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3 Acta Theriologica

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Body composition and gut length of Akodon azarae [Muridae: Sigmodontinae]: relationship with energetic requirements

100%

Del V. J. C. , Busch C.

nr 3

Akodon azarae (J. Fischer, 1829) is a small omnivorous murid rodent that lives in environments with seasonal fluctuations of food. Seasonal variation in its body composition and gut length, in relation to reproductive status, was studied. Physical Condition Index (PCI) and body composition showed seasonal differences, however, there were no differences in intestine length. The PCI was higher for both mature males and reproductive females compared to immature males and non-reproductive females. Lipid, protein and ash content showed differences in relation to reproductive status. The results suggest that A. azarae meets the additional costs of pregnancy and lactation by increasing energy intake, without relying on reserves.

The relationship among diet, alimentary tract morphology, and life history for five species of rodents from the central Argentine pampa

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Ellis B. A. , Mills J. N. , Kennedy E. J. T. , Maiztegui J. I. , Childs J. E.

tom 39

nr 4

A suite of characters describing digestive tract structure has been hypothesized to reflect the relative degree of specialization of the digestive system of rodent species along a continuum from a proteinaceous diet of seeds and insects to a cellulosic diet of vegetation. Similarly, it has been proposed that life history traits might reflect diet and digestive tract structure, with the most opportunistic species consuming the most energy-rich diets of seeds and insects. The five members of the rodent assemblage of agroecosystems of the Argentine pampa were found to be omnivores and varied in the relative proportions of seeds, insects, and vegetation consumed. On a gross level, diet reflected life history; the most opportunistic species (smallest body size, highest fecundity, preference for disturbed habitats) consumed the most energy-rich diet, while the least opportunistic species consumed the most vegetation. However, comparative digestive tract structure was generally converse to that predicted, based on diet. Failure to observe predicted correlations may be due in part to seasonal variability in diet, lack of evolutionary relevance of crop habitats, or flaws in the underlying hypotheses. Alternatively, the observed variation in digestive tract structure may reflect the spectrum of variation encountered within an omnivore rodent guild rather than the degree of food specialization.

Seasonal variations in microhabitat use and feeding habits of the pampas mouse Akodon azarae in agroecosystems of central Argentina

100%

Bilenca D. N. , Kravetz F. O.

nr 2

We studied microhabitat use and food habits of the pampas mouse Akodon azarae (Fischer, 1829), in agroecosystems of central Argentina. In summer (breeding season), A. azarae inhabited both cropfields and their weedy borders. Sexually active females were caught at microhabitats with 31% more green cover than inactive ones. The percentage of invertebrates in the stomach contents of females was 104% greater than in males. In winter (non-breeding season), individuals were restricted to the borders, selecting sites with high levels of forbs and green cover, There were no significant differences between the sexes, either in microhabitat use or in food habits. We propose that reproduction of females is influenced by green cover, and that a decrease of this resource may reduce breeding activity of females in the borders after most plants enter senescence. Females would feed on insects as a source of proteins, which are required for reproduction. In winter, when the population is sexually inactive, individuals of both sexes have similar requirements and occupy the most suitable sites available within a low-quality habitat. We conclude that seasonal variations in microhabitat use and food habits of A. azarae are mainly influenced by changes in resource availability and reproductive status of individuals.