Ines Klemme

Variation in predation risk and vole feeding behaviour: a field test of the risk allocation hypothesis

Many prey animals experience temporal variation in the risk of predation and therefore face the problem of allocating their time between antipredator efforts and other activities like feeding and breeding. We investigated time allocation of prey animals that balanced predation risk and feeding opportunities. The predation risk allocation hypothesis predicts that animals should forage more in low- than in high-risk situations and that this difference should increase with an increasing attack ratio (i.e. difference between low- and high-risk situations) and proportion of time spent at high risk. To test these predictions we conducted a field test using bank voles (Clethrionomys glareolus) as a prey and the least weasel (Mustela nivalis nivalis) as a predator. The temporal pattern and intensity of predation risk were manipulated in large outdoor enclosures and the foraging effort and patch use of voles were measured by recording giving-up densities. We did not observe any variation in feeding effort due to changes in the level of risk or the proportion of time spent under high-risk conditions. The only significant effect was found when the attack ratio was altered: the foraging effort of voles was higher in the treatment with a low attack ratio than in the treatment with a high attack ratio. Thus the results did not support the predation risk allocation hypothesis and we question the applicability of the hypothesis to our study system. We argue that the deviation between the observed pattern of feeding behaviour of bank voles and that predicted by the predation risk allocation hypothesis was mostly due to the inability of voles to accurately assess the changes in the level of risk. However, we also emphasise the difficulties of testing hypotheses under outdoor conditions and with mammals capable of flexible behavioural patterns.

Reproductive success of male bank voles (Clethrionomys glareolus): the effect of operational sex ratio and body size

The operational sex ratio (OSR) may influence the intensity of competition for mates and mate choice and is therefore thought to be a major factor predicting the intensity and direction of sexual selection. We studied the opportunity for sexual selection, i.e., the variance in male reproductive success and the direction and intensity of sexual selection on male body mass in bank vole (Clethrionomys glareolus) enclosure populations with experimentally manipulated sex ratios. The opportunity for sexual selection was high among male-biased OSRs and decreased towards female-biased OSRs. Paradoxically, selection for large male body mass was strongest in female-biased OSRs and also considerable at intermediate OSRs, whereas at male-biased OSRs, only a weak relationship between male size and reproductive success was found. Litters in male-biased OSRs were more likely to be sired by multiple males than litters in female-biased OSRs. Our results suggest that the intensity and direction of sexual selection in males differs among different OSRs. Although the direction of sexual selection on male body mass was opposite than predicted, large body mass can be favored by sexual selection. Naturally varying OSRs may therefore contribute to maintain variation in male sexually selected traits.

Do female bank voles (Clethrionomys glareolus) mate multiply to improve on previous mates

Contrary to classical sexual selection theories, females of many taxa mate with multiple males during one reproductive cycle. In this study, we conducted an experiment on the “trade-up hypothesis”, which proposes that females remate if a subsequently encountered male is potentially superior to previous mates to maximize the genetic quality of their offspring. We presented bank vole females (Clethrionomys glareolus) sequentially with two males of known dominance rank in different orders, i.e., either first subordinate and second dominant, first dominant and second subordinate, or two males that were equal in dominance (high ranking) and observed their mating behavior. We found that 92% of the females mated multiply and did not base their remating decision on male social status. Therefore, polyandry cannot be explained by the “trade-up hypothesis” based on dominance rank in this species. However, we found that dominant males sired significantly more offspring than subordinate males. This varied according to mating order: dominant males sired more offspring when they were second than when they were first. Moreover, litter sizes were significantly smaller when the dominant male was first (smallest relative success of dominant males) compared to litter sizes when mating order was reversed or both males equal in status. Our results suggest that even though multimale mating includes males that are of poorer quality and thus potentially decreases the fitness of offspring, most of a female’s offspring are sired by dominant males. Whether this is due to cryptic female choice, sperm competition, or a combination of both, remains to be tested.

STRATEGIC ADJUSTMENTS IN SPERM PRODUCTION WITHIN AND BETWEEN TWO ISLAND POPULATIONS OF HOUSE MICE

Sperm production is physiologically costly. Consequently, males are expected to be prudent in their sperm production, and tailor their expenditure according to prevailing social conditions. Differences in sperm production have been found across island populations of house mice that differ in the level of selection from sperm competition. Here, we determined the extent to which these differences represent phenotypic plasticity and/or population divergence in sperm production. We sourced individuals from two populations at the extreme levels of sperm competition, and raised them under common‐garden conditions while manipulating the social experience of developing males. Males from the high‐sperm competition population produced more sperm and better quality sperm than did males from the low‐sperm competition population. In addition, males reared under a perceived “risk” of sperm competition produced greater numbers of sperm than males reared with “no risk.” However, our analyses revealed that phenotypic plasticity in sperm production was greater for individuals from the high‐sperm competition population. Our results are thus consistent with both population divergence and phenotypic plasticity in sperm production, and suggest that population level of sperm competition might affect the degree of adaptive plasticity in sperm production in response to sperm competition risk.

Polyandry enhances offspring survival in an infanticidal species

The adaptive significance of polyandry is an intensely debated subject in sexual selection. For species with male infanticidal behaviour, it has been hypothesized that polyandry evolved as female counterstrategy to offspring loss: by mating with multiple males, females may conceal paternity and so prevent males from killing putative offspring. Here we present, to our knowledge, the first empirical test of this hypothesis in a combined laboratory and field study, and show that multiple mating seems to reduce the risk of infanticide in female bank voles Myodes glareolus. Our findings thus indicate that females of species with non-resource based mating systems, in which males provide nothing but sperm, but commit infanticide, can gain non-genetic fitness benefits from polyandry.

