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Mastozoología neotropical

versão impressa ISSN 0327-9383versão On-line ISSN 1666-0536

Mastozool. neotrop. v.12 n.2 Mendoza jul./dez. 2005

 

Phallic Morphology of Kunsia tomentosus (Rodentia: Sigmodontinae)

Alexandra M. R. Bezerra

PPG Biologia Animal, Dept. de Zoologia, Universidade de Brasília, Brasília, DF (CEP 70910-900), Brazil, abezerra@fst.com.br

Key words. Baculum. Glans penis. Kunsia tomentosus. Scapteromyine group. Sigmodontinae.

   The genus Kunsia Hershkovitz, 1966 comprises two species, K. tomentosus (Lichtenstein, 1830) and K. fronto (Winge, 1887). The type locality of K. tomentosus was restricted by Hershkovitz (1966) to the Uruguay River, in southeastern Brazil. This species was also recorded in Goiás, Mato Grosso, Minas Gerais, and Rondônia States, Brazil, and in northeast Bolivia (Hershkovitz, 1966; Emmons, 1999; Miranda et al., 1999; Santos-Filho et al., 2001; Rodrigues et al., 2002). Kunsia tomentosus is the largest living sigmodontine; however, it is a very rare species, resulting in a few specimens deposited in scientific collections. Due to this scarcity, data on distribution, morphology, ecology, and biology are very rudimentary (Machado et al., 1998).
   Kunsia has been grouped with the genera Scapteromys Waterhouse, 1837 and Bibimys Massoia, 1979, in the tribe Scapteromyini, based on morphology of skull, teeth, and ecological characters (Massoia, 1979, 1980; Reig, 1980; Pardiñas, 1996). This tribe comprises sigmodontine rodents with long claws, palustrine, semi-aquatic and fossorial habits, which inhabits open areas of central and southeastern Brazil, Uruguay, and parts of Argentina and Bolivia (Hershkovitz, 1966; Massoia, 1980; Musser and Carleton, 1993; Miranda et al., 1999). However, Smith and Patton (1999) based on evidence from molecular characters (Cyt b) from Kunsia, Scapteromys, Akondontini spp., among other sigmodontine rodents, questioned their systematic relationship, which was corroborated recently by D'Elía (2003) using nuclear and mitochondrial DNA sequences (now including Bibimys), and verified that Scapteromyini is not monophyletic. Gonçalves et al. (2005) in a phenetic com parison using phallic morphology from K. fronto planaltensis, Bibimys labiosus, and Scapteromys tumidus discussed the affinity among them and also questioned the relationship of Bibimys with the other two genera.
   Studies on male phallus in rodents have often been done by authors with purely descriptive purposes (Didier, 1962) or addressing the taxonomic implications suggested by observed variability (e.g., Hooper and Hart, 1962; Hooper and Musser, 1964; Lidicker, 1968; Pessôa et al., 1996; Colak et al., 1998). The morphology of the baculum has been used in systematic studies of rodents (e.g., Hershkovitz, 1962; Patton, 1987; Pessôa et al., 1998; Silva, 1998), suggesting that the study of the phallus may be a source of morphologic information about the rodents diversification. The aim of this study is to describe the phallic morphology of K. tomentosus comparing and discussing the phallic similarities with other genera traditionally included in the Scapteromyini.
   The glans penis was taken from an adult specimen collected at the Emas National Park, Goiás State (18° 15' 50'' S, 52° 53' 33'' W), Brazil, now housed at the Coleção de Mamíferos, Departamento de Zoologia, Universidade de Brasília, Distrito Federal, Brazil, with the number UNB 1705. The baculum was deposited in the Museu Nacional - Universidade Federal do Rio de Janeiro, Brazil, with the number MN 62568.
