Environmental factors and population fluctuations of Akodon azarae (Muridae: Sigmodontinae) in central Argentina

The aim of this work was to explore the relationship between population density of Akodon azarae (Muridae: Sigmodontinae) and climatic and environmental variables, and determine which of them are associated to within and among‐year changes in rodent abundance in agro‐ecosystems from south Córdoba, Argentina. The study was carried out in a rural area of central Argentina, from 1983 to 2003. Density was estimated as a relative density index (RDI). Temperature, precipitation and humidity were obtained from records of the National University of Rio Cuarto. Normalized Difference Vegetation Index (NDVI) and Land Surface Temperature were recorded from National Oceanic and Atmospheric Administration (1983–1998) and Landsat (1998–2003) imagery data sets. We performed simple correlations, multiple regressions and distributed lag analysis. Direct association of climatic and environmental variables with RDI was in general, low. The amount of variability in seasonal changes in density explained by climatic and environmental variables altogether varied from 10% to 70%. Seasonal population fluctuations were influenced by NDVI and rainfall with one and two seasons of delay. Autumn maximum density of the species was also associated with vegetation and rainfall of previous seasons. There also seemed to be an indirect influence of rainfall through vegetation given that we found a positive correlation between them. Results were consistent with basic aspects of the ecology of the species, such as its strong preference for highly covered areas, which provide food and protection from predators, likely increasing its reproductive success. Therefore, in the rural area central Argentina, A. azarae showed seasonal fluctuations with delayed influence of rainfall and vegetation and indirect effects of rainfall.     

A new species of Liniscus (Nematoda: Trichuridae) from Oxymycterus rufus and Akodon azarae (Cricetidae: Sigmodontinae) in Buenos Aires Province, Argentina

Liniscus diazae n. sp. (Nematoda: Trichuridae) is described from the urinary bladder of Oxymycterus rufus and Akodon azarae (Cricetidae: Sigmodontinae) from Buenos Aires Province, Argentina. Liniscus diazae can be differentiated from its congeners by having a much longer spicule and without an elevated vulva. In addition, L. diazae can be distinguished from L. incrassatus and L. himizu by having a longer body length. Males are similar in posterior width to L. papillosus and L. himizu, but thinner than L. incrassatus and L. maseri. This is the second record of a capillarid from mammals in Argentina. An updated list of capillarid species of rodents and insectivores from North America, with their synonyms, hosts, sites of infection, and geographic distribution, is provided.     

A new species of Syphacia (Nematoda: Oxyuridae) from Akodon azarae (Rodentia: Cricetidae) in Argentina

Eight species of Syphacia (Nematoda: Oxyuridae) have been reported from South America in rodents of the Sigmodontinae, only 1 of which has been recorded in Argentina. Syphacia (Syphacia) carlitosi n. sp. is described from the ceca of Akodon azarae bibianae and Akodon azarae hunteri (Sigmodontinae: Akodontini) captured in 3 provinces in the northeast region of Argentina. The new species is differentiated principally by the shape of the cephalic plate; distribution of submedian papillae and amphids; presence, extent, and shape of cervical alae in females; absence of lateral alae; absence of deirids; spicular and gubernaculum length; shape and structure of accessory hook of gubernaculum; and distance of mamelons, excretory pore, and vulva from the anterior extremity. This is the second record of Syphacia parasitizing rodents of the tribe Akodontini.     

Inter-male aggression with regard to polygynous mating system in Pampean grassland mouse, Akodon azarae (Cricetidae: Sigmodontinae)

Based on the hypothesis that, in Akodon azarae, polygyny operates through female defence, we studied inter-male aggression in order to test the following predictions: during the breeding period (1) resident males are more aggressive than intruder males in the presence of females (FP), and (2) aggressive behaviour is independent of male condition (resident or intruder) in the absence of females (FA). To test our predictions, we used the resident male behavioural response towards an intruder male in relation to FP or FA. We conducted 30 encounters in FP and 27 in FA in 0.79-m² round enclosures placed in the Espinal Reservation. Our results support the prediction that, in FP, the intensity of aggressive behaviour exhibited by males varied in relation to resident or intruder condition. Resident males showed high levels of aggression towards intruders, and intruders exhibited the greatest values of submissive behaviours with residents. In FA, the intensity of aggressive behaviour did not vary in relation to resident or intruder condition. Both resident and intruder males exhibited low aggressive behaviour and inter-male encounters resulted mainly in non interactive behaviours. Our results support the hypothesis that, in A. azarae, the polygynous mating system operates through female defence.     

