Vol. 3, Nº 2

The known late Cretaceous-Present South American land mammals, more than any other land fossils, demonstrate that macroevolutionary processes can only be understood if we have a prior knowledge of the historic biogeographical events. On this basis, the main events marking the history of South American land mammals are analyzed in relation to the geodynamic events that produced the successive geographical and climate-environmental South American changes. It was found a close relationship between the abiotic and biotic phenomena succeeded during the late Cretaceous-Present span. The mammals up to date known representing the early and late Cretaceous time interval pertain only to non-tribosphenic and pre-tribosphenic groups. Further, they are markedly endemic (to the familial level, and even to higher levels, e. gr. Gondwanatheria), which are interpreted as related to the long isolation of the Gondwanan Continent. This is recognized as the Gondwanan Stage, which followed the still non-recorded Pangean Stage. It has been recognized that there exists a long unrecorded span, representing at least most of the Maastrichtian and Danian. The next record, estimated as spanning the time between ca. 63.2 – 61.8 Ma, indicated that there had occurred a marked turnover, characterized by the replacement of the pre- and non-tribosphenic native mammals by the immigrant tribosphenid marsupials and placentals. Furthermore, the Patagonian locality of this age records the last non-tribosphenic gondwanatherian (Sudamerica ameghinoi) together with the first non-Australian monotreme (Monotrematum sudamericanum). On the one hand, it is obvious that during the unrecorded Maastrichatian-Danian span there existed connections both with North America and Australia (via Antarctica). On the other hand, the so marked endemism of marsupials and placentals indicated that the isolation of the South American continent had been acting from some time ago. This Isolation Stage was broken up during the late Eocene, permitting the immigration of hystricognathan rodents and platyrrhini monkeys —apparently from Africa—, and during the late Oligocene-early Miocene, permitting that some megalonychoid edentates emigrated to the West Indies. The final connection with North America occurred by the late Pliocene and through the Panamanian Bridge. It led to the Great American Biotic Interchange. It was preceded by some premonitory interchange, as island-hoppers, having the procyonid carnivores and the tardigrade edentates as the main heralds, respectively from North America and South America. The Megafaunal Extinction, succeeded around 10000 years before Present, and the subsequent entrance of the Sciuridae, Heteromyidae, and Geomyidae rodents, the lagomorph Leporidae, and the Soricidae insectivores, built up the present scenario characterizing the Neotropical Region. We estimate that at least 50% of the present genera descended from North American immigrants. This paleobiogeographical history indicates that South America is the only continent that preserves, as fossil or living, land mammals originated in any of the major continental blocks that differentiated from the late Triassic on, when the oldest mammals was recorded. Surprisingly, none of its extant land mammals represent any order proved to be originated in the South American continent when differentiated as such.

Many South American murids are reservoirs of etiologic agents of endemic diseases that are transmitted to man. The study of their reproductive physiology is essential for the knowledge of these diseases. We studied the changes in the vaginal cell morphology to determine the characteristics of the estrous cycle in Akodon azarae and Calomys laucha in laboratory conditions. We used this information to establish in these species the distribution of the estrous cycle throughout the seasons during a year in field conditions. The length of the cycle most frequently observed was 6 and 7 days for A. azarae and C. laucha respectively. In both species vaginal smears were studied during the estrous cycle. The changes in the vaginal smears were also studied during gestation in C. laucha. Field data showed that there is not a complete interruption of estrous cyclicity in these rodents in winter, however pregnant females (as determined by abdominal palpation) were not captured in this season.

