Reproductive costs and litter size in the bank vole

Evolved high aerobic capacity has context-specific effects on gut microbiota

Gut microbiota is expected to coevolve with the host's physiology and may play a role in adjusting the host's energy metabolism to suit the host's environment. To evaluate the effects of both evolved host metabolism and the environmental context in shaping the gut microbiota, we used a unique combination of (1) experimental evolution to create selection lines for a fast metabolism and (2) a laboratory-to-field translocation study. Mature bank voles Myodes glareolus from lines selected for high aerobic capacity (A lines) and from unselected control (C lines) were released into large (0.2 ha) outdoor enclosures for longitudinal monitoring. To examine whether the natural environment elicited a similar or more pronounced impact on the gut microbiota of the next generation, we also sampled the field-reared offspring. The gut microbiota were characterized using 16S rRNA amplicon sequencing of fecal samples. The artificial selection for fast metabolism had minimal impact on the gut microbiota in laboratory conditions but in field conditions, there were differences between the selection lines (A lines vs. C lines) in the diversity, community, and resilience of the gut microbiota. Notably, the selection lines differed in the less abundant bacteria throughout the experiment. The lab-to-field transition resulted in an increase in alpha diversity and an altered community composition in the gut microbiota, characterized by a significant increase in the relative abundance of Actinobacteria and a decrease of Patescibacteria. Also, the selection lines showed different temporal patterns in changes in microbiota composition, as the average gut microbiota alpha diversity of the C lines, but not A lines, was temporarily reduced during the initial transition to the field. In surviving young voles, the alpha diversity of gut microbiota was significantly higher in A-line than C-line voles. These results indicate that the association of host metabolism and gut microbiota is context-specific, likely mediated by behavioral or physiological modifications in response to the environment.

Artificial selection for predatory behaviour results in dietary niche differentiation in an omnivorous mammal

The diet of an individual is a result of the availability of dietary items and the individual's foraging skills and preferences. Behavioural differences may thus influence diet variation, but the evolvability of diet choice through behavioural evolution has not been studied. We used experimental evolution combined with a field enclosure experiment to test whether behavioural selection leads to dietary divergence. We analysed the individual dietary niche via stable isotope ratios of nitrogen (δ¹⁵N) and carbon (δ¹³C) in the hair of an omnivorous mammal, the bank vole, from four lines selected for predatory behaviour and four unselected control lines. Predatory voles had higher hair δ¹⁵N values than control voles, supporting our hypothesis that predatory voles would consume a higher trophic level diet (more animal versus plant foods). This difference was significant in the early but not the late summer season. The δ¹³C values also indicated a seasonal change in the consumed plant matter and a difference in food sources among selection lines in the early summer. These results imply that environmental factors interact with evolved behavioural tendencies to determine dietary niche heterogeneity. Behavioural selection thus has potential to contribute to the evolution of diet choice and ultimately the species' ecological niche breadth.

Parental Behavior in Carnivores

The mammalian order Carnivora is generally defined as species that feed exclusively or to some degree by eating other animals. The Carnivora comprise around 280 species, divided into 16 families, 13 of which are terrestrial and 3 aquatic. Carnivores are spread across the entire planet, including the two polar regions and on land and sea. Consistent with such diverse ecologies, there is no typical pattern of parental care distinguishing carnivores from other mammals. Using examples from different taxonomic families, our aim is to illustrate the diversity of parental care in Carnivora. Major topics include parental care before and after birth of the young, paternal, and alloparental care and the process of weaning. Given the position of many carnivores at the apex of food chains, a greater understanding of their patterns of parental care as a vital part of reproductive biology is essential to conservation programs.

Bank vole alarm pheromone chemistry and effects in the field

Chemical communication plays an important role in mammalian life history decisions. Animals send and receive information based on body odour secretions. Odour cues provide important social information on identity, kinship, sex, group membership or genetic quality. Recent findings show, that rodents alarm their conspecifics with danger-dependent body odours after encountering a predator. In this study, we aim to identify the chemistry of alarm pheromones (AP) in the bank vole, a common boreal rodent. Furthermore, the vole foraging efficiency under perceived fear was measured in a set of field experiments in large outdoor enclosures. During the analysis of bank vole odour by gas chromatography–mass spectrometry, we identified that 1-octanol, 2-octanone, and one unknown compound as the most likely candidates to function as alarm signals. These compounds were independent of the vole’s sex. In a field experiment, voles were foraging less, i.e. they were more afraid in the AP odour foraging trays during the first day, as the odour was fresh, than in the second day. This verified the short lasting effect of volatile APs. Our results clarified the chemistry of alarming body odour compounds in mammals, and enhanced our understanding of the ecological role of AP and chemical communication in mammals.