A new angiostrongylid (Nematoda) species from the pulmonary arteries of Akodon azarae (Rodentia: Cricetidae) in Argentina

Angiostrongylus morerai n. sp. (Nematoda: Angiostrongylidae) is described from the pulmonary arteries of Azara's grass mouse Akodon azarae (Rodentia: Cricetidae) in Argentina. It is distinguished from its congeners principally by the morphology of the dorsal ray, which is as long, or longer, than the externodorsals and has 2 long branches; the spicule lengths are also greater (400-465 microm). This is the first record of a metastrongyloid from sigmodontine rodents in Argentina.     

Ectoparasite occurrence associated with males and females of wild rodents Oligoryzomys flavescens (Waterhouse) and Akodon azarae (Fischer) (Rodentia: Cricetidae: Sigmodontinae) in the Punta Lara wetlands, Argentina

The objective of this research was to study infestation parameters and indexes of ectoparasites associated with each sex of the wild rodents Oligoryzomys flavescens (Waterhouse) andAkodon azarae (Fischer) in the Punta Lara wetlands, Argentina. A trend towards higher mean abundance (MA) and ectoparasite specific richness was observed in males of O. flavescens whereas those values were similar for both A. azarae sexes. The prevalence of the following ectoparasites was significantly higher on males (P<0.05): Mysolaelaps microspinosus Fonseca (65.2%) and Hoplopleura travassosi Werneck (73.9%) on O. flavescens, and Ixodes loricatus Neumann (71.4%) on A. azarae. Only H. travassosi mean abundance was significantly higher on males (MA=44.1). Since I. loricatus and Hoplopleura spp. are involved in the transmission of pathogens that cause diseases in animals and humans, and whose reservoirs are rodent hosts, these results are epidemiologically important.     

Return to preferred habitats (edges) as a function of distance in Akodon azarae (Rodentia, Muridae) in cropfield-edge systems of central Argentina

We studied the ability of Akodon azarae (Rodentia, Muridae) to return to their preferred habitat, when released at a perpendicular distance (25, 50 or 75 m) from the edge towards the cropfield, and a parallel distance (100 m) away from the site of first capture within the edge habitat. Return success was estimated as the proportion of animals recovered in edges. The recapture rate between the field and the border was significantly higher than the recapture rate estimated according to successive captures in the border. Successful returns did not decrease significantly with increasing release distance, but animals released at 50 m from edges were less successful in returning to borders than the other release-distance groups. Although the median time taken to the first recapture in edges did not differ among the release-distance groups, rodents released at 25 m and 50 m returned to edges faster than those released at 75 m. A. azarae showed both a successful return to the edge and a trend to return to the home range area. We conclude that A. azarae can return to edges from cropfields at distances that are larger than those they usually travel, allowing the use of fields when they present good conditions for reproduction and survival. Successful return is probably the result of direct movements rather than random wandering.     

Chromosomal aspects and inheritance of the XY female condition in Akodon azarae (Rodentia,Sigmodontinae)

The populations of several species of Akodon present, besides XX females, a variable proportion of fertile XY females. In Akodon azarae, a correspondence exists between the X-chromosome C-banding pattern and the sexual phenotype of XY individuals: males carry a determinate X-chromosome type, defined by its C-banding pattern, and XY females, any of two others. To confirm the relation between X-chromosome type and the XY female condition and to investigate the hereditary transmission of these different X-chromosomes, we analyzed 50 animals captured in the field and 95 individuals corresponding to the F₁ and F₂ offspring of 16 crosses.It was seen that the correlation between X type and the sexual phenotype of XY animals is retained, and that the three X types are transmitted to the progeny. It was also observed that the male offspring of XY females receive the X-chromosome from their male parents and the Y from their mothers. These results strongly support the causal role of an X-borne mutation in A. azarae XY sex reversal, and discard a mutation of the Y-chromosome as the sole basis of this phenomenon.     

