Tapio Mappes

COST OF REPRODUCTION IN THE WILD: MANIPULATION OF REPRODUCTIVE EFFORT IN THE BANK VOLE

For three years, we manipulated litter size by adding or subtracting pups in eight wild populations of the bank vole, Clethrionomys glareolus, to examine reproductive costs and allocation of reproductive effort between offspring number and size. In general, litter enlargements did not increase the number of weanlings per mother and significantly decreased the size of weanlings. Reproductive effort and the breeding success of individuals varied within breeding seasons, but time of breeding and litter manipulation did not interact to affect reproductive trade-offs. Our 3-yr field experiment revealed that litter enlargements also reduced survival and fecundity of mothers. Small mammals have been considered typical income breeders, in which potential reproductive costs may be masked due to their ability to compensate for increased energetic demands of reproduction. Our results provide evidence that, in the wild, females may be incapable of escaping the costs of reproduction.

Quantitative measure of sexual selection with respect to the operational sex ratio: a comparison of selection indices

Despite numerous indices proposed to predict the evolution of mating systems, a unified measure of sexual selection has remained elusive. Three previous studies have compared indices of sexual selection under laboratory conditions. Here, we use a genetic study to compare the most widely used measures of sexual selection in natural populations. We explored the mating and reproductive successes of male and female bank voles, Clethrionomys glareolus, across manipulated operational sex ratios (OSRs) by genotyping all adult and pup bank voles on 13 islands using six microsatellite loci. We used Batemans principles (Is and I and Bateman gradients) and selection coefficients (s′ and β′) to evaluate, for the first time, the genetic mating system of bank voles and compared these measures with alternative indices of sexual selection (index of monopolization and Morisitas index) across the OSRs. We found that all the sexual selection indices show significant positive intercorrelations for both males and females, suggesting that Batemans principles are an accurate and a valid measure of the mating system. The Bateman gradient, in particular, provides information over and above that of other sexual selection indices. Male bank voles show a greater potential for sexual selection than females, and Bateman gradients indicate a polygynandrous mating system. Selection coefficients reveal strong selection gradients on male bank vole plasma testosterone level rather than body size.

Territorial behaviour and reproductive success of bank vole Clethrionomys glareolus females

1. Although spacing behaviour in small mammals has been studied extensively, the ultimate function of territoriality in females is frequently under debate. Furthermore, almost no data exists on how territorial behaviour of females changes during reproductive cycles and how that affects their reproductive success. We studied these questions in large outdoor enclosures using the bank vole Clethrionomys glareolus as a study species. 2. Home range dynamics of females and behaviour of territory owners against intruder females were determined during four distinct periods of the reproductive cycle: (i) when females were non-pregnant, (ii) in early pregnancy, (iii) in late pregnancy and (iv) when females were lactating. 3. Home range size and home range overlap of females decreased from period (i) to period (iv) simultaneously when nearest neighbour distance increased. So, at the time of parturition female home range size (foraging area) was at its smallest but territory size (breeding area) at its largest. 4. Attacks of owner females increased and amicable behaviour decreased during the reproductive cycle. Attacks tended to correlate positively and amicable behaviour negatively with home range size. 5. The size of a females home range significantly correlated with her litter size. However, the weight of females did not correlate with their home range size or litter size. 6. During lactation, nearest neighbour distance correlated negatively with the number of weanlings. Further, home range overlap did not have an effect on the number of weanlings. So, in contrast to an earlier study by Mappes et al. we did not find support for infanticide directly affecting reproductive success of females. That might be due to synchronous breeding in our study females in contrast to females in Mappes et al. 7. Our results indicate that space-resource (large home range) and thus probably food may be an important determinant of litter size in the bank voles. Aggressive defence of a territory may have a role in determining the amount of resources available for reproduction and thus the reproductive success. 8. Our study gives new evidence both for and against the two hypotheses for function of female territoriality and suggests that they are not necessarily mutually exclusive.

ENDEMIC HANTAVIRUS INFECTION IMPAIRS THE WINTER SURVIVAL OF ITS RODENT HOST

The influence of pathogens on host fitness is one of the key questions in infection ecology. Hantaviruses have coevolved with their hosts and are generally thought to have little or no effect on host survival or reproduction. We examined the effect of Puumala virus (PUUV) infection on the winter survival of bank voles (Myodes glareolus), the host of this virus. The data were collected by monitoring 22 islands over three consecutive winters (a total of 55 island populations) in an endemic area of central Finland. We show that PUUV infected bank voles had a significantly lower overwinter survival probability than antibody negative bank voles. Antibody negative female bank voles from low-density populations living on large islands had the highest survival. The results were similar at the population level as the spring population size and density were negatively correlated with PUUV prevalence in the autumn. Our results provide the first evidence for a significant effect of PUUV on host survival suggesting that hantaviruses, and endemic pathogens in general, deserve even more attention in studies of host population dynamics.

Between-group competition and human cooperation

A distinctive feature of human behaviour is the widespread occurrence of cooperation among unrelated individuals. Explaining the maintenance of costly within-group cooperation is a challenge because the incentive to free ride on the efforts of other group members is expected to lead to decay of cooperation. However, the costs of cooperation can be diminished or overcome when there is competition at a higher level of organizational hierarchy. Here we show that competition between groups resolves the paradigmatic ‘public goods’ social dilemma and increases within-group cooperation and overall productivity. Further, group competition intensifies the moral emotions of anger and guilt associated with violations of the cooperative norm. The results suggest an important role for group conflict in the evolution of human cooperation and moral emotions.