   The methodology to clear, stain and preserve the penis, and the terminology of structures follows Hooper (1958) and Lidicker (1968). The penis was taken from a specimen fixed in 10% formalin in the field and maintained in 70% ethanol. The external morphology of glans penis was analyzed before the clearing process because structures, such as spines, may be damaged or lost because of KOH action. After the analysis of external morphology, the penis was cleared and stained to examination of position of the baculum inside of the penis. KOH 4% was used for two days to clear the specimen. The process of staining was done in 0.003% Alizarin Red, and took three days. The baculum was dissected from surrounding tissues under a stereo microscope. Next, the baculum was stocked successively in 20, 40, 80 and finally in 100% glycerin, each step taking one day.
   The measurements were taken with a 9X Opaque Base Magnifier Edmund Scientific@ with millimeter precision (0.1 mm). The measures, modified from Hooper (1958), are as follows: 1) Total bacular length (TBL), inclusive terminal digits; 2) proximal baculum length (PL), osseous part; 3) distal baculum length (DL), cartilaginous part; 4) lateral digit length (LD); 5) middle digit length (MD); 6) proximal baculum osseous breadth (PB), greater breadth in bottom of osseous baculum; 7) distal baculum osseous breadth (DB), greater breadth in top of osseous baculum. All illustrations were made with the aid of a camera lucida.
   The comparisons of the phallic morphology among the genera were performed using: Hooper and Musser (1964) and Hershkovitz (1966) for Scapteromys, and Gonçalves et al. (2005) for Bibimys and K. fronto planaltensis.
   Glans. The glans penis is long, sub-cylindrical and straight (Fig. 1). The measurements are: total length, with the apical mound included, 7.5 mm; apical mound length 0.5 mm; greatest diameter 3.2 mm. The epidermis of the ventral surface is smooth from its base as far as approximately the middle of the penis; the remainder of the epidermis up to the external rim of the terminal crater is covered with single-pointed, uniformly distributed spines. On its dorsal surface, but from the middle of the glans towards the base of the penis, the spines are distributed in the medial convergence, giving a "V" aspect to the distribution of the spines. On the middle of the ventral surface arises a raphe that ranges from the prepuce to the external edge of terminal crater, giving the impression of a bisected penis. The external edge of the terminal crater is not corrugate, but presents single-pointed, uniformly distributed spines. The internal edge of the terminal crater is smooth and without spines, and contains the urethral flap, the bacular mound and the dorsal papilla. The urethral flap extends beyond the crater rim, is long, not much higher than wide, bifurcate (two lobules) and without spines. The bacular mound, whose extremity is above the edge of terminal crater, is a unique mass that has three delicate projections, that probably refers to the lateral and medial digits of the distal portion of the baculum. The lateral projections are a little smaller than the medial bacular mound, and are dorsally curved. The dorsal papilla, which is localized immediately behind the medial flank of the bacular mound, is a simple structure, lacking spines, and flattened dorsum-ventrally. The dorsal papilla is much smaller than the urethral flap, having almost all its extent on the inside of the terminal crater, only exposing the apical end above of edge of the crater.


