Seasonal Changes in Microhabitat Use and Niche Overlap between Akodon azarae and Calomys laucha (Rodentia,Muridae) in Agroecosystems of Central Argentina

We examined seasonal changes in microhabitat use by the murid rodents Akodon azarae and Calomys laucha in agroecosystems of the former pampa region of Argentina. In summer, trap data showed that both species had low densities and were almost equally distributed between the mature cropfields and their surrounding weedy borders. Analysis of vegetation at the trap sites revealed no selection for microhabitats at the borders, whereas in the cropfields both species shared preferences for covered microhabitats. In contrast, winter data revealed a sharp habitat segregation, being Calomys numerically dominant in post-harvest cropfields and Akodon more abundant in borders. Moreover, there were clear differences in microhabitat selection at the borders, A. azarae occupying the more covered microhabitats and C. laucha the less covered. Breadth and overlap of niches were calculated using principal component analysis, in order to recognize interspecific competition and its influence on community niche space. Available evidence indicates that the structure of this rodent community is strongly influenced by seasonal changes in habitat stucture and rodent abundance. The socially dominant A. azarae retains the best part of the available niche space, a fact becoming more evident under the harsh winter conditions.     

Evaluation of liquid nitrogen and dry ice–alcohol refrigerants for freeze marking three mammal species

Degus (Octodon degus), Patagonian cavies (Dolichotis patagonum), and mouflon (Ovis musimon) were freeze marked with instruments refrigerated in liquid nitrogen or a mixture of dry ice and alcohol. The instruments were pressed against the skin of the animal for various trial durations. Recommended exposure durations with dry ice-alcohol refrigerant were: degus—28 sec; cavies—≈ 18 sec (estimated); mouflon—12 sec. While unsatisfactory results were obtained with the liquid nitrogen refrigerant on degus and cavies, an exposure duration of 8 sec was recommended for mouflon. With mouflon, the site of the mark influenced the appropriate exposure duration for both refrigerants. With degus, moderate overexposure resulted in acceptable marks as much of the length of peripheral white hairs was exposed due to the lack of hair in the central bare area. The dry ice-alcohol refrigerant yielded much greater exposure latitude and consistency, indicating its superiority for freeze marking, especially in a zoo setting.