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Androlaelaps fahrenholzi is a mite with a large distribution and associated with a wide range of hosts. To assess morphometric variation of A. fahrenholzi associated with different host species (Phyllotis xanthopygus and Akodon albiventer, both from Putre, Chile) and localities (Las Chinchillas National Reserve, Fray Jorge National Park, and Llanos de Challe National Park, all in Chile and all from the host Phyllotis darwini), 41 morphological characters of each specimen were measured, and principal component analysis and discriminant analysis were used. Both analyses showed two groups separated for species of rodents. Mites associated with P. xanthopygus are smaller than those of A. albiventer. The analysis by location indicates two groups: group 1 comprises all mites collected from Fray Jorge National Park and Las Chinchillas National Reserve, and group 2 comprises only mites from Llanos de Challe National Park. These results indicate that A. fahrenholzi population can vary between hosts and geographical areas. Molecular analysis would be necessary to validate these results and determine whether they are subspecies or different species., Facultad de Ciencias Naturales y Museo, Centro de Estudios Parasitológicos y de Vectores

Helminth parasites are still not a well-known component of animal biodiversity. In this study, we describe and compare the endoparasite assemblages of three native rodent species from a semiarid Mediterranean ecosystem. A total of eight species of gastrointestinal helminth parasites were detected. The endoparasitic communities of the rodents Phyllotis darwini (5 species), Octodon degus (4 species), and Abrothrix olivaceus (3 species) were described in terms of their prevalence and mean abundance. In addition, endoparasite records by rodent species were submitted to rarefaction analyses to verify sampling robustness and used to compare richness and similarity among rodent species. O. degus presented the highest prevalence and parasitic load, but was of low diversity, while P. darwini presented the highest diversity, but had a low prevalence rate. Conversely, A. olivaceus presented the lowest prevalence and diversity. This study contributes to the existing knowledge of the endoparasitic fauna of rodent species from semiarid Mediterranean zones. Resumo Os helmintos parasitas não são um componente ainda bem conhecido da biodiversidade animal. Neste trabalho, descreveu-se e comparou-se as redes de endoparasitas de três espécies de roedores nativos de um ecossistema mediterrâneo semiárido. No estudo foram detectadas um total de oito de espécies de helmintos gastrointestinais parasitas. As comunidades endoparasíticas dos roedores Phyllotis darwini (5 espécies), Octodon degus (4 espécies) e Abrothrix olivaceus (3 espécies) foram descritas por meio da sua prevalência e abundância média. Além disso, os registros de endoparasitas achados nas diferentes espécies de roedores foram submetidos a uma análise de rarefação para verificar a robustez da amostragem, e utilizados para comparar a riqueza e a similaridade entre espécies de roedores. Octodon degus apresentou a maior prevalência e carga parasitária, menor diversidade, enquanto P. darwini apresentou a maior diversidade, mas uma baixa prevalência. Porém, A. olivaceus apresentou a menor prevalência e diversidade de parásitas. Este estudo representa uma contribuição ao conhecimento da fauna endoparasitária das espécies de roedores próprias das zonas semi-áridas do Mediterrâneo chileno.

Replacement of native forests by forest plantations may change the composition and abundance of small mammals, thus influencing the foraging behavior of mesocarnivores in these human-created habitats. We assessed how differences in prey abundance between native forests and exotic plantations in southern Chile may explain the prey selection of four mesocarnivores, as analyzed from their scats. Using a spatial zero-inflated Poisson model, we determined that the abundance of most small mammals was lower in plantations than native forests, except for three common species, which had similar or larger abundances in exotic plantations. We assessed mesocarnivores' prey selection by assessing the coefficients and log-ratios of use and availability of a Bayesian Resource Selection Function. We determined that in native forest, the preferences of kodkod ( Leopardus guigna ) for arboreal prey was stronger, whereas chilla fox ( Pseudalopex griseus ) and Darwin's fox ( Pseudalopex fulvipes ) exhibited a selective preference for ground prey. Darwin's fox also exhibited a habitat-dependent changes in their selection for Darwin's leaf-eared mouse ( Phyllotis darwini ), from a positive log ratio in native forest to a negative ratio in exotic plantations. Conversely, culpeo fox ( Pseudalopex culpaeus ) selected long-tailed colilargo ( Oligoryzomys longicaudatus ) and Chilean climbing mouse ( Irenomys tarsalis ) in plantations only, even though these prey were more abundant in native forests. Although mature commercial forest plantations may provide feeding grounds for mesocarnivores, depending on their species-specific ability to capture available prey, the decline of small mammal availability in plantations may modify the prey selection of mesocarnivores.

American trypanosomiasis is a chronic disease transmitted mainly by vectors. The hematophagous triatomine vectors transmit Trypanosoma cruzi to a wide variety of mammals, which usually are their food source. This study determined the feeding profile of Mepraia spinolai, a sylvatic triatomine vector, present in endemic areas of Chile. Vectors were captured in the north-central area of Chile. Samples of intestinal contents were analyzed by an Enzyme-linked immunosorbent assay (ELISA) that identifies and discriminates the presence of serum antigens from Homo sapiens and nine animal species (Canis familiaris, Felis catus, Capra hircus, Mus musculus, Gallus gallus, Octodon degus, Thylamys elegans, Phyllotis darwini and Oryctolagus cuniculus). Our data indicate the most frequent feeding source in this area was P. darwini, followed by O. degus, O. cuniculus, M. musculus, G. gallus, T. elegans, C. familiaris, F. catus and C. hircus. Mixed food sources were also identified.