The sex-determining zinc finger sequences in XY females of Akodon azarae (Rodentia, Cricetidae)

Wild populations of Akodon azarae comprise females with a karyotype indistinguishable from that of males. These individuals were formerly assumed to be Xx, the x being an X chromosome with a deletion of most of its long arm. By using a DNA probe derived from the testis-determining region of the human Y chromosome (comprising a candidate gene for the testis-determining factor, Y-linked zinc finger [ZFY]), we demonstrate that A. azarae gonosomally variant females are XY and not Xx. The ZFY sequences in A. azarae are amplified and located in two different families of EcoRI fragments derived from Y-chromosome DNA. No rearrangement or change in the state of methylation of ZFY or ZFX (X-linked zinc finger) sequences were found in XY females. We propose that sex reversal in A. azarae may be mediated by a gene or genes other than ZFX or ZFY.     

Diversity of the helminth community of the Pampean grassland mouse (Akodon azarae) on poultry farms in central Argentina

This paper describes the helminth community of the Pampean grassland mouse (Akodon azarae) inhabiting poultry farms in central Argentina. Winter diversity (season of high rodent abundance) has been compared to spring diversity (season of low rodent abundance). Species richness was seven in both seasons: five nematodes (Syphacia carlitosi, Stilestrongylus spp., Trichuris laevitestis, Pterygodermatites (Paucipectines) azarai and Protospirura numidica criceticola) and two cestodes (adults of Cyclophyllidea and Taenia taeniaeformis hepatic cysts). No difference in richness was detected between host sexes in each season or among host age classes. However, the helminth community showed 67% similarity between winter and spring, with diversity being significantly higher in spring (H = 0.873) than in winter (H = 0.546; P < 0.0005). This could be attributed to different factors, such as host abundance, host diet or environmental factors, that affect the transmission of each species differently. On the other hand, Stilestrongylus spp. and S. carlitosi showed higher dominance and intensity in both seasons compared to their cohabiting species, P. (P.) azarai and T. laevitestis, respectively. The lower values of the latter two species may be related to a crowding effect due to their large body sizes. This is the first report of cestodes in A. azarae. The finding of T. taeniaeformis strobilocerci could be important in the epidemiology of parasitosis in domestic animals of the farms.     

Phenotypic plasticity in response to low quality diet in the South American omnivorous rodent Akodon azarae (Rodentia: Sigmodontinae)

We studied the responses in the omnivorous rodent A. azarae submitted to a low quality diet at morphological, physiological and biochemical levels. At short term, a decrease in body mass occurred. A later increase in food consumption constituted a strategy that allowed a temporal recovery of physical condition. However, hyperphagia appeared not to be enough to maintain physical condition after 30 days of low quality diet consumption. At the morphological level, an increase in length (9%) of the anterior portion of the gut occurred, the part of the gut where digestion and absorption take place. A decrease in small intestine weight could be related with the long-term impairment of body condition. Inhibition of sucrase specific activity in small intestine would indicate a down-regulation of sucrase-isomaltase complex. Total maltase specific activity in small intestine was not affected suggesting an up-regulation of sucrase-independent maltase specific activity. A down-regulation of protease specific activity in small intestine occurred in response to low quality diet. The specific activity of disaccharidases in caecum and large intestine was down-regulated. The strategies and constraints at different levels of A. azarae upon low quality diet are discussed.     

Effect of removing a dominant competitor,Akodon azarae (Rodentia,Sigmodontinae) on community and population parameters of small rodent species in Central Argentina

Abstract We studied the effect of removing Akodon azarae (Muridae, Sigmodontinae) on community and demographic parameters of rodent species in crop field borders of Central Argentina. We applied three replicated treatments: enclosure and removal of A. azarae, enclosure without removal, and a control without removal or enclosure. We conducted 14 monthly capture–mark–recapture samplings between October 1988 and March 1990. During the study period we observed an increase in Mus domesticus (Muridae, Murinae) abundance in removal areas. Other species did not respond to A. azarae removal. When A. azarae was removed, M. domesticus appeared to be competitively dominant over the remaining species of the community, increasing its relative abundance. Reproductive parameters of M. domesticus were more sensitive to treatments than survival parameters.     