Trade-off between offspring mass and number: the lightest offspring bear the costs

Life-history theory predicts a trade-off between offspring size and number. However, the role of intra-litter phenotypic variation in shaping this trade-off is often disregarded. We compared the strength of the relationship between litter size and mass from the perspective of the lightest and the heaviest yearling offspring in 110 brown bear litters in Sweden. We showed that the mass of the lightest yearlings decreased with increasing litter size, but that the mass of the heaviest yearling remained stable, regardless of litter size. Consistent with a conservative reproductive strategy, our results suggest that mothers maintained a stable investment in a fraction of the litter, while transferring the costs of larger litter size to the remaining offspring. Ignoring intra-litter phenotypic variation may obscure our ability to detect a trade-off between offspring size and number.

In utero behavioral imprinting to predation risk in pups of the bank vole

Abstract

In the predator–prey arms race, survival-enhancing adaptive behaviors are essential. Prey can perceive predator presence directly from visual, auditory, or chemical cues. Non-lethal encounters with a predator may trigger prey to produce special body odors, alarm pheromones, informing conspecifics about predation risks. Recent studies suggest that parental exposure to predation risk during reproduction affects offspring behavior cross-generationally. We compared behaviors of bank vole (Myodes glareolus) pups produced by parents exposed to one of three treatments: predator scent from the least weasel (Mustela nivalis nivalis); scent from weasel-exposed voles, i.e., alarm pheromones; or a control treatment without added scents. Parents were treated in semi-natural field enclosures, but pups were born in the lab and assayed in an open-field arena. Before each behavioral test, one of the three scent treatments was spread throughout the test arena. The tests followed a full factorial design (3 parental treatments × 3 area treatments). Regardless of the parents’ treatment, pups exposed to predator odor in the arena moved more. Additionally, pups spend more time in the center of the arena when presented with predator odor or alarm pheromone compared with the control. Pups from predator odor–exposed parents avoided the center of the arena under control conditions, but they spent more time in the center when either predator odor or alarm pheromone was present. Our experiment shows that cross-generational effects are context-sensitive, depending on the perceived risk. Future studies should examine cross-generational behavioral effects in ecologically meaningful environments instead of only neutral ones.

Significance statement

We exposed bank voles to odors signaling predation risk to assess the effects parental predation exposure on the behavior of their offspring. Besides predator odor, we also assessed the role of a conspecific alarm cue as a novel way of spreading the predation risk information. Pup behaviors were assessed in the open-field arena, a standard way of assessing animal behavior in a wide range of contexts. We found that also alarm pheromone increased the time pups spend in the center of the arena similarly to predator odor. While previous studies suggested that offspring would be more fearful, our results indicate that the cross-generational effects are very context-dependent; i.e., they differ significantly depending on which scent cue is presented in the open-field arena. This shows the need for better tools or measurements to translate laboratory results into ecologically meaningful frameworks.

Evolution of ageing as a tangle of trade-offs: energy versus function

Despite tremendous progress in recent years, our understanding of the evolution of ageing is still incomplete. A dominant paradigm maintains that ageing evolves due to the competing energy demands of reproduction and somatic maintenance leading to slow accumulation of unrepaired cellular damage with age. However, the centrality of energy trade-offs in ageing has been increasingly challenged as studies in different organisms have uncoupled the trade-off between reproduction and longevity. An emerging theory is that ageing instead is caused by biological processes that are optimized for early-life function but become harmful when they continue to run-on unabated in late life. This idea builds on the realization that early-life regulation of gene expression can break down in late life because natural selection is too weak to optimize it. Empirical evidence increasingly supports the hypothesis that suboptimal gene expression in adulthood can result in physiological malfunction leading to organismal senescence. We argue that the current state of the art in the study of ageing contradicts the widely held view that energy trade-offs between growth, reproduction, and longevity are the universal underpinning of senescence. Future research should focus on understanding the relative contribution of energy and function trade-offs to the evolution and expression of ageing.

