NOTES ON THE LITTER SIZE OF MICROTUS-ARVALIS (ARVICOLIDAE) FROM THE EAST SLOVAKIA LOWLAND

Reproduction in Microtus arvalis has been evaluated by examining 130 o bviously pregnant females collected from the East Slovakia lowland. Th e average litter size was 5.41 embryos per pregnant female (range 2-9) , the midseason average (May - August) was 5.48. The litter size incre ased in relation to body weight of females from 4.79 in the lightest f emales (to 20.0 g) to 5.89 in the heaviest ones (30.5 and above). The differences between weight groups were statistically significant in mo st of the cases. Similarly, the litter size increased in relation to t he length of the pregnant female from 5.17 in the smallest females (to 100.0 mm) to 5.81 in the biggest ones (110.5 mm and above). There wer e 13 embryonic sets (10.0%) and 17 embryos (2.4%) affected by embryoni c resorption. Both uterine horns were affected by equal embryonic mort ality. Highest embryonic mortality was registrated in the lightest and the smallest females.