Fernando O. Kravetz

Effects of Food Supplementation on Rodents in Winter

We evaluated the role of food availability on population parameters of rodents in agroeco-systems in the Pampas of Argentina by mark-recapture studies in six crop-field borders; three controls and three areas with supplemental food. Akodon azarae , the pampean grassland mouse, in supplemented borders exhibited increased density, more immigration, greater persistence, greater proportion of pregnant females, and increase in mean mass and length. We conclude that food availability is a factor contributing to winter reduction and slow springtime increase in density of A. azarae .

Density-dependent habitat selection between maize cropfields and their borders in two rodent species (Akodon azarae and Calomys laucha) of Pampean agroecosystems

We studied habitat preferences and intra and interspecific density-dependent effects on habitat selection by Akodon azarae and Calomys laucha between maize fields and their adjacent borders, during different developmental stages of the crop. Akodon azarae detected quantitative differences between habitats, using preferentially borders throughout the year, while C. laucha perceived borders and cropfields as quantitatively similar during spring and summer and it detected borders as quantitatively better at the high density period (autumn and winter). These results support the prediction of differential habitat preferences as a model of community organisation at the low density period, while they are consistent with shared habitat preferences during autumn and winter when both species apparently coexist in the better habitat (border). Akodon azarae showed intraspecific density-dependent habitat selection throughout the year, except in spring, while habitat selection by C. laucha was density-dependent in spring, autumn and winter. The effect of interspecific density on habitat selection was detected in both habitats and changed seasonally. The effect of A. azarae over C. laucha by resources exploitation was detected in borders, while competitive effects of C. laucha over A. azarae was observed within cropfields. Both species were more affected by exploitation competition than interference, which was more common in borders than in maize fields. We conclude that seasonally have a profound effect in habitat selection of these species because it changes the intensity of intra and interspecific competition and affects different habitat preferences and basic suitability of habitats.

SCURVY IN CAPYBARAS BRED IN CAPTIVITY IN ARGENTINE

In order to determine if the absence of vitamin C in the diet of capybaras (Hydrochoerus hydrochaeris) causes scurvy, a group of seven young individuals were fed food pellets without ascorbic acid, while another group of eight individuals received the same food with 1 g of ascorbic acid per animal per day. Animals in the first group developed signs of scurvy-like gingivitis, breaking of the incisors and death of one animal. Clinical signs appeared between 25 and 104 days from the beginning of the trial in all individuals. Growth rates of individuals deprived of vitamin C was considerably less than those observed in the control group. Deficiency of ascorbic acid had a severe effect on reproduction of another population of captive capybaras. We found that the decrease in ascorbic acid content in the diet affected pregnancy, especially during the first stages. The results obtained suggest that it is necessary to supply a suitable quantity of vitamin C in the diet of this species in captivity.

Transmission of food selectivity from mothers to offspring in Akodon azarae (Rodentia, muridae)

In mammals, selection of the appropriate food types from the available ones in the environment, is one of the main problems which offspring have to solve at weaning. Investigation of the mothers mouth by the offspring could be the best strategy for getting this information. Under laboratory conditions we studied the role played by the mother in the transmission of food selectivity to offspring in Akodon azarae. Offspring spent more time smelling the mothers mouth when she had eaten unfamiliar food (squash or potato) than when mothers had eaten familiar food (sunflower seeds). After a period of interaction between the mother and the offspring, pups ate larger quantities of being the food type that had been the most selected by their mothers, independently of preference ranking (squash and potato the less preferred type of food). We concluded that mothers in A. azarae play an active role in the transmission of food selectivity to their offspring.

Characteristics of Dispersing Pampean Grassland Mice (Akodon azarae) in Agroecosystems of Central Argentina

We studied the characteristics of dispersing Akodon azarae in agroecosystems of central Argentina. The landscape of the study area consists of cropfields surrounded by weedy margins, which provide good quality habitat for A. azarae. We sampled six times between June 1991 and May 1993. We defined a central area (center) where all A. azarae were removed, while capture-mark-release trapping was conducted in the adjacent areas (laterals). Individuals were characterized as residents if they did not change their home range between trapping sessions, or colonizing dispersers if they changed their home range and colonized the centers. We compared dispersers and residents with respect to sex ratio, proportion of reproductively active individuals, body length and weight. In general, residents were larger than dispersers but they did not differ with respect to reproductive activity and sex ratio. Differences were greater for females in the reproductive period and for males in the nonreproductive period, suggesting that during reproductive period smaller females of lower competitive ability may disperse to obtain a territory and reproduce, while male dispersal in the nonreproductive period would be non adaptive because the smaller male dispersers have low chance of survival in an impoverished habitat with high rodent density.