Sympatric Ixodes-tick species: pattern of distribution and pathogen transmission within wild rodent populations

The generalist tick Ixodes ricinus is the most important vector for tick-borne pathogens (TBP), including Borrelia burgdorferi sensu lato, in Europe. However, the involvement of other sympatric Ixodes ticks, such as the specialist vole tick I. trianguliceps, in the enzootic circulations of TBP remains unclear. We studied the distribution of I. ricinus and I. trianguliceps in Central Finland and estimated the TBP infection likelihood in the most common rodent host in relation with the abundance of the two tick species. Ixodes trianguliceps was encountered in all 16 study sites whereas I. ricinus was frequently observed only at a quarter of the study sites. The abundance of I. ricinus was positively associated with open water coverage and human population density around the study sites. Borrelia burgdorferi s. l.-infected rodents were found only in sites where I. ricinus was abundant, whereas the occurrence of other TBP was independent of I. ricinus presence. These results suggest that I. trianguliceps is not sufficient, at least alone, in maintaining the circulation of B. burgdorferi s. l. in wild hosts. In addition, anthropogenic factors might affect the distribution of I. ricinus ticks and, hence, their pathogens, thus shaping the landscape of tick-borne disease risk for humans.

The scent of infanticide risk? Behavioural allocation to current and future reproduction in response to mating opportunity and familiarity with intruder

Abstract

The killing of young by unrelated males is widespread in the animal kingdom. In short-lived small rodents, females can mate immediately after delivery (post-partum oestrus) and invest in future reproduction, but infanticide may put the nestlings, their current reproductive investment, at risk. Here, we investigated the behavioural trade-offs between mating interest and nest protection in an arena experiment with bank voles (Myodes glareolus). Non-gravid females (n = 33) were housed at one end of a large structured arena with their nestlings. Different scents (cage bedding) were presented to each female in a replicated design. Three combinations of mating opportunities and male-female familiarity were simulated using different scent donors: mating opportunity with the sire of the nestlings with whom the female was familiar; mating opportunity with a male unrelated to the offspring and unfamiliar to the female, thus posing a higher risk to the offspring; and neither risk nor mating opportunity (clean control). Most females investigated male scents, regardless of familiarity, leaving their litter unprotected. During control treatment, females with larger litters spent less time at the scent area, indicating increasing nursing demands or better protection. Females with older litters visited scents more often, suggesting an increased interest in reproduction while they are non-gravid alongside the decreased risk of infanticide for older young. In the presence of unfamiliar scents, females spent more time protecting their nests, supporting the perceived association of unfamiliarity with infanticide risk. Thus, rodent females flexibly allocate time spent between searching for a mate and protecting their nest, which is modulated by their familiarity with a potential intruder.

Significance statement

Infanticide by conspecific males is an extreme form of sexual conflict and has large costs on females, abolishing their investment into current offspring. In an experimental approach, we exposed lactating female bank voles to different combinations of mating opportunity and familiarity to a (simulated) intruder: (1) the sire of the nestlings with whom the female was familiar and, therefore, potentially less risky in terms of infanticide; (2) a male which was unrelated and unfamiliar to the female and thus posed a higher risk to the offspring; or (3) as a control, cage bedding, which posed neither risk of infanticide nor a mating opportunity. We show that females flexibly allocated pup protection and mating interest based on their familiarity with the male, indicating that the unfamiliar males pose a threat to offspring, which is perceived by the females. Females further adjusted their behaviour to the size and/or age of their current litter, investing more time in male scents when offspring were older, thus balancing future and current investments into reproduction.

Limits to sustained energy intake. XXVII. Trade-offs between first and second litters in lactating mice support the ecological context hypothesis

Increased reproductive effort may lead to trade-offs with future performance and impact offspring, thereby influencing optimal current effort level. We experimentally enlarged or reduced litter size in mice during their first lactation, and then followed them through a successive unmanipulated lactation. Measurements of food intake, body mass, milk energy output (MEO), litter size and litter mass were taken. Offspring from the first lactation were also bred to investigate their reproductive success. In their first lactation, mothers with enlarged litters (n=9, 16 pups) weaned significantly smaller pups, culled more pups, and increased MEO and food intake compared with mothers with reduced litters (n=9, 5 pups). In the second lactation, no significant differences in pup mass or litter size were observed between groups, but mothers that had previously reared enlarged litters significantly decreased pup mass, MEO and food intake compared with those that had reared reduced litters. Female offspring from enlarged litters weaned slightly smaller pups than those from reduced litters, but displayed no significant differences in any of the other variables measured. These results suggest that females with enlarged litters suffered from a greater energetic burden during their first lactation, and this was associated with lowered performance in a successive reproductive event and impacted on their offspring's reproductive performance. Female 'choice' about how much to invest in the first lactation may thus be driven by trade-offs with future reproductive success. Hence, the 'limit' on performance may not be a hard physiological limit. These data support the ecological context hypothesis.

