; skull, taxonomy INTRODUCTION is studied through the analysis of variance and covariance of pairs of Geometric morphometric constitute linear measurements (Marcus 1990), a well-established mathematical geometric morphometrics tools describe framework of tools developed since and locate the areas of shape changes the 1990s that allow the description and more clearly and represent graphically representation of the geometry of the such changes (Rohlf and Marcus 1993). studied shape. While in “traditional” The use of geometric morphometrics as (=linear) morphometrics form a tool in mammalogy is now widespread, (understood as shape + size) variation and employed in systematics, evolution, 144 Cranial Morphometric of Akodon cursor and Akodon montensis biogeography and morphology analyses 2n = 52, with most of the members being (Adams et al. 2004, Kryštufek et al. nevertheless characterized by a low 2012, Sebastião and Marroig 2013). diploid number (Pardiñas et al. 2015). Several groups of small mammal There are species groups defined species, including the Neotropical rodent according to phylogenetic analysis cricetid subfamily Sigmodontinae, (for a complete review, see Jayat et al. which is the most diverse mammal 2010 and Pardiñas et al. 2015). The group in South America, are widely only widespread species group at the distributed (Reig 1986). Craniodental Eastern part of South America is the characteristics define the Sigmodontinae Akodon cursor group, which includes tribes proposed by Vorontsov (1959, five cryptic species, one with the broader apud Weksler 2006), with some support latitudinal distribution – A. cursor of DNA sequences (Salazar-Bravo et al. (Geise 2012). Both A. montensis and 2013). The Akodontini, one of the most A. cursor, besides being cryptic in their diverse tribes, is also one of the most external and craniodental morphology widespread groups of Sigmodontinae, are found in sympatry in some localities occurring in most South American (Geise et al. 2005). The non-described habitats (Reig 1986). Despite the large third species, with a diploid number of geographic distribution and habitat 2n = 10 (Silva and Yonenaga Yassuda utilization, morphology between and 1998) has a disjunct distribution (Silva within genera shows a high degree et al. 2006). The karyotype of A. cursor of similarities and overlap. Species has a diploid number that varies from boundaries can be established with the 14 to 16 (Fagundes et al. 1998) and A. use of karyotypic data, mainly in the montensis presents diploid numbers Akodon genus (Ventura et al. 2012). equal to 24 or 25 (Rieger et al. 1995). The genus Akodon, now with 38 Internal characters, such as the absence recognized species, still presents a or presence of the gall bladder can also complex and unstable taxonomy, be used to distinguish them (Geise et al. including cryptic and sympatric 2005), or through molecular (Geise et species, in spite of being genetically al. 2001) and traditional morphometric defined. This genus possesses a wide analyses (Geise et al. 2005). distribution in South America, with The difficulty to identify specimens several species being described recently, of these two species that do not have such as A. polopi Jayat, Ortiz, Salazar- data on gall bladder or genetics raised Bravo, Pardiñas & D’Elía, 2010 and A. questions about the correct identification philipmyersi Pardiñas, D´Elía, Cirignoli of individuals included in ecological and Suarez, 2005. Species of Akodon can studies or the analysis of specimens be found in several biomes of Eastern deposited in collections. To increase South America (Savanna, Caatinga, the knowledge on the diversification Atlantic forest and Paraguayan Chaco), and differentiation of these two characterized by a wide variation of species, we analyzed size and shape diploid numbers, of 2n = 9 or 10 up to variation within and between Akodon Oecol. Aust., 19(1): 143-157, 2015 Astúa et al. 145 cursor and Akodon montensis using and lateral view, and of the mandible in geometric morphometrics tools, in lateral view, using the same orientation order to assess whether a geometric protocol for all specimens. morphometrics approach results in a Landmarks better discrimination than a traditional morphometric analysis. Landmark coordinates for all views of skull and jaw were captured MATERIAL AND METHODS using TPSDig2 (Rohlf 2006). Two- dimensional homologous landmarks Sample were digitized on both sides of the skull We only used specimens identified to facilitate visualization in the graphic through their karyotypes, in order to representations of deformations grids. avoid misidentification. Specimens are For statistical analyses an average housed in the collection of Laboratório of the homologous landmarks after de Mastozoologia, Universidade do reflection on the symmetry axis was Estado do Rio de Janeiro, and will be used, to avoid the redundancy of the data deposited in the Mammal Collection (inflation of degrees freedom). Twenty- of the Museu Nacional, Rio de Janeiro. four landmarks were established for Only adults were included, based on dorsal, 22 for ventral and 25 for lateral patterns of dental eruption following of skull views. For mandibles, 13 Voss (1991). Specimens were collected landmarks were established. Landmarks in southeastern Brazil (Appendix). are illustrated in Figure 1 and are For all specimens, we recorded digital defined as follows. In bilateral views, images of the skull in dorsal, ventral pairs of numbers refer to right and left Figure 1. Landmarks used on the dorsal, ventral and lateral views of the skull and on the lateral view of the mandible. Refer to text for precise description of landmark locations. For each view, a scheme with lines linking landmarks, as used in subsequent graphs for clarity and visualization, is presented. Oecol. Aust., 19(1): 143-157, 2015 146 Cranial Morphometric of Akodon cursor and Akodon montensis landmarks, respectively (dorsal view) or 20) internal more concave curve point to left and right landmarks, respectively of maxillary root of zygomatic arch, (ventral view). (17, 21) internal more concave curve Dorsal view of the skull: (1) frontal tip point of inner extreme curvature of of internasal suture, (2) posterior border squamosal root of zygomatic arch, (18, of supraoccipital, at the nuchal crest, (3, 22) posterior inferior tip of squamosal 21) external border of maxillary at the root of zygomatic bar, (23) tip of nasals most anterior curvature of zygomatic at their anterior suture in front of the arch, (4, 20) more concave curve point gnathic process, (24) posterior midline of zygomatic notch, (5, 19) internal suture of palatines, (25) midpoint border of maxillary, at the most anterior of suture between basisphenoid and curvature of zygomatic arch, (6, 18) most basioccipital. anterior point of maxillary and frontal Lateral view of the skull: (1) anterior suture, (7, 17) more concave curve tip of the nasals, (2) skull most posterior point of squamosal root of zygomatic border, at supraoccipital bone, above arch, (8, 16) external posterior border the foramen magnum, (3) labial upper of squamosal root