Background Trypanosoma cruzi is the agent of Chagas disease, a major public health problem in Latin America. Many wild and domestic animals are naturally infected with T. cruzi; rodents are one of the groups which have been consistently detected infected in different countries. The aim of this work was to characterize blood T. cruzi load in naturally infected rodents from a Chagas disease endemic region in Chile. Methods Baited traps were set in domestic and peridomestic areas of rural dwellings. The rodents were anesthetized and blood sampled; DNA was extracted and the parasite load was quantified by T. cruzi satellite DNA real-time PCR assays. Results Seventy-one rodents of four species, Rattus rattus, Mus musculus, Phyllotis darwini and Octodon degus, were captured; R. rattus was the most abundant species. Fifty-nine samples (83.1%) were T. cruzi-positive and the median value of the parasite load was 2.99 parasite equivalents (par-eq)/ml. The comparison of frequency of infection or parasite load by species showed no differences. However, one R. rattus presented very elevated parasitemia (1644 par-eq/ml). Conclusions The overall levels of parasitemia were similar to those found in humans in Chile. The high infection levels in exotic and endemic rodents very near to rural settlements increases their relevance as T. cruzi hosts. Electronic supplementary material The online version of this article (10.1186/s13071-018-2771-2) contains supplementary material, which is available to authorized users.

The relationship between tooth roots and diet is largely unexplored, although a logical relationship between harder diets and increased root surface area is suggested. Existing studies of primates, carnivorans and phyllostomid bats have indicated a relationship between diet hardness, bite force and tooth root surface area. The goal of this study was to determine whether root surface area can act as a potential surrogate for bite force and diet in cricetid rodents. Using microcomputed tomography (microCT), tooth root morphology from six species of rodents, two grass eaters (Calomys callosus and Reithrodon auritus), two seed eaters (Phyllotis darwini and Ochrotomys nuttalli) and two insect eaters (Akodon azarae and Oxymycterus hispidus) were compared. Similar to other studies, these rodents did exhibit differences in tooth root surface area based on diet classification, but food hardness did not seem to be a factor. Grass-eating species showed significantly larger roots relative to the other diet groups (p = 0.001). Bite force was estimated using skull measurements. Seed eaters were found to have a larger bite force, followed by grass and insect eaters, though the trend did not reach statistical significance (p = 0.058). No strong relationship was found between estimated bite force and tooth root surface area. In this study, the mechanics of grass eating seem to have a stronger effect on tooth root surface area than bite force. microCT allows the nondestructive quantification of previously difficult-to-access tooth morphology; this method shows the potential for tooth roots to provide valuable dietary, behavioral and ecological information in rodents.

Located in the Atacama desert of northern Chile, Pampa del Tamarugal is a hyperarid ecosystem characterized by extreme arid conditions ( Prosopis tamarugo . These unfavorable environmental conditions can affect the quantity and diversity of prey available for carnivores, making Pampa del Tamarugal an interesting environmental scenario for the study of the trophic ecology of raptors. Our analyses of the seasonal diet of the burrowing owl Athene cunicularia (Molina, 1782) indicate that insects and scorpions were the most common prey (58.3% and 27.5%, respectively). However, the rodent Phyllotis darwini was the most important prey in biomass (49.5%). Significant differences were observed in diverse trophic statisticians between summer and winter seasons. Unlike similar studies carried out in arid areas of South America and North America, we observe that the diet of the burrowing owl in Pampa del Tamarugal shows a higher trophic niche breadth and trophic diversity, but a lower species-prey richness.

We report natural infections by pathogenic Leptospira of two rodent species endemic to Chile: the degu (Octodon degus) and Darwin's pericote (Phyllotis darwini). We detected Leptospira DNA in kidney and urine samples taken in different years and sites, reaching 33% infection. The effects of infection in these species requires further evaluation.

The South American grey fox Lycalopex griseus is a canid widely distributed in southern South America; however, some aspects of its biology are still poorly known. We studied the diet and density of L. griseus in the Lago Penuelas Biosphere Reserve, in Central Chile. The trophic niche breadth was B = 6.16 (B-sta = 0.47) and prey diversity was H = 2.46 (H-max' = 3.17, J' = 0.78). The highest proportions of prey consumed in the diet were Oryctolagus cuniculus (52.21%) and other mammals (32.78%). We compared these results with a latitudinal gradient of diet results for this species in Chile. L. griseus eats mostly mammals (>90% of total prey), consuming the rodent Phyllotis darwini and reptiles in the northern zone; Oryctolagus cuniculus, Octodon degus and Abrocoma bennetti in the central zone; Abrothrix spp. and lagomorphs in the southern zone; and Lepus capensis and Ovis aries in the austral zone. The estimated density of L. griseus in Lago Penuelas NR was 1.3 foxes/km(2)

