Paola L. Sassi

SPATIAL AND SEASONAL PLASTICITY IN DIGESTIVE MORPHOLOGY OF CAVIES (MICROCAVIA AUSTRALIS) INHABITING HABITATS WITH DIFFERENT PLANT QUALITIES

Abstract Ecological physiology examines how animals cope with changing environmental demands. In low-productivity desert habitats, small mammals should consume low-quality, high-fiber food as a consequence of necessity rather than by choice. Diet quality of populations can differ at both spatial and temporal scales. Nevertheless, spatial and temporal variation in the digestive system has not been extensively studied in the field. We captured individuals from 4 populations of Microcavia australis and measured their digestive morphological traits. Fieldwork was carried out in 4 localities belonging to arid and semiarid regions, in dry and wet seasons. We also estimated diet quality for each population and season. We found significant effects of sex, season, and site on the size of digestive organs. The concentration of fiber and nitrogen in the plants consumed differed between populations and varied seasonally: dietary fiber was higher in the dry season and nitrogen concentration was higher in the wet season. As predicted by theory, the cecum, the organ most closely related to cellulose fermentation, was significantly larger in animals facing the lowest quality diet. The other organs also were affected by reproductive state and water requirements. Intraspecific variation in the digestive morphology of M. australis probably helps this species cope with remarkable seasonal and geographical variability.

Effect of Herbivory and Disturbances by Tuco-Tucos (Ctenomys Mendocinus) on a Plant Community in the Southern Puna Desert

ABSTRACT We studied the differences between areas undisturbed and disturbed by Ctenomys in the structure, diversity, and composition of a Puna desert plant community. We also studied differences in nutrient distribution in the soil produced by the activity of tuco-tucos. Within the plant community, plant and soil samples were taken from different sites, and at each site, both disturbed and undisturbed areas were sampled. We hypothesized that the activity of this rodent affected plant community structure, specific composition, diversity, and nutrient distribution (N, K, and P). Results at the plant population and community level and for soil nutrient concentration suggest that Ctenomys mendocinus could be a keystone species, capable of orienting the dynamics of the plant community studied in this ecosystem. Nevertheless, further manipulative experiments are necessary to confirm that the differences found between disturbed and undisturbed areas are indeed caused by the activity of Ctenomys.

Geographic and seasonal variability in feeding behaviour of a small herbivorous rodent

Foraging strategies have traditionally been modelled as a result of food selection in response to one factor, as for instance resource availability, deterrent compounds or nutrients. Thus, a trade-off is assumed between plasticity (generalist strategy) and efficiency (specialist strategy). Nevertheless, several studies have demonstrated that animals cope behaviourally with food supply variation. For instance, desert-dwelling rodents partially compensate for nutritional bottlenecks through diet selection. The aim of our study was to test how foraging behaviour matches spatial and temporal variations in the trophic environment and how modelling hypotheses help us to understand the resultant foraging strategy. Our animal study model was the small cavy Microcavia australis, a widely distributed herbivorous rodent. Fieldwork was carried out in four places, in wet and dry seasons. We found significant differences in plant cover, plant diversity and niche breadth, and diet selection revealed a complex foraging strategy. M. australis shows a behavioural repertoire that exceeds single-criterion categories; therefore, we appeal to theoretical models that consider ecological and physiological perspectives. We classified the small cavy as a facultative specialist displaying a thoroughly opportunistic strategy based on the plasticity of the behavioural phenotype. We finally discuss the evolutionary relevance of our results and propose further investigation avenues.

Plasticity in food assimilation, retention time and coprophagy allow herbivorous cavies (Microcavia australis) to cope with low food quality in the Monte desert

Energy balance depends on the efficiency with which organisms make use of their trophic resources, and has direct impact on their fitness. There are environmental variations that affect the availability as well as the quality of such resources; energy extraction also depends on the design of the digestive tract. It is expected that features associated with food utilization will be subjected to selective pressures and show some adjustment to the variability of the environment. Since energetic constraints challenge animals to display digestive compensatory mechanisms, the objective of this study is to determine the physiological and behavioral responses to spatial and seasonal heterogeneity in food quality. We investigated digestive strategies (digestive efficiency and coprophagy) in cavies inhabiting two different populations, and hence naturally experiencing different levels of diet quality. Cavies under experimentally different quality diets showed changes in dry matter digestibility and intake, digesta retention time and coprophagy. Our results partially support the expectations from theory and also reveal interpopulation differences in the ability to cope with changes in food quality, and may explain the capability of Microcavia australis to colonize extreme habitats.

