Phyllotis xanthopygus (Waterhouse, 1837) is an Andean rodent endemic to South America. Despite its wide geographical distribution in Argentina, few individuals have been studied on the cytogenetic level and only through conventional staining. In this work, chromosome characterization of Argentine samples of this species was performed using solid staining, C-banding and base-specific fluorochromes. Twenty two specimens were analyzed, collected in the provinces of Jujuy, Catamarca, and the north and south of Mendoza. All studied specimens showed 2n=38, having mostly the bi-armed autosomes, metacentric or submetacentric. Fundamental Number varied between 70 and 72. These changes were due to the presence of chromosome heteromorphisms in individuals from southern Mendoza and Jujuy. C-banding revealed pericentromeric blocks of constitutive heterochromatin in most chromosomes. Acrocentric chromosomes involved in heteromorphisms showed high variation in the amount of heterochromatin within and among populations. Additionally, banding with fluorochromes (DAPI and chromomycin A3) revealed homologous localization of AT and GC rich regions among chromosomes of the different populations analyzed. Comparisons among heteromorphic pairs suggested, however, that the variation might be the result of complex chromosome rearrangements, involving possibly amplifications and/or deletions of heterochromatic segments. These results are in accordance with molecular studies that indicate genetic variability within and among the populations of this taxon.

Understanding why social relationships vary among conspecifics is central to studies of animal behaviour. For many species, patterns of space use provide important insights into social behaviour. To characterize the social organization of the highland tuco-tuco (Ctenomys opimus), we used visual observations and radiotelemetry to quantify spatial relationships among adults in a population at Laguna de los Pozuelos, Jujuy Province, Argentina. Specifically, we sought to confirm anecdotal reports that these subterranean rodents are social, meaning that adults share burrow systems and nest sites. Our data indicate that the animals live in spatially distinct groups, although the number of individuals per group varies markedly. Although these relationships were robust with regard to location (above vs. below ground) and type of data (visual vs. telemetry), some groups identified during the daytime fissioned during the night. We suggest that the population of C. opimus at Pozuelos is facultatively social, meaning that individuals display predictable, adaptive differences in social relationships with conspecifics. More generally, our findings add to the growing number of subterranean species of rodents recognized as social, thereby generating new opportunities for comparative studies of these animals aimed at assessing the causes and consequences of variation in social organization.

RESUMENEl ratón delicado de los salares (Salinomys delicatus) es un pequeño roedor endémico de Argentina. Ha sido considerado como vulnerable a la extinción dada su distribución restringida y en parches, tamaño poblacional pequeño y especialización en hábitats salinos. A pesar de que el ratón delicado tiene adaptaciones morfológicas y fi siológicas para la supervivencia en ambientes áridos y salitrosos, poco es lo que se conoce de su historia natural. El presente trabajo tiene como objetivo estudiar la historia natural de S. delicatus, reportar nuevas localidades de ocurrencia, caracterizar el uso de hábitat, su dieta y el ensamble acompañante, y describir su biología reproductiva y morfología. Se registraron dos nuevos sitios de ocurrencia en ambientes de bosques en galería y bajos salinos en la región noreste de la provincia de Mendoza (Argentina), siendo las primeras citas para esta provincia. S. delicatus presenta una dieta omnívora, como es frecuente en otros roedores de desierto, y consume en igual medida semillas, artrópodos y plantas halófi tas. El ensamble acompañante está compuesto por no más de tres especies de pequeños mamíferos que varían según la localidad, siendo esta riqueza similar a otros sitios del Monte. La proporción de sexos y el patrón de dimorfi smo sexual mostraron un marcado desvío hacia las hembras (H:M = 3.6:1.1 y H:M = 1.151 respectivamente). La morfología externa y craneal presenta una importante variabilidad intra e interpoblacional, sugiriendo que el aislamiento juega un rol importante. Cuando se consideraron adultos y juveniles, conjuntamente dos de las seis medidas externas (longitud total y longitud de cabeza y cuerpo) y siete de las 22 medidas craneales resultaron signifi cativamente mayores en hembras que en machos. Cuando se compararon solo los adultos, cuatro características externas (longitud total, longitud de cabeza y cuerpo, largo de la cola y peso) y seis medidas craneales fueron signifi cativamente mayores en hembras que en machos. Este trabajo refuerza la importancia del desarrollo de nuevos estudios más detallados sobre la historia de vida de S. delicatus y permite reafi rmar la importancia de conservación de esta especie.Palabras clave: conservación, historia de vida, ratón delicado, salares, zonas áridas. ABSTRACTThe delicate mouse (Salinomys delicatus) is a small rodent endemic of Argentina. It has been considered vulnerable to extinction because of its restricted and patchy distribution, small population size and specialization on salty habitats. Besides its morphological and physiological adaptations to survive in arid and salty habitats, there is still little information about the natural history of the delicate mouse. The objectives of our study are to study the natural histor y of S. delicatus, report new localities of occurrence, characterize its habitat use, diet and accompanying assemblages and describe its reproductive biology and morphology. We report two new localities of occurrences in gallery woodlands and salt fl ats in the northeast of Mendoza prov...

Salinomys delicatus is considered a rare species due to its restricted and patchy distribution, poor records and low abundances. It is also the phyllotine with the lowest known diploid chromosome number (2n = 18), however its sex chromosome system has never been described. Here, we studied the chromosomes of six females and three males with bands G, C, DAPI/CMA(3) and meiosis. In males, the chromosome number was 2n = 19, with one large metacentric X-chromosome and two medium-sized acrocentrics absent in females. The karyotype of females was the same as previously described (2n = 18, FN = 32), with X-chromosomes being metacentric and the largest elements of the complement. In males, the two acrocentrics and the large metacentric form a trivalent in meiotic prophase. This indicates that S. delicatus has XY(1)Y(2) sex chromosomes, which is confirmed by G and DAPI bands. Constitutive heterochromatin (CH) is restricted to small pericentromeric blocks in all chromosomes. The X-chromosome shows the largest block of centromeric CH, which could favor the establishment of this X-autosome translocation. This sex chromosome system is rare in mammals and, compared with other phyllotine rodents, S. delicatus seems to have undergone a major chromosome restructuring during its karyotypic evolution.

Despite striking diversity in mammalian social behavior, studies of social organization have often dichotomized species by identifying them as either solitary or social (i.e., group living). This tendency has been particularly pronounced for subterranean rodents, the majority of which have long been assumed to be solitary. As a result, variation in social organization has likely been underestimated for these animals, particularly for species in which patterns of space use suggest limited or temporally dynamic opportunities for interactions among conspecifics. Here, we characterize patterns of space use in a population of tuco-tucos (Ctenomys sp.) from Anillaco, La Rioja Province, Argentina. Although these animals have been the subject of extensive research regarding circadian patterns of activity, spatial and social relationships among free-living individuals have not been documented. Analyses of radiotelemetry data from 17 individuals monitored during the breeding season (December 2015) revealed that partial overlap of individual home ranges was common, occurring between male–female as well as female–female pairs of animals. Spatial relationships, however, were dynamic, with both home range sizes and overlap changing on a daily basis. Although members of the study population did not meet the criteria typically used to identify group living in subterranean species, they were not completely solitary. Instead, the animals displayed an intermediate form of social organization characterized by persistent partial overlap of the areas occupied by different adults. These data add to the growing comparative picture of social variation in Ctenomys and suggest that further studies of these animals should contribute to improved understanding of the factors underlying differences in mammalian social systems.