Fig. 1. Glans penis of Kunsia tomentosus (MN 62568) in views: a) ventral, b) dorsal, c) right lateral, and d) apical end. Bar=1 mm.

   Baculum. The length of the baculum is nearly equal to the length of the glans, and is composed by a long and osseous part, the proximal portion, and a short and cartilaginous part, the distal portion (Fig. 2). The proximal portion of the baculum is a convex shaft, which becomes narrow at the distal end and wide at the proximal end. The proximal end of the osseous baculum is ventrally concave and has a medial notch that seems to divide it into two parts, giving it the appearance of an inverted heart. The distal end is almost straight, with a tenuous median depression, and narrow, capped by the distal portion of baculum. A small and tenuous back-ventral projection emerges laterally on the ventral surface near the middle of the shaft. The dorsal part of the osseous baculum has a delicate medial crest extended from the distal end until not much past the middle of the baculum.


Fig. 2. Baculum of Kunsia tomentosus (MN 62568) in views: a) ventral, b) dorsal, and c) right lateral. Bar=1 mm.

   The distal portion of the baculum is cartilaginous and has three digits that are not divided in the base, the lateral digits are smaller than the medial digit, which is long and ventral, and flattened dorsally. It was not observed the presence of small bones inside the digits. The measures of baculum analyzed were TBL = 7.0 mm, PL = 5.8 mm, DL = 1.2 mm, LD = 1.0 mm, MD = 1.3 mm, PB = 1.8 mm and DB = 0.9 mm.
   The phallic morphology of K. tomentosus shares with K. fronto, Scapteromys, and Bibimys only the bacular mound slightly trifurcated and a reduction of the cartilaginous capsule of the baculum (Table 1), as already observed and discussed by Gonçalves et al. (2005), being a derived condition among South American Sigomodontinae (Sportono, 1992). The dorsal papilla showed two conditions, simple and multilobate. All the genera showed the simple dorsal papilla, a condition common to other Sigmodontinae, while the multilobate dorsal papilla was present in Bibimys and Scapteromys only. This condition is shared with microtines (see Hooper and Musser, 1964), and Gonçalves et al. (2005) suggested that this can be a plesiomorphic character or may have arisen independently in Bibimys and Scapteromys. Neither Kunsia species showed this condition of character (present work and Gonçalves et al., 2005), nevertheless only one individual of each species of Kunsia was taken to analyses of phallic morphology, not being possible to assess any pattern of variability in this character.

   Two-lobed urethral flap was the only exclusive character shared between K. tomentosus and Bibimys, but this was with spines in the latter. The phallic characters shared between Kunsia spp. and Scapteromys are the convex gross baculum basally expanded and the presence of three cartilaginous digits undivided in the base. The general shape of the glans penis in K. tomentosus and Scapteromys spp. (long, sub-cylindrical and straight, with the bacular mound extending little beyond the crater rim) is more alike than Bibimys, which showed a conspicuous bacular mound in a flame-like shape. Moreover, Bibimys presents the bacular mound covered with spinous epidermis, a condition not present in K. tomentosus and Scapteromys spp.
   K. tomentosus presents the glans wall epidermis covered with spines throughout almost all of its body and the edge of crater, which differs from Bibimys and Scapteromys, which have a collar of non-spinous epidermis on the edge of crater (condition present in others sigmodontines, e. g., Akodon, Calomys, Holochilus and Neacomys; Hooper and Musser, 1964). The condition of this character is unknown for K. fronto.
   D'Elía (2003) and D'Elía et al. (2005) using molecular characters showed that the Scapteromyini tribe sensu Massoia (1979) is not monophyletic and cannot be grouped with any other group less inclusive than Akodontini, although K. tomentosus and Scapteromys spp. formed a natural group. Gonçalves et al. (2005), based on phenetic comparisons of the phallic morphology of B. labiosus, found one specimen with well-differentiated glans diverging in several characters from K. fronto planaltensis and S. tumidus.
   Kunsia spp. and Scapteromys shared more characters between each other than any of these shared with Bibimys, as this last genus seems to be more divergent. However, these similarities are ambiguous because the polarity of these characters are unknown and several of them occur in the other sigmodontine rodents, as previously observed by Gonçalves et al. (2005). Any conclusion based on morphology from only one individual is preliminary because little could be known about the intraspecific variability. At present, few genera and representative species of each division of Akodontini sensu D'Elía (2003) have the phallic morphology described, and is usually based in small samples (e.g., this work; Hooper and Musser, 1964; Hershkovitz, 1966; Massoia, 1979; Gonçalves et al., 2005). Therefore, for a more comprehensive approach to the morphologic patterns of phallic apparatus in the tribe Akodontini sensu D'Elía (2003), larger and more representative samples of both genera and species of the tribe are necessary.

I thank Reginaldo Constantino and Ulyses Pardiñas for special comments to this paper, and Daniel O. Mesquita for reviewing the earlier versions of the text. Also I am grateful to reviewers Guillermo D'Elía, Pablo R. Gonçalves, Pablo Teta, and Marcelo Weksler for making helpful comments that greatly improved the manuscript. This study is one of the results of the project "Vertebrates of Emas National Park" led by Flávio H. G. Rodrigues and supported by Fundação O Boticário de Proteção à Natureza/ MacArthur Foundation, Wildlife Conservation Society, BP Conservation/ BirdLife International/ Fauna and Flora International and Conservation International do Brasil/ Anhauser-Bush. The Instituto Brasileiro do Meio Ambiente and Recursos Naturais Renováveis - IBAMA provided research permission and the support at the Emas National Park. The Conselho Nacional de Desenvolvimento Científico e Tecnológico provided fellowship to the author (CNPq Procs. 141899/2004-2).

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Recibido 8 diciembre 2004.
Aceptación final 28 junio 2005.

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