V. I. Evsikov

Behaviour, chemosignals and endocrine functions in male mice infected with tick-borne encephalitis virus

Odour attractiveness, social behaviour and endocrine status of male mice (outbred ICR strain) were examined 6-7 days after inoculation with subclinical dose of tick-borne encephalitis virus (TBE). According to RT-PCR control of efficiency of infection, males injected with TBE were divided on the two subgroups: TBE+ (males with viral RNA) and TBE- (males without viral RNA). Susceptible males (TBE+ subgroup) showed the higher level of plasma testosterone in comparison with both control and nonsusceptible (TBE- subgroup) males. TBE+ males had also more odour attraction for oestrus females and more aggressiveness in social conflict. Higher sexual attractiveness and aggressiveness of the infected host benefit the pathogens distribution in the host population.

Body condition and reproductive characteristics of female water voles (Arvicola terrestris L.)

The relationship between long-term dynamics of reproductive parameters and body condition were studied in female water voles from a cycling population. Body condition was estimated from the deviation of body weight (excluding the weight of the uterus with embryos) from the theoretically expected weight calculated from the equation of body weight regression with respect to body length. As the index of body condition increases upon transition from decline to peak in the population cycle, the numbers of corpora lutea and live embryos also increase and the risk of resorption of the entire litter becomes lower.

Effects of selection for olfactory preferences in an inbred mouse strain.

Sexual reproduction, a major evolutionary advantage, ensures an increase in the latent mutation reserve and, in addition, improves the overall population gene pool by testing the adaptive value of new mutations and stabilizing their advantageous combinations [1]. Since individual organisms ceased to be self-reproducing units, interindividual relatedness became more important, and “the population level” appeared, manifesting itself as a family, which is a new, triune reproductive unit (male, female and their offspring). As population biology developed starting with Darwin’s works [2], it became increasingly evident that the formation of the reproductive “cells” is not accidental in natural populations, because the advantages of combinatorial variability proved to be actually beneficial for the species’ evolutionary fate. Both the “latent” genetic features and physiological state of mates are known to be distinguished by animals and determine their mating preferences [3]. Gradual progress in the phenotypical expression of vitally important traits is thus supported [1, 4, 5]. In any case, when a female has an opportunity to choose the mate, reproduction is optimized (a higher fecundity and better offspring development are observed [6, 7]). In inbred animals, the range of phenotypical variability is similar to that observed in heterogeneous populations and influences the mating preferences. Several hypotheses were put forward to explain this phenomenon by an a priori preserved genetic variability or “phenotypical instability” [8, 9]. Because of genetic and/or phenotypical variability, discrimination is possible between the potential sexual partners, as well as mating preferences and the effects of epigamic selection. In this study, we attempted to determine the ethological and physiological effects of the odorous choice of a male by females of inbred strains, because the “chemical sense” is the leading one in mammalian communications [10]. The BALB/c mice from the Laboratory of Biomedical Modelling (Tomsk), which are maintained at the Institute of Animal Systematics and Ecology, Russian Academy of Science, were used. Two weeks prior to the experiments, each male was placed into an individual cage to smooth out the differences caused by the behavioral and physiological effects of the keeping in groups [10]. When testing for mating preference as described earlier [6, 7], it was determined what time it takes estrous females to examine the bedding from two males. Formation of a mating pair was considered to be completed when the stimulus from one of the males was examined for a longer time and repeated preference was displayed by the female. Then, both male and female were kept in one cage to obtain about three litters of young mice, which were weaned at the age of three weeks. Next day after the young mouse birth, the fathers were tested for parental care [7]. For this purpose, they were removed from the cage, in which the young mice were replaced from the nest into the opposite corner. Then, the father was returned to the nest, and, if it did not carry any of the young mice into the nest for 10 min, it was considered to exhibit no paternal care, and vise versa. After each of eight selection cycles, ten pairs were selected to form the “preferential olfactory stimulus” (POS) experimental group (the selection rate being 50%). Ten control (C) pairs were formed from animals bred without the selection for the POS; some of these were brother–sister pairs.