Can but don't: olfactory discrimination between own and alien offspring in the domestic cat

Mammalian maternal care usually comes at a large energetic cost. To maximize their fitness, mothers should preferentially care for their own offspring. However, the majority of studies of mother-offspring recognition have focused on herd- or colony-living species and there is little information on maternal discrimination in more solitary-living species. Olfaction has been found to play a major role in mother-offspring recognition across various taxa. Therefore, our aim was to study this in a species evolved from a solitary-living ancestor, the domestic cat. We asked whether cat mothers distinguish between their own and alien offspring when providing maternal care, and whether cat mothers use olfactory cues in the offspring discrimination process. Results of Experiment 1 showed that cat mothers do not discriminate between own and alien young when retrieving them to the nest. They treated own and alien young similarly with respect to latency and order of retrieval. However, the results of Experiments 2 and 3, where we used an olfactory habituation-discrimination technique, showed that mothers were able to distinguish between the odours of their own and alien kittens. We discuss what ecological and/or behavioural factors might influence a mother's decision when faced with discriminating between own and alien young, and why mothers might not discriminate between them when they are able to do so. Our findings support the view that maternal care alone should not be used as a measure of offspring recognition, and equal maternal care of own and alien young should not be immediately interpreted as an inability to discriminate between them.

Current versus future reproduction and longevity: a re-evaluation of predictions and mechanisms

Oxidative damage is predicted to be a mediator of trade-offs between current reproduction and future reproduction or survival, but most studies fail to support such predictions. We suggest that two factors underlie the equivocal nature of these findings: (1) investigators typically assume a negative linear relationship between current reproduction and future reproduction or survival, even though this is not consistently shown by empirical studies; and (2) studies often fail to target mechanisms that could link interactions between sequential life-history events. Here, we review common patterns of reproduction, focusing on the relationships between reproductive performance, survival and parity in females. Observations in a range of species show that performance between sequential reproductive events can decline, remain consistent or increase. We describe likely bioenergetic consequences of reproduction that could underlie these changes in fitness, including mechanisms that could be responsible for negative effects being ephemeral, persistent or delayed. Finally, we make recommendations for designing future studies. We encourage investigators to carefully consider additional or alternative measures of bioenergetic function in studies of life-history trade-offs. Such measures include reactive oxygen species production, oxidative repair, mitochondrial biogenesis, cell proliferation, mitochondrial DNA mutation and replication error and, importantly, a measure of the respiratory function to determine whether measured differences in bioenergetic state are associated with a change in the energetic capacity of tissues that could feasibly affect future reproduction or lifespan. More careful consideration of the life-history context and bioenergetic variables will improve our understanding of the mechanisms that underlie the life-history patterns of animals.

Reproduction is not costly in terms of oxidative stress

One of the core assumptions of life-history theory is the negative trade-off between current and future reproduction. Investment in current reproduction is expected to decrease future reproductive success or survival, but the physiological mechanisms underlying these costs are still obscure. To test for a role of oxidative stress, we measured oxidative damage to lipids and proteins in liver, heart, kidneys, and muscles, as well as the level of antioxidants (total glutathione and catalase), in breeding and non-breeding bank voles. We used females from lines selected for high aerobic metabolism and non-selected control lines and manipulated their reproductive investment by decreasing or increasing litter size. Unlike in most previous studies, the females reared four consecutive litters (the maximum possible during a breeding season). Contrary to predictions, oxidative damage in reproducing females was decreased or not changed, and did not differ between the selected and control lines. Oxidative damage to lipids and proteins in liver was lower in females that weaned enlarged litters than in non-breeding ones, and was intermediate in those with reduced litters. Oxidative damage to proteins in the heart also tended to be lower in breeding females than in non-breeding ones. A negative relationship between the level of oxidative damage and activity of catalase in kidneys indicated a protective action of antioxidants. In conclusion, our study falsified the hypothesis that oxidative stress is a part of the proximate physiological mechanism underlying the fundamental life-history trade-off between current and future reproduction.