We evaluated if two sigmodontine rodent taxa (Abrothrix olivacea and Phyllotis darwini) from the Andes and Coastal mountaintops of central Chile, experienced distributional shifts due to altitudinal movements of habitat and climate change during and after the Last Glacial Maximum (LGM). We tested the hypothesis that during LGM populations of both species experienced altitudinal shifts from the Andes to the lowlands and the coastal Cordillera, and then range retractions during interglacial towards higher elevations in the Andes. These distributional shifts may have left remnants populations on the mountaintops. We evaluated the occurrence of intraspecific lineages for each species, to construct distribution models at LGM and at present, as extreme climatic conditions for each lineage. Differences in distribution between extreme climatic conditions were interpreted as post-glacial distributional shifts. Abrothrix olivacea displayed a lineage with shared sequences between both mountain systems, whereas a second lineage was restricted to the Andes. A similar scenario of panmictic unit in the past was recovered for A. olivacea in the Andes, along with an additional unit that included localities from the rest of its distribution. For P. darwini, both lineages recovered were distributed in coastal and Andean mountain ranges at present as well, and structuring analyses for this species recovered coastal and Andean localities as panmictic units in the past. Niche modeling depicted differential postglacial expansions in the recovered lineages. Results suggest that historical events such as LGM triggered the descending of populations to Andean refuge areas (one of the A. olivacea's lineages), to the lowlands, and to the coastal Cordillera. Backward movements of populations after glacial retreats may have left isolates on mountaintops of the coastal Cordillera, suggesting that current species distribution would be the outcome of climate change and habitat reconfiguration after LGM.

A major goal of evolutionary physiology is to understand the intrinsic and the extrinsic factors that impose limitations on an animal's energy budget. Although natural selection acts upon organismal traits such as performance (e.g., burst, sustained metabolic rates), from a mechanistic perspective, organismal performance results from the integrated functioning of different levels of biological organization. Hence, a better understanding of whole-animal performance must necessarily incorporate an explicit analysis of the integration between those different levels. Although this topic has been under intense scrutiny, overall there have been very few consistent patterns. Here, we explore the phenotypic integration between organ masses and the overall energy budget under routine capacities by statistically decomposing the covariance matrix (using path analysis and canonical correlation analysis) between organ masses and thermoregulatory burst and sustained metabolisms in cold acclimated individuals of Phyllotis darwini. Our results suggest that (a) central organs associated with the processing of food (cecum and liver), residuals (kidneys) and pumping of O(2) (heart) are tightly integrated to sustained expenditure and between themselves; (b) with the exception of the heart, central energy supplying organs are weakly related to burst expenditures; (c) sustained and burst metabolisms refer to complete different strategies and (d) basal metabolic rate is not related to any of the physiological or morphological traits considered in this study. Overall, our results support the hypothesis of an economic phenotype: animals maintain their excess capacities to face those critical extreme events, but their physiology and internal morphology are tightly integrated to function under routine needs.

1. In the altricial rodent, Phyllotis darwini, we found higher body temperatures and faster developmental rates of the thermoregulatory capacity in neonates born from cold- than warm-acclimated mothers. 2. This difference could be explained by maternal effects on the litter, such as high levels of catecholamines and thyroxin levels, high concentration of the uncoupled protein and larger quantity of brown adipose tissue as a consequence of cold acclimation. 3. The exposition of mothers and the maintenance of cold condition during the early development might be responsible of the high metabolism and better thermoregulatory capacity of newborns.

Lower-elevation habitats of north-central Chile are characterized by semiarid vegetation that is strongly influenced by rainfall related to El Nino Southern Oscillation (ENSO) events. During the ENSO warm phase (i.e., El Nino), heavy rains occur resulting in small mammal irruptions or ratadas. During prevailing drier periods, pronounced droughts result in reductions in small mammal densities and local extinctions of some species. Within the dominant thorn-scrub habitat of a national park, we identified 2 other habitat types, aguadas (riverine shrublands) and fog-forest patches that serve as refuges for small mammals during dry years. We divide small mammal species of the thorn scrub into 2 groups: core species (Octodon degus, Phyllotis darwini, Abrothrix olivaceus, and Thylamys elegans) maintain populations at all times, whereas opportunistic species (Oligoryzomys longicaudatus and A. longipilis) are present only after rains. The latter group appears to maintain refuge populations in aguadas and fog forests and to opportunistically exploit the thorn scrub when conditions are favorable. Aguadas also may play an important role in the persistence of less-common species, such as Octodon lunatus and Abrocoma bennetti. Despite being small and patchily distributed, aguadas are important for the maintenance of regional biodiversity.

In the present study the diet of the barn owl Tyto alba was analysed in the ecosystem of Pampa del Tamarugal, in the Atacama Desert, northern Chile. The area is characterised by extremely dry conditions, and relatively homogeneous and poor vegetation, dominated mainly by tamarugo forests (Prosopis tamarugo). The results indicated that small mammals were the greatest proportion (76.2%) in the diet of the barn owl, which predated only four species, of which the rodent Phyllotis darwini (approximately 62%) represented the major proportion. Nevertheless, reptiles and arthropods were also relevant prey for the barn owl, with a proportion of 5% and 15.1%, respectively. The general dietary composition of the tamarugos barn owl showed a wide-ranging diet pattern, characterised mainly by a poor diversity of small mammals and a significant consumption of reptiles and arthropods, in contrast to the sites in the central region and south Chile, where the diet included a greater diversity of small mammals, especially rodents. This pattern might reflect the conditions of extreme aridity, and low primary productivity in the ecosystem of Pampa del Tamarugal, restricting the abundance and diversity of the preferential prey (e.g. rodents). Hence, T. alba tends to increase its trophic diversity, adding other kinds of alternative prey to compensate for the low proportion of preferential prey available in the field.

