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Abstract. Since both reproduction and immunocompetence are costly, the negative reciprocal relationships between these functions were found in many species. Due to reproductive immunosuppression, some fraction of seasonally breeding populations of small mammals reproduces in the first breeding season, while others reproduce in the next one. Elevation of breeding efforts under the increased risk of mortality and the reproductive delay until recovery are the polar variants of mating behavior of parasitized individuals. However the parasite-induced changes of odor, visual or acoustic signals limit the mating success of the infected hosts. The direct influence of the immune system, regularly activated by infections, to chemicals signals can be answer to the question: why these signals are honest? Decrease of a strange infection risk by kin breeding can be a satisfactory strategy of an isolated population. Nevertheless, many species follow the inbreeding avoidance strategy, where the major histocompatibility complex (MHC) genes play key role in kin recognition. The advantage of MHC heterozygosity was found at the all steps of the breeding cycle; including mating choice, fertilization, pre-and postnatal development. So, the relationships between immune system and neuroendocrine regulation of behavior give proximate explanations of the evolutionary stable strategies of breeding behavior.

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The contribution of the maternal environment (gestational weight gain, month of birth, and litter size) to the formation and realization of the reproductive potential of adjacent European water vole genera tions has been studied for years in the natural population and in captivity. The accumulation of body fat and other reserves during pregnancy plays an essential role in the regulation of reproductive processes. Realization of the reproductive potential of females, growth rate, and the sexual maturation of the offspring depends on the ability of the mothers to properly utilize the energy reserves of their body both during pregnancy and at the next stage of the reproduction cycle, lactation.

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