Identification of factors influencing the Puumala virus seroprevalence within its reservoir in aMontane Forest Environment

Puumala virus (PUUV) is a major cause of mild to moderate haemorrhagic fever with renal syndrome and is transmitted by the bank vole (Myodes glareolus). There has been a high cumulative incidence of recorded human cases in South-eastern Germany since 2004 when the region was first recognized as being endemic for PUUV. As the area is well known for outdoor recreation and the Bavarian Forest National Park (BFNP) is located in the region, the increasing numbers of recorded cases are of concern. To understand the population and environmental effects on the seroprevalence of PUUV in bank voles we trapped small mammals at 23 sites along an elevation gradient from 317 to 1420m above sea level. Generalized linear mixed effects models(GLMEM) were used to explore associations between the seroprevalence of PUUV in bank voles and climate and biotic factors. We found that the seroprevalence of PUUV was low (6%–7%) in 2008 and 2009, and reached 29% in 2010. PUUV seroprevalence was positively associated with the local species diversity and deadwood layer, and negatively associated with mean annual temperature, mean annual solar radiation, and herb layer. Based on these findings, an illustrative risk map for PUUV seroprevalence prediction in bank voles was created for an area of the national park. The map will help when planning infrastructure in the national park (e.g., huts, shelters, and trails).

A genetic tool to manipulate litter size

INTRODUCTION

Experimental litter size manipulations are often not problem free. Typically conducted shortly after birth or oviposition, they do not account for the energy already invested into the production of the offspring. Such effects make it difficult to interpret the results from experimental litter size manipulations and therefore to study optimality of litter or clutch size, a long debated topic in evolutionary biology.

RESULTS

We propose the use of a mating design based on a selfish genetic element, the t haplotype, to reduce litter size in an eutherian mammal, the house mouse. Most t haplotypes are recessive lethal and therefore lead to the death of all homozygous embryos. Litter sizes can be reduced by up to 50% by pairing a +/t female with a +/t male instead of a +/+ male.

CONCLUSIONS

This method allows litter size manipulation before birth without the use of invasive techniques, therefore providing an excellent tool for studying optimal litter size and ultimately helping to understand life history strategies.

Interactive effects of past and present environments on overwintering success-a reciprocal transplant experiment

Life-history traits are influenced by environmental factors throughout the lifespan of an individual. The relative importance of past versus present environment on individual fitness, therefore, is a relevant question in populations that face the challenge of temporally varying environment. We studied the interacting effects of past and present density on body mass, condition, and survival in enclosure populations of the bank vole (Myodes glareolus) using a reciprocal transplant design. In connection with the cyclic dynamics of natural vole populations, our hypothesis was that individuals born in low-density enclosures would do better overwintering in low-density enclosures than in high-density enclosures and vice versa. Our results show that the effect of summer (past) density was strong especially on survival and body mass. The response of body mass to summer density was negative in both winter (present) density groups, whereas the response of survival probability was nonlinear and differed between the winter density groups. In particular, our data show a trend for higher overwintering success of individuals originating from the lowest summer densities in low winter density and vice versa. We therefore conclude that the capacity of individuals to respond to a change in density was constrained by the delayed density-dependent effects of environment experienced in the past. These effects have the potential to contribute to vole population dynamics. Possible mechanisms mediating the effects of past environment into present performance include both intrinsic and environmental factors.

Is reproduction costly? No increase of oxidative damage in breeding bank voles

According to life-history theory, investment in reproduction is associated with costs, which should appear as decreased survival to the next reproduction or lower future reproductive success. It has been suggested that oxidative stress may be the proximate mechanism of these trade-offs. Despite numerous studies of the defense against reactive oxygen species (ROS) during reproduction, very little is known about the damage caused by ROS to the tissues of wild breeding animals. We measured oxidative damage to lipids and proteins in breeding bank vole (Myodes glareolus) females after rearing one and two litters, and in non-breeding females. We used bank voles from lines selected for high maximum aerobic metabolic rates (which also had high resting metabolic rates and food intake) and non-selected control lines. The oxidative damage was determined in heart, kidneys and skeletal muscles by measuring the concentration of thiobarbituric acid-reactive substances, as markers of lipid peroxidation, and carbonyl groups in proteins, as markers of protein oxidation. Surprisingly, we found that the oxidative damage to lipids in kidneys and muscles was actually lower in breeding than in non-breeding voles, and it did not differ between animals from the selected and control lines. Thus, contrary to our predictions, females that bred suffered lower levels of oxidative stress than those that did not reproduce. Elevated production of antioxidant enzymes and the protective role of sex hormones may explain the results. The results of the present study do not support the hypothesis that oxidative damage to tissues is the proximate mechanism of reproduction costs.