The study of the relative importance of the feedback structure (intrinsic processes) and exogenous (climatic or environmental) factors in determining population dynamics, in particular the interaction between density-dependence and climate, is a major question in population ecology. We sought to explain the numerical fluctuations of 2 sympatric rodent species at one well-studied site in semi-arid Chile using simple theoretically based population dynamics models and Royama's theoret- ical framework for analyzing the dynamics of populations influenced by exogenous climatic forces. We found that rainfall effects appear to operate in a different manner on the 2 rodent species. For one species (Phyllotis darwini), rainfall appeared to influence the carrying capacity of the environment, whereas for a second (Akodon olivaceus) the rainfall effect had a primarily additive influence on the maximum per capita growth rates.

In order to investigate the genomic compatibility between allopatric rodent species, Phyllotis darwini and Phyllotis magister, we have studied several cytogenetic and reproductive features of their laboratory hybrids. Of thirty-one pairings between species, only five were successful, producing eleven newborns. Like parents, hybrids had 38 metacentric chromosomes, except for the subtelocentric Y chromosome inherited from P. magister. There was almost total C and G band correspondence between homeologous autosomes. However, parental sex chromosomes had different morphology, C and G bands. Ag-NOR bands appeared as small telomeric Ag+ regions, distributed in four chromosomal pairs of darwini, three of magister and four homeologous chromosomes of the hybrids. The three forms had similar indexes of NOR activity per cell, in spite of the variability in NOR expression which was always detected. Usually, only one member of parental homologous chromosomes showed AgNOR+; nevertheless, both homeologous chromosomes were active in many hybrid cells. The frequencies of cells that expressed their ribosomal genes in the two homologous or homeologous NOR chromosomes were similar in parental and hybrid cells. These results strongly suggest that ribosomal genes of both parental genomes would function codominantly in the hybrids. The gonad histological and morphometric analyses showed that hybrids conformed to Haldane's rule, since females were fertile and males were infertile. Our results indicate that P. darwini and P. magister genomes can function in relative harmony and compatibility when they are placed together in their laboratory generated hybrids, suggesting that these species have few genetic differences, probably because they have recently diverged.

We investigated predation risk and competition as they affected small-mammal foraging behavior in semiarid north-central Chile. Giving-up densities (GUD) of seeds were used to measure the foraging activity of the three most common small mammals at the site: degu (Octodon degus), Darwin's leaf-eared mouse (Phyllotis darwini), and the olivaceous field mouse (Akodon olivaceus), under shrubs (cover) and in the open on predator-excluded and competitor-excluded (Octodon) plots. Experiments were conducted during both new and full moons. Monthly small-mammal censuses using standard mark–recapture techniques provided data on movement, reproduction, and long-term fluctuations in density between 1989 and 1994. Diurnal Octodon foraged more (had lower GUD) in the absence of predators (although this was confounded by a numerical increase resulting from predator exclusion), and foraged more under shrubs than in the open. However, the lack of a significant cover × predator exclusion interaction and thermoregulation studi...

Population dynamics of small mammals and predators in semi-arid Chile is positively correlated with rainfall associated with incursions of El Nino (El Nino Southern Oscillation: ENSO). However, the causal relationships between small mammal fluctuations, predator oscillations, and climatic disturbances are poorly understood. Here, we report time series models for three species of small mammal prey and two species of owl predators. The large differences in population fluctuations between the three small mammal species are related to differences in their respective feedback structures. The analyses reveal that per capita growth rate of the leaf-eared mouse is a decreasing function of log density and of log barn owl abundance together with a positive rainfall effect. In turn, per capita population growth rate (R-function) of the barn owl is a negative function of log barn owl abundance and a positive function of leaf-eared mouse abundance, suggesting a predator–prey interaction. The dramatic population fluctuations exhibited by leaf-eared mouse (Phyllotis darwini) are caused by climate effects coupled with a complex food web architecture.

