Akodon caenosus Thomas 1918
Creators: name: J. Pablo Jayat
name: Pablo E. Ortiz
name: Jorge Salazar-Bravo
name: Ulyses F. J. Pardiñas
name: Guillermo D'Elía
Publisher: name: Zenodo
Year: 2010
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Mammalia
Rodentia
Muridae
Akodon
Akodon caenosus

description: Akodon caenosus Thomas, 1918
Akodon diminutus Barquez, Díaz and Goytia, 1994. Nomen nudum (see Galliari et al., 1996). Akodon aliquantulus Díaz, Barquez, Braun and Mares, 1999. Journal of Mammalogy, 80:788.
Holotype: BMNH 18.1.1.38, adult male.
Type locality: León, Jujuy, 1500 m (Thomas, 1918).
Description: Detailed morphological description in Myers et al. (1990) (as A. puer caenosus). Additional morphological and morphometric data for some populations of northwestern Argentina can be found in Thomas (1918; original description of A. puer caenosus), Barquez et al. (1980) and Díaz (1999). Here we summarize the characteristics of populations along the latitudinal and altitudinal gradients of northwestern Argentina.
Akodon caenosus is the smallest species of the boliviensis group in northwestern Argentina. Dorsal coloration uniform and highly variable, ochraceous brown with yellowish, rufous, or olivaceous casts. Ears of the same color than the dorsum and with a tuft of hairs in front. Laterals clearer than dorsum and more richly colored in some specimens. Venter paler, whitish gray, buffy gray, yellowish or even ruddy, clearly contrasting with dorsum. Chin with few white hairs that do not form a conspicuous patch. Inguinal region with a more intense tinge than the rest of the venter in some specimens. Fore and hind feet covered with whitish or buffy hairs. Tail strongly bicolored, dorsally blackish-brown and ventrally whitish or buffy, densely or sparsely covered depending on the analyzed populations and individuals.
The skull is the smallest among the Akodon species of northwestern Argentina. Rostrum short, with very narrow and shallow zygomatic notches and frontal sinuses not well developed. Interorbital region comparatively broad, with hourglass shaped and with margins rounded or slightly squared. Zygomatic arches not flared, braincase small and inflated, with temporal and lambdoid crests not well developed. Zygomatic plate narrow with anterior border straight or slightly concave and generally slopes gently backward from bottom to top. In most of the studied specimens the posterior ascending process of alisphenoid does not reach the squamoso-alisphenoid groove or only touch their lower margin. Hamular process of squamosal delicate, but in a few individuals it is strongly built. Incisive foramina generally extended to the protocone of M1, but in some individuals its reach the hypoflexus or even further back. Mesopterygoid fossa very narrow, its anterior margin rounded or slightly squared and with lateral margins straight and slightly divergent backward. Parapterygoid fossa generally broader than mesopterygoid, with convex border and slightly divergent backward. Mandibular ramus very delicate. Masseteric crest extends slightly behind the anterior margin of m1. Capsular projection generally poorly developed and situated at the same level or behind the coronoid process. Angular process is less extended backward than the condyle.
Upper incisors orthodont or slightly opistodont. The procingulum and anteromedian flexus of M1 are well developed. Anteroloph and mesoloph are always present and in some individuals an enteroloph can be observed. The M2 has a conspicuous mesoloph and the M3 shows great variability. In old individuals it is completely oval shaped but in young ones anteroflexus, metaflexus and hipoflexus are present. The m1 has the procingulum and anteromedian flexid well developed and presents protostylid and ectostylid. In some individuals a remnant of metastylid and mesostylid can be observed. The m2 retains only the labial stylid although some specimens have a very small ectostylid. The m3 presents deep labial and lingual flexids. Some individuals show a small protostylid.
Karyotype: 2n = 34, FN = 40, based on six specimens from León, Jujuy, and two from El Cadillal, Tucumán (Barquez et al., 1980; Vitullo et al., 1986 as A. puer).