Ciclo estral en condiciones de laboratorio y de campo en dos especies de múridos sudamericanos (Akodon azarae y Calomys laucha). Muchos múridos sudamericanos son reservarlos de agentes etiológicos de enfermedades endémicas que son transmitidas al hombre. El estudio de su fisiología reproductiva es esencial para el conocimiento de estas enfermedades. Estudiamos los cambios en la morfología celular vaginal para determinar las características del ciclo estral en Akodon azarae y Calomys laucha en condiciones de laboratorio. Usamos esta información para establecer en estas especies la distribución del ciclo estral en condiciones de campo a través de las estaciones de un año. El largo del ciclo observado más frecuente fue de 6 y 7 días para A. azarae y C. laucha respectivamente. En ambas especies los extendidos vaginales fueron estudiados durante el ciclo estral. Los cambios en los extendidos vaginales fueron también estudiados durante la gestación en C. laucha. Los datos de campo mostraron que no hay una completa interrupción de la ciclicidad estral en estos roedores en invierno, sin embargo no se capturaron hembras preñadas (determinadas por palpación abdominal) en esta estación.

The burrowing behavior of Ctenomys eremophilus in substrates of different hardness was studied in a glass terrarium. We found that both the type and frequency of use of burrowing tools are closely associated with the degree of hardness of the substrate. The study of the burrowing sequence showed that Ctenomys eremophilus uses both forelegs and incisors when burrowing in a hard substrate, but only forelegs in a soft substrate. Hence, this species is a “mixed” digger according to Dubost’s classification, and a “scratch- and chiseltooth digger” according to Hildebrand’s.

Comportamiento excavador de Ctenomys eremophilus (Rodentia, Ctenomyidae) en relación a la dureza del substrato. Se estudió el comportamiento excavador en un terrario vertical y en diferentes durezas de substrato. Encontramos que, tanto el tipo de herramienta como la frecuencia de uso, estaba estrechamente relacionada con el grado de dureza del substrato ofrecido. El estudio de la secuencia de excavación mostró que Ctenomys eremophilus en un substrato duro usa los miembros anteriores y los incisivos, pero solamente los miembros anteriores en uno blando. Así, esta especie es un excavador «mixto» de acuerdo a la clasificación de Dubost, y un excavador «rasguñador» y excavador «con dientes» de acuerdo a la clasificación de Hildebrand.

Lestodelphys halli is a small, terrestrial didelphid marsupial known only from a few widely scattered, semidesert localities from Mendoza south to Santa Cruz in Argentina. We have updated the distribution map based on 17 traditional specimens and 31 catalogued and more than 15 uncatalogued individuals and lots of osteological remains recovered from owl pellets. Images taken from a videotape of a captive L. halli document the killing and partial consumption of a mouse, strongly supporting Thomas’ (1921) hypothesis that the species is predaceous. The karyotype consists of 14 chromosomes and is generally similar to that of other 2N = 14 marsupials. Study of a 281-base pair region of the cytochrome b gene compared to the same region in other South American didelphids suggests that Lestodelphys is most closely related to Thylamys, another small South American didelphid that occurs in southern Argentina, but does not elucidate the relationship of this clade to other genera previously hypothesized to be closely related to Lestodelphys.

Lestodelphys halli: nueva información sobre un marsupial argentino muy poco conocido. Lestodelphys halli es un marsupial didélfido de pequeño tamaño conocido solo por unas pocas y dispersas localidades en semidesiertos de Argentina, desde Mendoza hasta Santa Cruz. Nosotros hemos actualizado el mapa de distribución de la especie sobre la base de los 17 especímenes regulares y sobre evidencias adicionales obtenidas de 31 especímenes y lotes catalogados y al menos 15 especímenes obtenidos ambos de regurgitados de lechuzas. Una serie de fotografías obtenidas de una videofilmación de un Lestodelphys halli en cautiverio documenta la muerte y consumo parcial de un ratón, apoyando la sugerencia de Thomas (1921) sobre los hábitos de predador de esta especie. El cariotipo es de 14 cromosomas y es similar al 2N = 14 de otros marsupiales. Estudios utilizando regiones del ADN mitocondrial de esta especie, comparados con las mismas regiones del ADN mitocondrial de otros didélfidos sudamericanos, sugieren que Lestodelphys es la especie más cercanamente relacionada a Thylamys, otro pequeño didélfido que habita el sur de Argentina. Las relaciones filogenéticas de este ciado con otros géneros, previamente hipotetizados como cercanamente emparentados, permanecen sin resolver en nuestro estudio.