Can number and size of offspring increase simultaneously?--a central life-history trade-off reconsidered

Background

To maximize their fitness, parents are assumed to allocate their resources optimally between number and size of offspring. Although this fundamental life-history trade-off has been subject to long standing interest, its genetic basis, especially in wild mammals, still remains unresolved. One important reason for this problem is that a large multigenerational pedigree is required to conduct a reliable analysis of this trade-off.

Results

We used the REML-animal model to estimate genetic parameters for litter size and individual birth size for a common Palearctic small mammal, the bank vole (Myodes glareolus). Even though a phenotypic trade-off between offspring number and size was evident, it was not explained by a genetic trade-off, but rather by negative correlations in permanent and temporary environmental effects. In fact, even positive genetic correlations were estimated between direct genetic effects for offspring number and size indicating that genetic variation in these two traits is not necessarily antagonistic in mammals.

Conclusions

Our results have notable implications for the study of the life-history trade-off between offspring number and size in mammals. The estimated genetic correlations suggest that evolution of offspring number and size in polytocous mammals is not constrained by the trade-off caused by antagonistic selection responses per se, but rather by the opposing correlative selection responses in direct and maternal genetic effects for birth size.

A trade-off between current and future sex allocation revealed by maternal energy budget in a small mammal

Sex-allocation theories generally assume differential fitness costs of raising sons and daughters. Yet, experimental confirmation of such costs is scarce and potential mechanisms are rarely addressed. While the most universal measure of physiological costs is energy expenditure, only one study has related the maternal energy budget to experimentally controlled offspring sex. Here, we experimentally test this in the bank vole (Myodes glareolus) by simultaneously manipulating the litter's size and sex ratio immediately after birth. Two weeks after manipulation, when mothers were at the peak of lactation and were pregnant with concurrent litters, we assessed their energy budget. We found that maternal food consumption and daily energy expenditure increased with the size of the litters being lactated. Importantly, the effects of offspring sex on energy budget depended on the characteristics of the simultaneously gestating litters. Specifically, the mothers nursing all-male litters and concurrently pregnant with male-biased litters had the highest energy expenditure. These had consequences for the next generation, as size of female offspring from the concurrent pregnancy of these mothers was compromised. Our study attests a higher cost of sons, consequently leading to a lower investment in them, and reveals the significance of offspring sex in moulding the trade-off between current and future maternal investment.

Maternal investment in relation to sex ratio and offspring number in a small mammal - a case for Trivers and Willard theory?

Optimal parental sex allocation depends on the balance between the costs of investing into sons vs. daughters and the benefits calculated as fitness returns. The outcome of this equation varies with the life history of the species, as well as the state of the individual and the quality of the environment.

We studied maternal allocation and subsequent fecundity costs of bank voles, Myodes glareolus, by manipulating both the postnatal sex ratio (all-male/all-female litters) and the quality of rearing environment (through manipulation of litter size by −2/+2 pups) of their offspring in a laboratory setting.

We found that mothers clearly biased their allocation to female rather than male offspring regardless of their own body condition. Male pups had a significantly lower growth rate than female pups, so that at weaning, males from enlarged litters were the smallest. Mothers produced more milk for female litters and also defended them more intensively than male offspring.

The results agree with the predictions based on the bank vole life history: there will be selection for greater investment in daughters rather than sons, as a larger size seems to be more influencial for female reproductive success in this species. Our finding could be a general rule in highly polygynous, but weakly dimorphic small mammals where females are territorial.

The results disagree with the narrow sense Trivers & Willard hypothesis, which states that in polygynous mammals that show higher variation in male than in female reproductive success, high-quality mothers are expected to invest more in sons than in daughters.

Island selection on mammalian life-histories: genetic differentiation in offspring size

Background

Since Darwin's pioneering work, evolutionary changes in isolated island populations of vertebrates have continued to provide the strongest evidence for the theory of natural selection. Besides macro-evolutionary changes, micro-evolutionary changes and the relative importance of natural selection vs. genetic drift are under intense investigation. Our study focuses on the genetic differentiation in morphological and life-history traits in insular populations of a small mammal the bank vole Myodes glareolus.