Variation: We have observed substantial morphological variation for this species. Within the same population, we find pale and dark individuals, some of them with striking rufous tones, particularly lactating females. Among the cranial characters, the development of zygomatic notches, zygomatic plate, and the thickness of hamular process show some variability. We also observed morphometric variations for individuals of the same age and population, with remarkable differences among some of the studied localities. In addition, consistent variation in the coloration pattern with elevation and some geographic variation for some morphometric variables are also apparent. The specimens from high altitude populations are conspicuously paler than those from lower localitites, showing more buffy tinges in the coloration of the fur and more developed eyerings. These populations come from open environments characterized as the ecotone between humid grasslands and semiarid areas (e.g., the Prepuna or impoverished grasslands of high-Andean region). Specimens from central Salta and central Tucumán provinces are on average smaller than individuals from Bárcena and Reyes, southern Jujuy, near the type locality.
Comparisons: Externally this species can be confused with A. boliviensis and young individuals of A. spegazzinii. The differences between A. caenosus and A. boliviensis were described in detail by Myers et al (1990) and are summarized above.
In areas of sympatry A. caenosus and A. spegazzinii follow similar variation patterns in coloration through altitudinal gradients. However, many morphometric values and several cranial characters are useful in specimen determination. In 15 of the 20 analyzed morphometric variables, A. caenosus is significantly smaller than A. spegazzinii (Tables 1 and 2). This is also verified by the minimum overlap in the PCA analysis (Fig 2) and the lack of missclasifications in the DA. In addition, Akodon caenosus shows narrower and shallower zygomatic notches, comparatively broader interorbital constriction, temporal and mastoid ridges less developed, narrower mesopterygoid fossa and less hypsodont molars. The average percentage of genetic divergence between these species is the greatest (7.7%) observed for the boliviensis group in northwestern Argentina (Table 12).
Akodon caenosus and A. sylvanus represent two extremes in the morphometric variation observed within the boliviensis group in northwestern Argentina (Tables 1 and 2; figure 2). Akodon caenosus is, furthermore, paler than A. sylvanus, showing a more evident contrast between dorsum and venter, and has more developed eyerings. The skull of A. caenosus has a shorter and narrower rostrum, less swollen frontal sinuses, narrower braincase and interorbital region and a narrower mesopterygoid and parapterygoid fossa. The upper incisors in A. caenosus are less orthodont and the molars less hypsodont than A. sylvanus. Additionally, these species show a high average percentage of genetic divergence (6.7%).
The differences between A. caenosus and the new species are listed under the treatment of the latter.
Distribution: Akodon caenosus is the more broadly distributed species of the boliviensis group in northwestern Argentina, with records from northernmost Salta to southern Catamarca, from 400 m to 3100 m elevation (Fig. 7).
Habitat: Most of the records come from Yungas environments, from the lower altitudinal belt to high altitude grasslands. Notwithstanding, we have recorded the species in Chacoan environments near the ecotone with Yungas and in the lower altitudinal limit of the High Andean grasslands.
Natural history: Although we have recorded specimens in reproductive condition throughout the year, most of the individuals were active between November and January. We observed the highest proportion of molting animals in fall and winter. In their broad distributional range A. caenosus can be found coexisting with many different species depending on the altitudinal or latitudinal sector in northwestern Argentina. Thus, in the north A. caenosus was captured alongside Akodon sylvanus, A. simulator, Oxymycterus paramensis, Oligoryzomys cf. O. flavescens and Oligoryzomys sp. in high altitudinal grasslands at 1400 m. At the same latitude in upper belts above 2000 m, A. caenosus is sympatric with A. boliviensis, Necromys lactens, N. amoenus, Calomys musculinus (Thomas), Phyllotis caprinus Pearson, P. osilae and P. xanthopygus. In the south, in Yungas areas of Tucumán province, the species was caught together with Akodon spegazzinii, A. simulator, Necromys lactens, N. lasiurus (Lund), Oxymycterus paramensis, Oxymycterus wayku Jayat et al., Oligoryzomys cf. O. flavescens, Oligoryzomys sp., Phyllotis anitae, P. osilae, Andinomys edax and Abrothrix illutea. At this same latitude in Chacoan environments in transition with Yungas A. caenosus is sympatric with Akodon spegazzinii, A. simulator, Necromys sp., Oligoryzomys cf. O. flavescens, Oligoryzomys sp., Calomys sp. and Graomys centralis.