The red vizcacha rat Tympanoctomys barrerae is a rare octodontid rodent endemic to the desert ecosystem of central western Argentina. Our report is based on recent surveys of two isolated populations in the desert habitats of Mendoza province, Argentina. The red vizcacha rat lives in or near salt basins (“salares”) (Arroyito site) and sand dunes (Trintrica site). This species lives in complex burrow systems built in sand mounds, with at least three different gallery levels, and several entrances. In Arroyito, halophytic chenopods such as Heterostachys, Alternanthera and Suaeda (Chenopodiaceae) represent more than 80% of its diet. In Trintrica the red vizcacha rat preferred Atriplex (Chenopodiaceae) and Prosopis flexuosa (Leguminosae). Our data on the natural history of T. barrerae expand the understanding of this poorly known and highly adapted desert rodent.

Observaciones ecológicas de la rata vizcacha colorada, Tympanoctomys barrerae, en hábitats de desiertos de Argentina. La rata vizcacha colorada, Tympanoctomys barrerae, es un roedor octodóntido raro, endémico del ecosistema desértico central oeste de Argentina. Nuestro trabajo se basa en estudios recientes de dos poblaciones aisladas de hábitats desérticos de la provincia de Mendoza. La rata vizcacha colorada habita salares o sus alrededores (en Arroyito) y dunas (en Trintrica). Esta especie vive en sistemas de galerías complejos, de al menos tres niveles y con varias entradas, construidos en montículos de arena. En Arroyito, varias quenopodiáceas halofíticas tales como Heterostachys, Alternanthera y Suaeda (Chenopodiaceae) constituyen más del 80% de la dieta. En Trintrica, la rata vizcacha colorada prefiere Atriplex (Chenopodiaceae) y Prosopis flexuosa (Leguminosae). Nuestros datos acerca de la historia natural de T. barrerae amplían el conocimiento de esta especie pobremente estudiada y altamente adaptada a la vida en el desierto.

El presente trabajo tuvo como objetivo estudiar la dieta del ciervo de los pantanos y los aspectos nutricionales de las especies vegetales más utilizadas. Se realizaron inventarios florísticos y se detectaron las principales especies utilizadas por el ciervo de los pantanos a lo largo del año. Además, se realizaron observaciones del comportamiento alimentario. En el laboratorio se efectuó el análisis químico de siete especies, incluyendo la determinación de humedad y materia seca, proteína bruta, fibra bruta, fósforo, calcio y magnesio. Los valores obtenidos indicarían ciertas carencias minerales en la dieta, especialmente de fósforo y calcio, en algunas categorías de la población.

Diet of the Marsh Deer, Blastocerus dichotomus, at lberá reserve, Corrientes, Argentina. The present study on the marsh deer’s diet was carried out at lberá Natural Reserve. The nutritional quality of the botanical species most commonly used by this cervid has also been investigated. Botanical inventories were made and the main species in the deer’s year round diet were detected. In addition, observations of the deer’s behavior were made during different seasons of the year. Twenty-two species were identified as components of the marsh deer’s diet in the lberá lake area; 72.7% are herbs, 18.1% are shrubs and 9% are subfruits. Of these 22 species, 14 probably constitute the “base” of the diet. The feeding activity of the deer varies with the season of the year according to the weather. Chemical analyses performed on seven plant species induded the determination of water content and the remaining dry residue, total protein, total fiber, phosphorus, calcium and magnesium. The different laboratory values obtained for the 7 selected species were compared with different ruminants (Odocoileus spp.) and cattle. The results indícate the lack of certain minerals in the diet of some individuals, especially phosphorus and calcium. The importance of the aquatic habitats in the marsh deer’s feeding habits is mentioned.