Results

Our results do not support the earlier findings for larger adult size or lower reproductive effort in insular populations of small mammals. However, the individuals living on islands produced larger offspring than individuals living on the mainland. Genetic differentiation in offspring size was further confirmed by the analyses of quantitative genetics in lab. In insular populations, genetic differentiation in offspring size simultaneously decreases the additive genetic variation (V_A) for that trait. Furthermore, our analyses of differentiation in neutral marker loci (F_st) indicate that V_A is less than expected on the basis of genetic drift alone, and thus, a lower V_A in insular populations could be caused by natural selection.

Conclusion

We believe that different selection pressures (e.g. higher intraspecific competition) in an insular environment might favour larger offspring size in small mammals. Island selection for larger offspring could be the preliminary mechanism in a process which could eventually lead to a smaller litter size and lower reproductive effort frequently found in insular vertebrates.

Younger bank voles are more vulnerable to avian predation

The importance of predation on prey populations is mainly determined by the number of eaten prey. However, the total impact of predation might also be determined by the selection of certain prey individuals, e.g., different sexes or age categories. Here we tested selective predation by an avian predator, the pygmy owl ( Glaucidium passerinum (L., 1758)), on bank voles ( Myodes ( Clethrionomys ) glareolus (Schreber, 1780)). We compared the sex, age, and mass of hoarded prey with the animals snap-trapped from the field. There were no differences in the sex ratio between hoarded bank voles and those available in the field. However, hoarded voles were significantly younger than ones in the field sample. There was no statistically significant difference in mass between animals from larders and from the field. We suggest that the greater vulnerability of younger animals to predation might be due to their higher activity, or alternatively, they might be forced to forage in less safe habitats.

Producing sons reduces lifetime reproductive success of subsequent offspring in pre-industrial Finns

Life-history theory states that reproductive events confer costs upon mothers. Many studies have shown that reproduction causes a decline in maternal condition, survival or success in subsequent reproductive events. However, little attention has been given to the prospect of reproductive costs being passed onto subsequent offspring, despite the fact that parental fitness is a function of the reproductive success of progeny. Here we use pedigree data from a pre-industrial human population to compare offspring life-history traits and lifetime reproductive success (LRS) according to the cost incurred by each individual's mother in the previous reproductive event. Because producing a son versus a daughter has been associated with greater maternal reproductive cost, we hypothesize that individuals born to mothers who previously produced sons will display compromised survival and/or LRS, when compared with those produced following daughters. Controlling for confounding factors such as socio-economic status and ecological conditions, we show that those offspring born after elder brothers have similar survival but lower LRS compared with those born after elder sisters. Our results demonstrate a maternal cost of reproduction manifested in reduced LRS of subsequent offspring. To our knowledge, this is the first time such a long-term intergenerational cost has been shown in a mammal species.

Frequency and density-dependent selection on life-history strategies--a field experiment

Negative frequency-dependence, which favors rare genotypes, promotes the maintenance of genetic variability and is of interest as a potential explanation for genetic differentiation. Density-dependent selection may also promote cyclic changes in frequencies of genotypes. Here we show evidence for both density-dependent and negative frequency-dependent selection on opposite life-history tactics (low or high reproductive effort, RE) in the bank vole (Myodes glareolus). Density-dependent selection was evident among the females with low RE, which were especially favored in low densities. Instead, both negative frequency-dependent and density-dependent selection were shown in females with high RE, which were most successful when they were rare in high densities. Furthermore, selection at the individual level affected the frequencies of tactics at the population level, so that the frequency of the rare high RE tactic increased significantly at high densities. We hypothesize that these two selection mechanisms (density- and negative frequency-dependent selection) may promote genetic variability in cyclic mammal populations. Nevertheless, it remains to be determined whether the origin of genetic variance in life-history traits is causally related to density variation (e.g. population cycles).

Large litter size increases maternal energy intake but has no effect on UCP1 content and serum-leptin concentrations in lactating Brandt's voles (Lasiopodomys brandtii)

Lactation is the most energetically demanding period in the female mammal's life. We measured maternal energy intake, uncoupling protein 1 (UCP1) content in brown adipose tissue (BAT), serum-leptin concentration, and litter growth in lactating Brandt's voles (Lasiopodomys brandtii) with different litter sizes. Litter mass was positively related to litter size but there was no difference in pup mass at birth. Maternal gross energy intake at peak lactation was positively correlated with litter size and litter mass. Maternal resting metabolic rate (RMR) was positively correlated with litter mass, but not with litter size. No significant differences were detected in body-fat mass, serum-leptin concentration, or UCP1 in lactating voles with different litter sizes. Serum-leptin concentration was negatively correlated with energy intake during lactation. Our data did not support the hypothesis that there is a trade-off in energy allocation between maternal maintenance and offspring growth in lactating Brandt's voles, but support the idea that if the mothers with ten pups should have less energy available for their maintenance than mothers raising fewer pups. Also, leptin is probably not the only factor that induces the high energy intake in mothers with large litter sizes, although it was involved in the regulation of energy intake during lactation.