Comments: Although originally described as a subspecies of Akodon puer (Thomas 1918), the form caenosus was soon considered as a valid species by Thomas (1920); this latter suggestion was followed by Cabrera (1961), Barquez et al. (1980), and Mares et al. (1981). Vitullo et al. (1986) and Apfelbaum & Reig (1989) considered specimens from León, Jujuy Province (type locality of caenosus) as A. puer and Myers et al. (1990) followed this position (but see Hershkovitz 1990), considering caenosus as the subspecies of A. puer inhabiting northwestern Argentina (see also Barros et al. 1990). Anderson (1997) also recognized three subspecies for A. puer but noted the priority of the name lutescens over puer. Since then, the nomenclature used for this species has been constantly changing. Capllonch et al. (1997) and Mares et al. (1997) listed caenosus as valid species whereas Díaz et al. (1999, 2000) recognized caenosus as subspecies of puer for the populations of the northwestern Argentina. Notwithstanding, Díaz (1999) and Díaz & Barquez (2007) considered again the nominal form caenosus as representing a valid biological species including A. lutescens puer as an additional sigmodontine species in northwestern Argentina. Musser & Carleton (2005) and Pardiñas et al. (2006) followed the conclusions of Myers et al. (1990).
Díaz et al. (1999) described Akodon aliquantulus, from the ecotone between upper Yungas forest and highland grassland in Tucumán province (Las Agüitas, Cumbres del Taficillo, 1700 m, 26º 42’ S, 65º 22’ W), and related it to the A. boliviensis group on the basis of its morphology. The unimpressive morphological distinction from A. lutescens, based only on multivariate ordinations and univariate overlap, induced some authors to recommend a revision of the status of this form (Musser & Carleton 2005; Jayat et al., 2008a). This species was diagnosed and differentiated from A. caenosus (as A. puer caenosus) and A. spegazzinii on the account of being smaller in a number of measurements and the “weakly developed eye ring” of two individuals originally preserved in fluid and then prepared as skin plus skull. We could not distinguish the type specimens of A. aliquantulus from A. caenosus following the diagnosis provided by Díaz et al. (1999). Most of the characters used for the diagnosis of A. aliquantulus (centered, as said, on a few external and cranial dimensions) are subtle, highly variable in many sigmodontine species, and what is most important are age-dependent. Although Díaz et al. (1999) considered the two specimens as old individuals (age-class 5) our observations indicate that they must be assigned to the age class 4 of Myers (1989) (Figure 8). The morphometric distinction between A. aliquantulus and A. caenosus was based on a PCA analysis with a relatively low explained variance (63.3%, 7.9% and 5.1% for components 1, 2 and 3 respectively) and the statistic significance of the observed differences was not provided by the authors. We think that the erroneous assignment of age for the two specimens by Díaz et al. (1999) misinform their conclusions. The analysis of table 1 of Díaz et al. (1999) clearly indicates the overlap between A. aliquantulus specimens and A. caenosus exemplars of ageclass 4 (see also Table 9 of this work). Remarkably, even some of the diagnostic skull measurements (e.g. braincase breadth, maxillary toothrow length) are included in the observed range for age-class 5 individuals of A. caenosus. Moreover, all the characters mentioned in the description of A. aliquantulus are within the levels of morphological variation observed in A. caenosus of Northwestern Argentina. Therefore, we find no defensible argument to consider A. aliquantulus as a distinct species and consider it a junior synonym of A. caenosus.
description: Published as part of J. Pablo Jayat, Pablo E. Ortiz, Jorge Salazar-Bravo, Ulyses F. J. Pardiñas & Guillermo D'Elía, 2010, The Akodon boliviensis species group (Rodentia: Cricetidae: Sigmodontinae) in Argentina: species limits and distribution, with the description of a new entity, pp. 1-61 in Zootaxa 2409 on pages 23-25, DOI: 10.5281/zenodo.293461