We report the first record of Litomosoides pardinasi in native and exotic rodents from Chile. Litomosoides pardinasi, described in the Argentine Patagonia parasitizing Phyllotis xanthopygus and Oligoryzomys longicaudatus, was found in Chile parasitizing the peritoneal and thoracic cavities of O. longicaudatus (prevalence (P)= 18.9%, mean intensity (MI)= 57.3) and Phyllotis darwini (P= 4.2%, MI= 10), and in the peritoneal cavity of Rattus rattus (P= 1.24%; MI= 22.5). Total P in native rodents from Chile was significantly lower than in Argentina, while the total MI was higher. Prevalence and MI of L. pardinasi in O. longicaudatus from Chile and Argentina showed non-significant differences; prevalence in P. darwini from Chile was significantly lower than in P. xanthopygus from Argentina and than in Chilean O. longicaudatus. Our results, together with those from Argentina, support the hypothesis that L. pardinasi is well established in O. longicaudatus, but seems to be recently acquired by P. darwini and the exotic R. rattus. Considering the known host distribution of Litomosoides species among the sigmodontines, our results also support the hypothesis that L. pardinasi first colonized the Oryzomyini tribe and later, by different phenomena of host-switching, colonized the Phyllotini tribe and the exotic R. rattus., Litomosoides pardinasi es registrada por primera vez en roedores nativos y exóticos de Chile. La especie Litomosoides pardinasi, parásita de Phyllotis xanthopygus y Oligoryzomys longicaudatus en la Patagonia Argentina, fue encontrada en Chile parasitando la cavidad torácica y abdominal de O. longicaudatus (prevalencia (P)= 18.9%, intensidad media (MI)= 57.3) y de Phyllotis darwini (P= 4.2%, MI= 10), y la cavidad abdominal del roedor exótico Rattus rattus (P= 1.24%; MI= 22.5). La P total en roedores nativos de Chile fue significativamente menor que la P de Argentina, en tanto la MI total fue significativamente mayor. La P y MI de L. pardinasi en O. longicaudatus de Chile y Argentina no mostraron diferencias significativas; la P en P. darwini de Chile fue significativamente menor que la de P. xanthopygus de Argentina y que la de O. longicaudatus de Chile. Considerando la distribución de las especies de Litomosoides entre los sigmodontinos, estos resultados apoyan la hipótesis que L. pardinasi está bien establecida en O. longicaudatus y que posiblemente haya sido recientemente adquirida por P. darwini y R. rattus. Así, L. pardinasi colonizó primero a la Tribu Oryzomyini y posteriormente por un fenómeno de cambio de hospedero a la Tribu Phyllotini y a R. rattus., Facultad de Ciencias Naturales y Museo

Since 1989, effects of biotic interactions including predation and herbivory have been examined in a replicated experimental study in a north-central Chilean semiarid thorn scrub community. Strong responses of small mammals and plants to El Nino Southern Oscillations (ENSO) have also been documented suggesting that ''bottom- up'' factors related to high rainfall are important. To simulate increased primary productivity effects on small mammals, ad lib rabbit pellet additions were initiated in mid-1997 on unfenced grids near the experimental complex. Following the 1997 El Nino event with three times normal precipitation, numbers of small mammals during pre-addition months and the first treatment year were similar on control and food addition grids. During the second year (1998 -1999), a period of severe drought, food additions had significant positive effects on numbers of two predominantly herbivo- rous ''core'' (resident) species, Octodon degus and Phyllotis darwini, and an omnivo- rous ''quasi-core'' (resident but highly fluctuating) species, Akodon oliaceus; however, all three species declined towards the end of the second treatment year. Two ''opportunistic'' (temporarily resident) species, Abrothrix longipilis (an insectivore) and Oligoryzomys longicaudatus (a granivore), showed no responses to food addi- tions. An insectivorous marsupial, Thylamys elegans (also a ''core species''), had significantly lower numbers on food addition grids. Changes in body weight distribu- tions and proportions of reproductive individuals particularly in O. degus indicate in situ responses. Whereas no differences in residency, numbers of stations visited, and trappability were observed, energy compensation ratios greater than one suggest significant immigration in the second year. Thus, food additions elicited strong responses by herbivorous/omnivorous ''core'' and ''quasi-core species'' whereas they had no effects on ''opportunistic species''. These results reinforce the view that ''bottom-up'' factors influencing food availability exert prevailing control on numer- ically important small mammal species by temporarily increasing carrying capacity, and that ''top-down'' factors (i.e., biotic interactions) become important when small mammal numbers are at or near their carrying capacity. Spatial dynamics may be important in explaining declines of species populations exhibiting initially positive responses to food additions.

Summary 1 The leaf-eared mouse (Phyllotis darwini) exhibits large numerical fluctuations associated with high- and low-rainfall years in semi-arid Chile. Using capture–mark–recapture (CMR) statistical modelling, we provide a detailed description of the demographic variation in this species. We studied between-year and seasonal variation of demography, and tested for the relative importance of endogenous and exogenous factors as covariates of survival, recruitment, maturation and reproduction over a 12-year period. 2 The demographic variables studied show larger between-year than within-year (seasonal) variation, emphasizing the importance of this source of external variability. We found that both feedback structure and exogenous factors operate together on different demographic processes of this species. 3 We found positive direct feedback effects on survival rate and negative feedback on recruitment, fraction of reproductive individuals and female maturation rates. The ratio between barn and magellanic horned owls showed an important effect on most of the demographic variables of leaf-eared mice, in particular by influencing negatively survival rates and the fraction of reproductive individuals, thus suggesting the importance of non-lethal predation. 4 Hence, feedback intrinsic mechanisms (density-dependence), owls and climate (rainfall and Southern Oscillation Index (SOI)) operate simultaneously in determining the demography of leaf-eared mice. We hypothesize that trophic interactions with owls (and perhaps other predators) may cause second-order feedback responses in demography and population dynamics of this small rodent species.