New eco-morphological-behavioural approach of the chinchilla rats in the pre-Andean foothills of the Monte Desert (Argentina)

This study characterizes the ecology, behaviour and morphology of populations of Abrocoma schistacea and A. uspallata in north-western Argentina. We carried out live-trapping, tagging and recapture during 2006–2008. Burrow structure and vegetation were described. Diet composition and the activity of chinchilla rats were defined. Both species inhabit rock crevices in the pre-Andean foothills of the Monte Desert. Some of their morphological specializations for saxicolous life, such as foot pads for better traction on rocky surfaces and reduced nails, are convergent with those of other saxicolous species. Both species appear to be diurnal, with activity peaking in the early morning. Dietary analyses suggest trophic specialization by both species. Density of A. schistacea was of 0.15 ± 0.06 individuals ha−1; with individuals living in groups of 3–4 animals. Given their low abundance, restricted distribution and high level of habitat and food specialization, we urge that populations of this species be protected through effective management.

High genetic differentiation among populations of the small cavy Microcavia australis occupying different habitats

The small cavy Microcavia australis, a social and fossorial rodent, inhabits a large distribution range in South American arid zones. The species is versatile in coping with the seasonal and spatial variability typical of these environments through changes in morphology, physiology, and behavior. In order to explore whether phenotypic variations are related to the evolutionary history of the species, we analyzed the levels of genetic variability and divergence among four populations that differ in climate and habitat characteristics, two belonging to highlands and the other two from lowlands. We sequenced the mitochondrial control region and used the Inter Simple Sequence Repeats technique to study variability in the noncoding nuclear genome. Results from both genetic markers were consistent. Variability levels were high for all populations, and even higher for lowland ones. Pairwise genetic differentiation varied greatly, all comparisons being statistically significant except for the two highland populations. Seventeen haplotypes were detected which displayed three clear lineages: two corresponding to each lowland population and one to those in the highlands. Levels of genetic differentiation between population pairs varied widely. Haplotypes showed a mean sequence divergence of 1.4% between lowland populations and 0.2% between highland ones, whereas divergence was around 9% when populations from different altitudes were compared. Results from BEAST analysis support extant hypotheses suggesting that lowland forms are clearly older than the highland group. The deep genetic divergence between lineages poses the need to search for new evidence for properly defining the taxonomic status of divergent populations of M. australis.

Effect of climatic variables on seasonal morphological changes in the testis and epididymis in the wild rodent Microcavia australis from the andes mountains, Argentina

It has been shown that seasonal changes, especially in arid areas have a large influence on gonadal changes of the species that inhabit these areas. We studied a wild hystricomorph Microcavia australis in its natural habitat in the arid Andes Mountains. Sampling of adult males was carried out every 2 months. After autopsy, testes and epididymides were weighed and processed to obtain histological samples. Testes were analyzed with a microscope to measure seminiferous tubule area and diameter for each sampled month. Epididymides were used to determine spermatozoon storage in the cauda region. Results illustrate morphological changes in the testis and epididymis along the year. A high output of sperm cells was detected from middle winter to middle summer and a complete shutdown of spermatogenesis at the end of summer. The initiation of testicular activity coincided with month with the shortest day length, in dry season and very low temperature. On the other hand, gonadal regression started in the middle of summer with long day length, in the wet season, and high temperatures. Rainfall, temperature, and day length seem to be important for the testis cycle. We suggest that photoperiod could be a good predictor for an oncoming period suitable for breeding, and males may probably use it as a signal to regulate gonadal activity.

Abrocoma schistacea (Rodentia: Abrocomidae)

Abstract: Abrocoma schistacea Thomas, 1921 is an abrocomid rodent commonly known as the Sierra del Tontal chinchilla rat. This medium-sized chinchilla rat is a rock specialist that inhabits the pre-Andean foothills of southern San Juan Province (Argentina) within an elevation range of 1,100–2,900 m. Its saxicolous specializations include padded feet with leathery tubercles on the pads. Incisors are large and orthodont; molariform teeth appear broad. A. schistacea is a creosotebush dietary specialist that can be distinguished from other chinchilla rats by living in groups of 3–4 individuals. This species is categorized as of low conservation concern in Argentina but is currently listed as “Data Deficient” by the International Union for Conservation of Nature and Natural Resources.