Phenotypic flexibility of digestive morphology and physiology of the South American omnivorous rodent Akodon azarae (Rodentia: Sigmodontinae)

We have determined the occurrence of responses at different levels (morphological, physiological and biochemical) in the omnivorous rodent Akodon azarae upon cold acclimation (15 degrees C). A short-term enhancement in food consumption appeared to account for the maintenance of both mass and body composition. At the morphological level, the main response was an increase in the dimensions of small intestine, which constitutes the section of the gut where absorption and secretion take place. An increase in sucrase specific activity was only found in small intestine. Sucrose independent maltase activity was very low since 99.8% of total maltase activity was due to sucrase-isomaltase (SI) complex. Protease specific activities were not affected. The fact that resting metabolic rates determined at 15 and 23 degrees C were similar in cold acclimated animals suggests a change in lower critical temperature. In conclusion, our results show that A. azarae exhibits different strategies to support cold environment that could lead to an enhancement in digestion and absorption efficiency. Furthermore, this work suggests that low temperature is an independent cue of other environmental factors to trigger the strategies allowing the maintenance of body condition in A. azarae.     

Temporal variation of allele frequencies in populations of Akodon dolores (Rodentia,Cricetidae)

Summary Six population samples of the South American cricetid rodent Akodon dolores were collected at the same site at six-month intervals over a three year period. Changes in density were detected. Seven out of 18 loci analyzed by means of starch gel electrophoresis were polymorphic. Only two of these loci (Est-4 and G6pdh) showed statistically significant variation in allele frequencies following a seasonal pattern. There was no correlation between allele frequencies and population density. When animals were grouped into two classes according to body weight, a clear difference in allele distribution at the Est-4 and G6pdh loci was observed between individuals 39 g or less and those heavier than 39 g. As the first group comprises predominantly younger animals, the data indicate that changes in the age-structure of population, rather than density variations, are responsible for the cyclic pattern of allele frequencies fluctuations.     

Description of a new Capillariinae (Nematoda: Trichuridae) from Scapteromys aquaticus (Cricetidae: Sigmodontinae) from Buenos Aires, Argentina

A new species of Capillaria is described on the basis of specimens recovered from the intestine of the swamp rat Scapteromys aquaticus (Cricetidae: Sigmodontinae) from Argentina. Capillaria alainchabaudi n. sp. and a peculiar species from Australian marsupials are the only 2 species of Capillaria sensu stricto parasitic in mammals. A comparison with the Australian species and with the 18 species of this genus described from other vertebrates from the Western Hemisphere is given. The separation of the new species is based on morphologic and morphometrical features, such as intestine ending in cloaca beside ejaculatory duct, 2 lateral nonmembranous caudal lobes, 2 pairs of caudal non-pedunculated papillae, terminal part of cylindrical cirrus ornamented with thin and thick spines, spicule with apex not well sclerotized in the males, a conspicuous vulvar appendage in the females, and 2 bacillary bands. A survey of the literature revealed that the species of Capillariinae from rodents belong to 9 genera, and the total number of species is low compared to the high diversity and abundance of the hosts, particularly if the modern Muroidea are considered.     

Climatic drivers of pinyon mouse Peromyscus truei population dynamics in a resource-restricted environment

Highly variable patterns in temperature and rainfall events can have pronounced consequences for small mammals in resource-restricted environments. Climatic factors can therefore play a crucial role in determining the fates of small mammal populations. We applied Pradel's temporal symmetry model to a 21-year capture–recapture dataset to study population dynamics of the pinyon mouse (Peromyscus truei) in a semi-arid mixed oak woodland in California, USA. We examined time-, season- and sex-specific variation in realized population growth rate (λ) and its constituent vital rates, apparent survival and recruitment. We also tested the influence of climatic factors on these rates. Overall monthly apparent survival was 0.81 ± 0.004 (estimate ± SE). Survival was generally higher during wetter months (October–May) but varied over time. Monthly recruitment rate was 0.18 ± 0.01, ranging from 0.07 ± 0.01 to 0.63 ± 0.07. Although population growth rate (λ) was highly variable, overall monthly growth rate was close to 1.0, indicating a stable population during the study period (λ ± SE = 0.99 ± 0.01). Average temperature and its variability negatively affected survival, whereas rainfall positively influenced survival and recruitment rates, and thus the population growth rate. Our results suggest that seasonal rainfall and variation in temperature at the local scale, rather than regional climatic patterns, more strongly affected vital rates in this population. Discerning such linkages between species' population dynamics and environmental variability are critical for understanding local and regional impacts of global climate change, and for gauging viability and resilience of populations in resource-restricted environments.