The daily locomotor activity pattern of the nocturnal sigmodontine rodent Phyllotis darwini was analysed when faced with a differential offer of food. Animals with restricted food supply were less active during the day compared to animals who had access to food ad libitum, but no differences in activity were observed during the night. Minimization of energy expenditure may account for this behavioural plasticity, and such flexibility is extremely important in the unpredictable and low productive environments inhabited by P. darwini.

It is well known that some rodent populations display dramatic density fluctuations in semiarid regions of western South America after the unusual rainfall levels associated with El Niño-southern oscillation (ENSO) disturbances. These correlated phenomena have led some ecologists to believe that rodent outbreaks are determined solely by density-independent factors (e.g., rainfall regime). However, demographic studies have detected strong delayed density-dependent effects in one of the most irruptive rodent species, the leaf-eared mouse Phyllotis darwini. We tested the effects of rainfall and delayed density-dependent factors by constructing a structured model based on demographic data estimated from a capture-mark-recapture study of this species in Chile. A model including both rainfall and delayed density-dependent effects predicts the observed population dynamics rather accurately over a 10-yr period. Interestingly, small changes in model parameters result in large changes in model dynamics, which strongly suggests that local variations in demographic features are important in explaining the asynchronous pattern in outbreak occurrences. These findings suggest that inextricably intertwined endogenous and exogenous forces cause rodent outbreaks in western South America. The former are characterized by delayed nonlinear feedbacks, whereas the latter are characterized by the positive effects of the El Niño phases and the negative effects of the La Niña phases of the ENSO disturbance.

In the last two decades, several researchers have noted rodent population outbreaks in semiarid South America, in association with unusually high precipitation that seemingly concurs with El Nifio events. To date, no studies have been conducted to determine the statistical relationships between ENSO (El Nifio Southern Oscillation) events, increased precipitation, and rodent irruptions. Here we show that: 1) there is a statistical association between ENSO events and increased precipitation in the semiarid region of northern Chile; 2) the occurrence of rodent outbreaks in that region is statistically related with the precipitation levels of the same year; 3) the multi-annual patterns of the total annual precipitation levels and population abundance of those rodents during the summer are positively associated. The putative chain of effects seems to start with unusually high rainfall brought by ENSO to semiarid environments, which thus respond with increased primary productivity

We analysed statistically the influence of density-dependent regulation, seasonality and precipitation on the realized population rate of change of the Neotropical rodent Phyllotis darwini (Waterhouse 1837) at an intra-annual time scale. We used four years of continuous live trapping at a semiarid locality of north central Chile. Results showed that density-dependence, seasonal effects and precipitation were important factors influencing population growth rates in this species. An empirical population model including a sine function for seasonal effects, a linear form for density-dependence, and precipitation was fitted to the full data set and to the data set with the first year removed (after an outbreak). The empirical model explained 33% and 48% of the variance in population growth. The natural rate of population increase, estimated from the empirical model, was r max = 2.51 or 5.06 years 1 . These estimates indicate a great potential for population increase and may explain the capability of this species to undergo large irruptions. We propose that merging empirical and theoretical modelling with field research is the most promising avenue to understand the outbreaks experienced by some rodent species in western South America.

It is widely believed that only precipitation levels (through increased primary production) determine irruptions of small mammals in semi-arid areas of western South America. Nevertheless, density-dependent factors may also drive population fluctuations. To test statistically these putative effects we analysed 11 years of population records on three sympatric species of small mammals at two different habitat types in north central Chile. We applied the classical diagnostic tools of time series analysis (the autocorrelation function: ACF) to the observed time series of three neotropical small mammals. We also used simple linear autoregressive time series models to reconstruct the endogenous dynamics of these populations. The analysis strongly suggests that population fluctuations of the three species have an important density-dependent component, with the most irruptive species (Phyllotis darwini, Waterhouse 1837) displaying stronger second order population feedbacks than the other two (Akodon olivaceus, Waterhouse 1837 and Thylamys elegans, Waterhouse 1839). The latter two species showed direct density-dependent feedbacks. We hypothesize that the frequent population outbreaks of P. darwini (and perhaps of other species) in semi-arid regions of western South America, may be the result of population-level (direct density- dependence) and community-level processes (delayed density-dependence), interacting with exogenous perturbations (rainfall and associated primary production).

We examined the influence of density-dependent and density-independent factors on reproductive processes of leaf-eared mouse Phyllotis darwini (Waterhouse, 1837) in a semiarid region of Chile subjected to El Nino-driven precipitation. This species undergoes periodic irruptions apparently triggered by unusually high precipitation. The effects of density and precipitation were analyzed statistically regarding the following reproductive parameters: fraction of reproductive females, reproductive number (juveniles + reproductive females), per capita reproductive rates, and reproduction index based on long-term data (10 years). The fraction of reproductive females was affected positively by precipitation during the preceding winter and negatively by population density one year before. The reproduction index and the reproductive rate was positively affected by population density the previous year. In addition, the reproductive number was positively correlated with precipitation levels. The P. darwini population studied was affected by delayed density-dependent and density-independent factors in reproductive parameters. We propose that both destabilizing effects on reproductive rates of P. darwini may be major factors underlying the frequent outbreaks of this mouse observed in semiarid regions of Chile.