Growth rate, reproductive capacity, and survival rate of European water voles taken from natural populations at different phases of the population cycle

Growth rate, reproductive characteristics, and survival rate of European water voles taken from the population at the peak, decline, depression, and increase phases of population cycle were compared under standard vivarium conditions. In November to March, the growth rate of decline-phase males was higher, while that of decline-phase females was lower than in other animals. Decline-phase males were also more successful in reproduction than males taken at other phases. The survival rate of decline-phase females was reduced in the winter period.

Sexual maturation of daughters depends on the mother's body condition during pregnancy: an example of the water vole Arvicola terrestris L.

The body condition of an animal directly depends on the amount, quality, and assimilation efficiency of available food and on the energy expenditures and may serve as a reliable criterion of both internal and external conditions favorable for the onset of reproduction. The body condition is known to determine the probability of participation in reproduction and its effectiveness [8, 11]. Unlike species that rely on stored body reserves during pregnancy and bringing up the offspring [7], females of many small nonhibernating rodents has to rearrange their energy budget during pregnancy to reach a positive balance. When the amount of energy consumed with food is insufficient to support pregnancy and lactation, the energy deficit is compensated for at the expense of metabolic reserves of the body, primarily, fat [6, 12]. Previous ecological and population studies showed that, in the water vole, the probability of pregnancy failure and the litter size depended on the energy balance. In the years when population density dropped because of reduced food supply and/or stressful situations, a higher concentration of free fatty acids was found in female blood, which testifies to the mobilization of energy substrates from the stores of fat. This was accompanied by a significantly higher frequency of embryo resorption at the postimplantation developmental stage [1]. During the perinatal period, the probability of death of the young also depends on the mother’s body condition. The female body weight after the birth of the offspring is found to exceed their body weight on the day of mating by about 10%. However, in females that display a 100% death rate of offspring, the increase in the body weight during pregnancy was significantly lower (3%) [4]. The effect of the mother’s body condition during gestation on the offspring’s postnatal development remains unstudied so far. The rate of sexual maturation is one of the most plastic parameter that is related to animal growth [5]. Analysis of the contribution of maternal factors to the variation of the rate of the daughters’ sexual maturation in the water vole Arvicola terrestris L. showed that the mother’s body condition during pregnancy had a significant effect on this characteristic. The animals used in our study were kept in a vivarium at the natural photoperiod and received food and water ad libitum. The index of body condition during gestation was defined as the difference between the logarithm of the postpartum body weight and the logarithm of the expected body weight. The logarithm of the expected body weight was calculated from the regression equation of the postpartum body weight on the body weight at mating ( R 2 = 0.81; p < 0.001):

Stabilizing function of sexual selection in relation to male aggressiveness in rodents.

A series of experiments on water voles ( Arvicola terrestris L.) and outbred (Swiss) and inbred (BALB/c and C57BL/6) strains of house mice was performed to estimate the dependence of the attractiveness of male rodents on their aggressiveness. The results showed that females preferred males with intermediate aggressiveness indices, which proved to have higher reproductive capacity. The aggressiveness (aggressiveness index) was measured as the mean ratio of aggressive acts to the total number of social interactions between males placed in the same cage. The males were divided into four groups according to their aggressiveness indices: low-aggressive (with aggressiveness indices varying in different series from 0.00‐0.20, M = 0.13), mediumaggressive (0.45‐0.65, M = 0.55), aggressive (0.65‐0.85, M = 0.75), and high-aggressive (0.85‐1.00, M = 0.93). Olfactory attractiveness was estimated by attractiveness index calculated as the proportion of tests in which the stimulus of the given male (the bedding from its cage) was preferred by a female. The stimulus was considered preferred if a receptive female (at the late proestrus or estrus stage) explored it for a longer time than other stimuli. From 17 to 112 males were studied in individual experimental series. The dependence of the olfactory attractiveness of males on their aggressiveness was not monotonic (figure). Low-aggressive males were the least attractive for females (their mean attractiveness index was M = 0.14), and aggressive males were the most attractive ( M = 0.86). The attractiveness of high-aggressive males ( M = 0.51) was significantly lower than that of aggressive and about the same as that of medium-aggressive ( M = 0.49). Although linear approximation can be applied to the observed dependence of attractiveness on aggressiveness ( r 14 = 0.63, R 2 = 0.40, t = 3.07, p = 0.001), which explains the widespread opinion that it is monotonic, a fourth-degree polynomial approximation considerably better fits the observed dependence ( r 14 = 0.93, R 2 = 0.87, t = 9.81, p < 0.000001).