Why do female bank voles, Clethrionomys glareolus, mate multiply?

Females of many species actively engage in multiple mating, with either a single male or several males, but the adaptive function of this behaviour is often unclear. We conducted a laboratory experiment on a small mammal species, the bank vole, testing the possible benefits of multiple mating on a females short-term reproductive success (pregnancy rate, litter size and early postnatal survival). Such benefits may affect a females fitness either directly or indirectly (genetic benefit). We assigned females to three treatments: a single mating treatment in which females mated once with a single male and two multiple mating treatments in which females mated either twice with a single male or twice with two different males. We found a significant reduction in pregnancy rate of females that mated only once compared to females that mated twice. This direct benefit is most likely explained by an increased stimulus gained from multiple mating. However, we found no difference in reproductive success of females mated twice with the same male or once with each of two males. Our study supports the importance of direct benefits of multiple mating. Although our study cannot rule out effects of genetic benefits on future survival and reproductive success of offspring, we found no fitness benefits of polyandry for the traits studied here.

Polyandrous females produce sons that are successful at post‐copulatory competition

Some of the genetic benefit hypotheses put forward to explain multiple male mating (polyandry) predict that sons of polyandrous females will have an increased competitive ability under precopulatory or post‐copulatory competition via paternally inherited traits, such as attractiveness or fertilization efficiency. Here, we tested these predictions by comparing the competitive ability of sons of experimentally monandrous and polyandrous female bank voles (Myodes glareolus), while controlling for potential material and maternal effects. In female choice experiments, we found no clear preference for sons of either monandrous or polyandrous mothers. Moreover, neither male type was dominant over the other, indicating no advantage in precopulatory male contest competition. However, in competitive matings, sons of polyandrous mothers significantly increased their mating efforts (mating duration, intromission number). In line with this, paternity success was biased towards sons of polyandrous mothers. Because there was no evidence for maternal effects, our results suggest that female bank voles gain genetic benefits from polyandry.

Reducing Mortality of Shrews in Rodent Live Trapping — a Method Increasing Live-Trap Selectivity with Shrew Exits

Shrews have very high metabolic rates and are often unintentionally starved in rodent live-traps during capture-mark-recapture (CMR) studies. Here, we suggest a shrew exit as a modification to rodent traps. To test whether this modification is (1) saving shrews and (2) not jeopardizing results of rodent captures, we compared captures in Ugglan traps with and without shrew exits, studying bank voles (Myodes glareolus) in a spruce forest in central Finland. Numbers of captured bank voles and body size of smallest juvenile bank voles were not affected by the shrew exit, while the number of captured common shrews (Sorex araneus) was reduced from 31 to 0 individuals per 100 trap nights. However, rare larger shrew species (> 8 g body weight) could not escape through the exit. A shrew exit can, therefore, save smaller shrew species in standard live-trapping of vole-sized rodents without affecting CMR data of the rodent.

Is reproduction really costly? Energy metabolism of bank vole (Clethrionomys glareolus) females through the reproductive cycle

ABSTRACT Energetic requirements during reproduction are important determinants of the onset of reproduction and of breeding strategy (e.g., breeding post-partum) and therefore affect female reproductive output in seasonally varying environments. To balance the energetic needs of breeding with energy availability, females must optimize energy allocation between their own energy use and energy allocated to their litter. Here, we studied energetic costs and potential energetic trade-offs of reproduction in female bank voles (Clethrionomys glareolus). We measured energy consumption, i.e., metabolic rates as determined from carbon dioxide production of females either with their pups (breeding unit) to find the total energy expenditure of breeding or alone (maternal energy use), to find potential trade-offs between energy allotted to offspring and to maintenance metabolism. We found that energy used by breeding units remained on the non-pregnant level during pregnancy and increased during lactation but did not increase further during post-partum pregnancy. Maternal energy use remained unchanged through the breeding cycle. Carbon dioxide produced per gram of tissue decreased throughout the reproductive cycle for both breeding units and females alone, suggesting that energy use efficiency improved with advancing pregnancy and lactation. Our study supports the idea that in small mammals reproduction does not increase female energetic costs beyond the costs involved in increasing body mass. We found further that offspring body mass did not differ between lactating non-pregnant and lactating pregnant females and that there were no trade-offs between female energy use and offspring body mass. We conclude that energy allocation by breeding iteroparous females allows for an optimal breeding strategy, i.e., fertilization of new ova post-partum.

Host infection history modifies co‐infection success of multiple parasite genotypes

Co-infections by multiple parasite genotypes are common and have important implications for host-parasite ecology and evolution through within-host interactions. Typically, these infections take place sequentially, and therefore, the outcome of co-infection may be shaped by host immune responses triggered by previous infections. For example, in vertebrates, specific immune responses play a central role in protection against disease over the course of life, but co-infection research has mostly focused on previously uninfected individuals. Here, we investigated whether sequential exposure and activation of host resistance in rainbow trout Oncorhynchus mykiss affects infection success and interactions between co-infecting parasite genotypes of the trematode eye-fluke Diplostomum pseudospathaceum. In accordance with earlier results, we show that a simultaneous attack of two parasite genotypes facilitates parasite establishment in previously uninfected hosts. However, we find for the first time that this facilitation in co-infection is lost in hosts with prior infection. We conclude that vertebrate host infection history can affect the direction of within-host-parasite interactions. Our results may have significant implications for the evolution of co-infections and parasite transmission strategies.