The population persistence and extinction probabilities of three small mammal species were analyzed by estimating growth and extinction properties obtained from 10 years of live-trapping data at two different habitat types in semiarid Chile. We used a stochastic formulation with an exponential growth model known as a Wiener-drift process, out of which growth and extinction quantities were estimated. The rodent Phyllotis darwini showed the lowest rates of growth, and the lowest infinitesimal variances, whereas the opposite trend was found for the rodent Akodonolivaceus. The marsupial Thylamys elegans showed intermediate values for growth rates and infinitesimal variances. The rodent P. darwini showed the lowest extinction risk in the study site. We also detected spatial differences between mesic and xeric habitats in the growth rates of P. darwini and T. elegans, and in the extinction risks of the three species studied. Although the population growth of these three species can be approximated by purely stochastic processes, the introduction of density dependence through autoregressive log-linear models reduced the extinction times of all species analyzed.

We addressed the role of density-dependent (direct and delayed) and density-independent (precipitation) factors in shaping the dynamics of fluctuating populations of three small mammal species. Using a stepwise regression procedure, we tested the effects of nonlagged population density (log 10 N t-1 ), lagged population density (log 10 N t-2 ), and annual precipitation on the per capita rate of population change of Phyllotis darwini, Akodon olivaceus, and Thylamys elegans in two habitat types of a semiarid region of Chile. The most irruptive species (P. darwini) showed direct and delayed density-dependent effects in equatorial subpopulation, and only direct density-dependence in polar subpopulation. The per capita rates of population change of A. olivaceus showed direct density-dependent and precipitation effects in both habitats types, while T. elegans showed direct density-dependence and precipitation effects in the equatorial subpopulation but only a marginal effect of direct density-dependence in the polar subpopulation. The presence of delayed density-dependent strongly suggests the importance of biological interactions in shaping the dramatic irruptions exhibited by P. darwini.

We analyzed the combined effect of subchronic dietary cellulose and tannic acid on the metabolic cost in the herbivorous burrowing caviomorph rodent Octodon degus, and the omnivorous Sigmodontine Phyllotis darwini. Both are sympatric rodent species living in open scrub subjected to summer droughts in the Mediterranean communities of northern and central Chile. We measured basal metabolic rate (BMR), maximum metabolic rate of thermoregulation (MMR) and the aerobic scope. As a consequence of the ingestion of comparative higher concentrations of dietary tannic acid and fiber, a higher MMR was observed in P. darwini than in the herbivorous O. degus. Metabolic costs were observed under maximal but not under basal conditions.

Four subpopulations of Phyllotis darwini were monitored through monthly censuses conducted over 48 months on opposite slopes of two creeks separated by 2 km at a semiarid locality in Chile. Nine response variables were measured to characterize the spatial and temporal variability of the subpopulations: number of individuals per grid; mean body mass; coefficient of variation in body mass; coefficient of variation in body mass of males; coefficient of variation in body mass of females; number of males; number of females; number of juveniles; number of adults. These variables were analyzed with regard to two main factors (creek and slope). The demographic characteristics were more variable at a lower spatial scale (slope exposure) than at a larger spatial scale (creek). In situ reproduction and recruitment on each slope, as well as the low level of migration between slopes, indicate that this system is not driven by source-sink dynamics. The spatial and temporal dynamics of these subpopulations can be characterized as a partially coupled metapopulation system whose component subpopulations are weakly interconnected by dispersal between opposite slopes and totally unconnected between creeks.

Authors report on body mass dynamics, field growth rates and age-specific growth curves for the leaf-eared mouse Phyllotis darwini (Waterhouse, 1837). Mark-recapture methods provided data for a population of P. darwini in a semi-arid region of Chile from 1987 to 1996. There were significant effects of sex, season, and slope exposure on body mass. In addition, authors found significant effects of sex, mass class, and season on field growth rates.

Predation risk influences differentially the foraging decisions of rodent spe- cies with different morphologies (body size and locomotion type). Explanations for different foraging patterns have been derived largely from the behavior of heteromyid rodents, and there is a lack of information regarding the behavior of other rodent groups with different morphologies. Nevertheless, the explanations are thought to be applicable to other groups. In order to study the generality of hypotheses about the effect of morphology on foraging ecology, I investigated the foraging decisions of Chilean sigmodontine rodents (Muridae) in the laboratory, experimentally manipulating nocturnal illumination (a surrogate of pre- dation risk) and food distribution. Three coexisting rodent species from the Chilean matorral were compared: two quadrupeds, one with large body size (Phyllotis darwini) and another with small body size (Abrothrix olivaceus), and one small species with bipedal escape behavior (Oligoryzomys longicaudatus). The species did not differ in patch utilization and food consumption in response to different food distributions. However, there was an overall significant effect of illumination on food consumption. When illumination was higher, A. olivaceus and P. darwini increased food transportation to the refuge and decreased patch consumption and total food consumption. They also increased significantly the number of trips between food patch and refuge. P. darwini showed the most cautious foraging behavior. 0. longicaudatus was insensitive to different conditions of illumination. It is argued that a large body size and quadrupedal locomotion seem to confer higher sensitivity to nocturnal illumination. Bipedality may confer less vulnerability to predation, and quadrupedal species show facultative central-place foraging, diminishing a possible greater vulnerability to potential risk. Former explanations for different foraging behaviors under the risk of pre- dation may not be as general as previously assumed. This study emphasizes the need to consider the particular features of the system under investigation.