Male sexual attractiveness and aggressiveness in rodents having different mating systems

Male aggressiveness can affect male reproductive success both directly by increasing competitiveness and indirectly through female preference. Assuming that significance of male aggressiveness in species having different mating systems can be different, we studied how male aggressiveness relates to sexual attractiveness in polygynous rodents, the water vole (Arvicola terrestris) and the house mouse (Mus musculus), and in a monogamous species, the steppe lemming (Lagurus lagurus). Our analysis revealed that the relation between odor attractiveness and aggressiveness is nonlinear. In polygynous species, males are more aggressive, so females opt for aggressive, albeit not too aggressive, males. In the monogamous steppe lemming, males show low level of intermale aggressiveness, and the most attractive are slightly aggressive males who have greater reproductive potential.

The effect of habitat hydrology on intraspecific competition, settlement structure, and reproduction in the water vole (Arvicola terrestris).

Analysis of long-term monitoring of the water vole population in Northern Baraba Lowland (Ubinskii raion, Novosibirsk oblast) has revealed correlation between flow intensity of the Om’ River in the study area and population numbers and population dynamics (estimated May through August), average number of live embryos in overwintered females, and percentage of mature young-of-the-year females in different years of study.

Interstrain Odor Preferences and Factors Influencing Growth Rates of Two Strains of Mice and Their Hybrids

We used C57BL/6J (B6) and BALB/cLac (C) mice strains to examine whether inter-strain odor preferences are correlated with differences in pup growth caused by the combination of genotypes of mothers and young. In our study, C females preferred B6 male odor (soiled bedding) over C male odor; C females raising such hybrid pups had improved milk quality (protein and lipid content). B6 females preferred B6 male odor and their milk quality did not change during gestation of hybrid pups. On the other hand, pup development may be influenced by the “graft-versus-host” reaction (GVHR) that is induced by maternal lymphocytes that are naturally transferred via milk. At weaning, weight indices of the spleen in hybrid pups raised by C dams were lower than in hybrids raised by B6 females. As a result of the inter-strain differences in these physiological mechanisms, hybrid pups grew faster if their mother was a C female than if she was a B6 female. Thus, preference of C females for dissimilar B6 males is supported by the long-term positive effects on hybrid pup development during lactation. B6 females have no effective mechanisms giving advantages for the hybrids and they do not prefer dissimilar males.

ATTRACTIVENESS OF MALE VOLE ODOR IS POSITIVELY CORRELATED WITH PUP VIABILITY

The aim of this study was to determine whether pup weight and growth rate are correlated with the “attractiveness” of the odor of the male that the female mated with. To determine male attractiveness, receptive water vole (Arvicola terrestris L.) females were exposed to odor stimuli from triads of males. The odor stimulus from each male was presented to 1-5 different females. The average time of odor investigation served as a measure of male attractiveness. For breeding, male-female pairs were chosen randomly. It was found that the attractiveness of a male’s odor was positively correlated with the mean birth weight of his progeny and their growth rate to weaning. A relationship between pup characteristics and father’s social rank or body weight was not observed. Because the measured attractiveness of the male odor was not based only on the preference of his mate or on the male’s social/physical status, we suggest that some mechanisms other than genetic dissimilarity, kinship or dominance are involved in odor attractiveness.