The social interactions between Apodemus agrarius and A. flavicollis, and their behavioral responses to conspecific and heterospecific odors, were studied in male-male and female-female interspecific dyadic encounters, and an attraction-avoidance test was used in order to clarify the behavioral mechanisms which control their relationships in wild populations. The experiments were carried out at the beginning and at the end of the breeding season -in spring and in autumn. In spring the aggressiveness was higher than in autumn. Males of both species showed attraction to conspecific odors from the opposite sex, while the females were indifferent. In autumn both males and females displayed attraction to conspecific odors from the same sex. However, mice of both species showed avoidance to heterospecific odors from the same and the opposite sex in spring, and indifference to heterospecific odors from the same and the opposite sex in autumn. Based on these findings, it could be assumed that the patterns of social interactions and responses to conspecific and heterospecific odors undergo seasonal changes in their life cycle. Probably the avoidance response to heterospecific odors could serve as a spacing mechanism to avoid aggressive encounters between A. agrarius and A. flavicollis in syntopic habitats during the breeding period.

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The social relationships of Mus spicilegus were studied in intraspecific cage groups for a month during the reproductive period to clarify the role of intraspecific aggressiveness in shaping social structure and in reproduction. The mound-building mice were captured from a wild population in northern Bulgaria. A total of 13 groups, 5 male, 4 female, and 4 male-female groups, were tested. Each group consisted of five unfamiliar adults. The experiments were carried out in 100100-cm glass cages. The patterns of behaviour and the dominantsubordinate structure within each group were investigated. A dominance index for every individual in the group was calculated. During the period of establishment of the hierarchical structure in the groups a high level of aggressiveness was registered among individuals. About half of the subdominants and subordinates were found dead, with or without obvious marks of injury. Male-male interactions were more aggressive compared to these of females. Dominant-subordinate relationships between individuals of the same sex were observed in the mixed groups. A link between dominance rank and reproduction was established. The dominant male and the dominant female formed a pair and started to breed. Based on these findings it can be assumed that the aggressiveness of M. spicilegus is an adaptation to its life in the fields, where dominance is likely to be an essential factor in individual reproductive success. It is likely that the successful rearing of offspring in natural populations, which are exposed to strong biotic and abiotic impact, necessitates the existence of monogamous relationships.

Mus spicilegus is an outdoor species from southern-eastern Europe, adapted to agroecosystems. In the beginning of autumn, to spend the winter, mound-building mice build complex mounds. These mounds are built using accumulation of seeds covered with earth. This behavior is unique among mice species. However, there is evidence that at higher temperatures and a larger availability of food in winter, mice can optimize this particular behavior in direction to simpler constructions or even not build them at all. To clarify if mice display building activity under laboratory conditions we studied behavior of M. spicilegus in 5 intraspecific cage groups during autumn-winter period. The observations were carried out in 100 x 100 cm glass cages provided with earth and sawdust as bedding and straw and hay as nesting material. The groups were formed by 3-4 mice captured from a wild population in northern Bulgaria in early September of 2008, where mounds were already built. Mice were grouped according to the mound they belong to. The results showed that mound-building mice display high level of building activity, especially in the first month of their time in the laboratory. In majority of groups mice built domed nests made from straw and hay, where they lived together. Seeds in the cages were covered with earth, as far as that was possible. However, mice in some groups made burrows of simple design. Based on these findings it could be assumed that building behavior of M. spicilegus is genetically determined, but as microtine rodents M. spicilegus can be quite flexible in employing behavior to suit the local environment.

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