The role of biotic interactions including vertebrate predation and interspecific competition was studied among members of a semiarid small mammal assemblage in a long-term field manipulation in north central Chile. Fences and netting selectively excluded mammalian and avian predators, and a putative rodent competitor, the degu (Octodon degus), from large, replicated grids. Small mammal populations were censused monthly over >4 yr, and effects of predator and degu exclusions on numbers and survival of small mammals were assessed with repeated measures ANOVAs. Marginally significant predator exclusion effects were detected on 0. degus, and highly significant year X predation interactions in both wet and dry seasons; degu survival (= persistence) was also significantly longer on exclusion grids. Predation had significant effects on numbers of Phyllotis darwini, and a significant month X predator interaction existed in the wet season; survival was also sig- nificantly higher on predator exclusion grids. There was a marginally significant year X competitor interaction in the dry season for this species. Akodon olivaceus had significant month X predator and month x competitor interactions in the wet season only; survival was significantly longer on degu exclusion grids. An irruptive species, Oligoryzomys lon- gicaudatus, showed no significant predator or competitor effects in 1991-1992; however, survival was significantly longer on predator + competitor exclusion (vs. control) grids. An uncommon species, Abrothrix longipilis, had marginally nonsignificant time x com- petitor interactions but no treatment differences in survival. Finally, a second uncommon rodent, Abrocoma bennetti (not analyzed numerically) had significantly longer survival on predator exclusion grids with degus. Degus and P. darwini were significantly overrepre- sented (relative to availability) in diets of culpeo foxes (Pseudalopex culpaeus) and three nocturnal owls, respectively, whereas A. bennetti was significantly overrepresented in pred- ator diets generally. Vertebrate predation has pervasive effects on some species that, how- ever, may be related to their intrinsic demographic characteristics. Competition may be mediated by subtle behavioral interactions among assemblage members. Long-term studies are necessary to elucidate the role of biotic and abiotic factors in semiarid environments.

Scatter-hoarding rodents can act as both predators and dispersers for many large-seeded plants because they cache seeds for future use, but occasionally forget them in sites with high survival and establishment probabilities. The most important fruit or seed trait influencing rodent foraging behavior is seed size; rodents prefer large seeds because they have higher nutritional content, but this preference can be counterbalanced by the higher costs of handling larger seeds. We designed a cafeteria experiment to assess whether fruit and seed size of Myrcianthes coquimbensis, an endangered desert shrub, influence the decision-making process during foraging by three species of scatter-hoarding rodents differing in body size: Abrothrix olivaceus, Phyllotis darwini and Octodon degus. We found that the size of fruits and seeds influenced foraging behavior in the three rodent species; the probability of a fruit being harvested and hoarded was higher for larger fruits than for smaller ones. Patterns of fruit size preference were not affected by rodent size; all species were able to hoard fruits within the entire range of sizes offered. Finally, fruit and seed size had no effect on the probability of seed predation, rodents typically ate only the fleshy pulp of the fruits offered and discarded whole, intact seeds. In conclusion, our results reveal that larger M. coquimbensis fruits have higher probabilities of being harvested, and ultimately of its seeds being hoarded and dispersed by scatter-hoarding rodents. As this plant has no other dispersers, rodents play an important role in its recruitment dynamics.

A prolonged El Nino Southern Oscillation (ENSO) event during 1991–1992 with three times the normal annual 85-mm rainfall was accompanied by major changes in numbers of small mammals at a semiarid Mediterranean site in northcentral Chile. Several demographic patterns were evident. Akodon olivaceas , an omnivore, had a rapid increase in population size of more than an order of magnitude. Phyllotis darwini , a granivore-herbivore, showed somewhat delayed, smaller increases superimposed on annual oscillations. Octodon degus , an herbivore, showed a delayed response with larger increases and extended breeding in 1992–1993. Finally, Oligoryzomys longicaudatus , a granivore, experienced increases during both a dry (1990, ppt = 32 mm) and a wet year (1992). Other species such as Abrothrix longipilis, Abrocoma bennetti , and Thylamys elegans had smaller, delayed demographic responses. Reproductive rates for the first three species were higher due to the ENSO event only in O. degus males and P. darwini as a quadratic function of time. Survival rates of all four principal species were significantly greater during the 1991–1992 ENSO. Finally, average movement between captures was lower during ENSO years, suggesting behavioral changes. Explanations for these patterns include rainfall-related increases in food-resource levels ( A. olivaceus and P. darwini ), the importance of source-sink processes in vagile species from more mesic, adjacent habitats ( O. longicaudatus ), and delayed responses to extrinsic events in species with long gestation ( O. degus, A. bennetti ). The heterogeneity of species responses suggests different capabilities for small mammals to respond to an extrinsic, large-scale event, and it emphasizes the importance of long-term studies in semiarid systems.