Maternal antibodies postpone hantavirus infection and enhance individual breeding success

The transfer of maternal antibodies from mother to progeny is a well-known phenomenon in avian and mammalian species. Optimally, they protect the newborn against the pathogens in the environment. The effect of maternal antibodies on microparasite transmission dynamics may have important consequences for both the fitness of the host and the epizootic processes of the pathogens. However, there is a scarcity of studies examining these effects in free-living wild species. We studied the influence of maternal antibodies against the zoonotic Puumala hantavirus (PUUV) on the fitness of bank voles (Clethrionomys glareolus) and on PUUV transmission by exposing young maternal antibody-positive (MatAb+) and negative (MatAb−) bank voles (n=160) to PUUV in experimental populations. PUUV-specific maternal antibodies delayed the timing of infection. Females were more susceptible to PUUV infection than males. Interestingly, both the females and the males with maternal antibodies matured earlier than the other individuals in the population. Our results highlight the significance of maternal antibodies in the transmission of a pathogen and in the breeding success of the carriers.

REPRODUCTIVE COSTS AND LITTER SIZE IN THE BANK VOLE

The potential reproductive costs for free-ranging bank vole (Clethrionomys Glareolus) offspring and mothers were assessed by manipulating litter size and by determining the effects of nursing varied numbers of offspring. Litter enlargement did not increase the number of weanlings per mother. The mass of juveniles was significantly lower in the enlarged litters and higher in the reduced litters, compared to the control group. However, the survival of juveniles from weaning aged three months did not depend on their mass at weaning. Data from a previous study (Mappes et al. 1995) indicated that a higher mass at weaning may increase juveniles’ abilities to maturate and breed during their summer of birth. Manipulation of litter size did not significantly affect the mass or survival of mothers or the success of subsequent breeding. The size of home ranges did correlate positively with the initial litter size. However, space use by females did not change with the degree of manipulation. Our results indicate that females nursing enlarged litters produce smaller offspring at weaning with no residual effects on future maternal survival or reproduction. Mothers did not seem to compensate the nursing costs with increased parental effort (which should be reflected in the condition of mothers or in the use of resources), for example, in an enlarged size of home range. Probably the possibility of obtaining a larger home range is constrained by the other breeding females, in a saturated breeding population. These results may support the optimal investment hypothesis that a female will produce a particular litter size which gives the best reproductive success in the particular environment where offspring are nursed.

Testosterone‐Mediated Effects on Fitness‐Related Phenotypic Traits and Fitness

The physiological and behavioral mechanisms underlying life‐history trade‐offs are a continued source of debate. Testosterone (T) is one physiological factor proposed to mediate the trade‐off between reproduction and survival. We use phenotypic engineering and multiple laboratory and field fitness‐related phenotypic traits to test the effects of elevated T between two bank vole Myodes glareolus groups: dominant and subordinate males. Males with naturally high T levels showed higher social status (laboratory dominance) and mobility (distance between capture sites) than low‐T males, and the effect of T on immune response was also T group specific, suggesting that behavioral strategies may exist in male bank voles due to the correlated responses of T. Exogenous T enhanced social status, mate searching (polygon of capture sites), mobility, and reproductive success (relative measure of pups sired). However, exogenous T also resulted in the reduction of immune function, but only in males from the high‐T group. This result may be explained either by the immunosuppression costs of T or by differential sensitivity of different behavioral strategies to steroids. Circulating T levels were found to be heritable; therefore, female bank voles would derive indirect genetic benefits via good genes from mating with males signaling dominance.

Limitation of reproductive success by food availability and litter size in the bank vole, Clethrionomys glareolus

Food limitation has been suggested as one of the most important factors affecting life history evolution in terrestrial vertebrates. However, this inference is based mainly on evidence from birds, and reproductive trade–offs may differ among groups with different forms of parental care. To study whether the costs of enlarged litters (decreased mass of offspring) would appear when food is not limiting, we performed outdoor enclosure experiments in which we manipulated simultaneously the litter size (control versus litter plus two pups) and food availability (control versus food–supplemented) of female bank voles, Clethrionomys glareolus. The weaning success of females increased significantly in response to supplementary food. When females were provided with extra food, no differences were observed in body masses of weanlings of control and enlarged litters. Further, food–supplemented females grew to larger sizes during nursing than unsupplemented females. Our experiment suggests that energetic requirements during nursing constrain the number of offspring that can be raised successfully in a particular breeding attempt. The results also indicate that unlimited food resources may increase future reproductive potential of females because they can use more energy for somatic growth.

Breeding suppression in voles under predation risk of small mustelids : laboratory or methodological artifact?

Many prey animals have life-history strategies that seem to have evolved to avoid predation from specialist predators. During the past few years. the hypothesis of breeding suppression (BSH) of voles as an adaptation to avoid small mustelid predation has prompted several empirical and theoretical studies. However, the validity of empirical work as well as justification of the conclusions in these studies have been criticised. Here we report results of an experiment in which we studied the breeding suppression of bank voles, Clethrionomvs glareolus. in four replicated enclosures. We found no effects of simulated least weasel, Mustela nivalis nivalis, predation risk on the reproductive output of female voles. In contrast to earlier laboratory studies, the weights of females did not differ between treatment groups after the experiment. We reanalysed results of our earlier laboratory studies in light of these results and criticism of the BSH. At present there is no direct evidence for brecding suppression of voles in field populations. Furthermore, the underlying assumptions of the BSH have not been tested experimentally. Thus the most parsimonious explanation for breeding suppression is that it may be a methodological or laboratory artifact.