The cost of reproduction induced by body size at birth and breeding density

Body size at birth has implications for the quality of individuals throughout their life. Although large body size is generally considered an advantage, the relationship between body size at birth and long-term fitness is often complicated. Under spatial or temporal variation in environmental conditions, such as the seasonally changing densities of Fennoscandian vole populations, selection should favor variation in offspring phenotypes, as different qualities may be beneficial in different conditions. We performed an experiment in which a novel hormonal manipulation method was used to increase phenotypic variance in body size at birth in the bank vole (Myodes glareolus). The effects of body size on the future fitness of young males and females were then studied at varying population densities in outdoor enclosures. Our results show that small body size at birth and high breeding density increase the survival costs of reproduction. However, there was no interaction between the effects of body size and density on survival, which suggests that the fitness effects of body size were strong enough to persist under environmental variation. Moreover, litter size and the probability of breeding were more sensitive to variation in breeding density than offspring size. Therefore, it is unlikely that individual fitness could be optimized by adjusting offspring body size to the prevailing population density through adaptive maternal effects. Our results highlight the significance of the costs of reproduction in the evolution of life-history traits, and give strong experimental support for the long-term fitness effects of body size at birth.

Signal reliability compromised by genotype-by-environment interaction and potential mechanisms for its preservation

Sexual selection based on signaling requires that signals used by females in mate choice are reliable indicators of a male's heritable total fitness. A signal and the preference for it are expected to be heritable, resulting in the maintenance of genetic covariance between these two traits. However, a recent article has proposed that signals may quickly become unreliable in the presence of both environmental variation and genotype-by-environment interaction (G x E) with crossing reaction norms, potentially compromising the mechanisms of sexual selection. Here we examine the heritability and plasticity of a male dominance advertisement in the bank vole, Clethrionomys glareolus, in stable and changing rearing environments from father to son. The bank vole is naturally exposed to considerable sources of spatial and temporal environmental variation and male reproductive success is determined by both intra- (male-male competition) and inter- (females prefer to mate with dominant males) sexual selection. Significant G x E for male dominance was found with crossing reaction norms. Plasma testosterone level (T), rather than condition, determined a male's dominance and T also showed a significant G x E. Dominance showed a considerable plasticity across environments, but was only heritable under stable conditions. We document a negative between-environments correlation of male dominance, suggesting that when the environment changes between father and son, the dominance signal is unreliable to females and sexual selection may be compromised. We discuss how G x E and environmental variation interacting with other mechanisms may preserve the reliability of signals and thus the mechanism of sexual selection itself.

The intestinal nematode Trichuris arvicolae affects the fecundity of its host, the common vole Microtus arvalis

Parasites have detrimental effects on host fitness. Consequently, they play a major role for host population dynamics. In this study, we investigated experimentally the impact of the nematode Trichuris arvicolae on the reproduction of its host, the common vole Microtus arvalis. Wild common voles were trapped in east of France and reared in standardized conditions before being experimentally infected. Infection with Trichuris arvicolae did not affect host consumption of food or water. Parasitized females gave birth to slightly less pups (mean 3.36 +/- 0.38) than unparasitized females (mean 3.60 +/- 0.40). Controlling for natal litter size using analysis of covariance (ANCOVA), T. arvicolae infection had a significant effect on the individual mass at birth, with pups from parasitized females having significantly lower mass (2.11 g +/- 0.01) than pups from unparasitized females (2.20 g +/- 0.01). Other measures of host reproductive outputs (time to first reproduction, mass of pups at weaning, litter survival) were not affected by maternal parasite infection. We discuss how these changes in M. arvalis reproductive investments associated with T. arvicolae infection must now be investigated in the context of physiological trade-offs.

VARIATION IN LITTER SIZE: A TEST OF HYPOTHESES IN RICHARDSON'S GROUND SQUIRRELS

We studied litter size variation in a population of Richardson's ground squirrels (Spermophilus richardsonii) in Alberta, Canada, from 1987 to 2004. Litter size at first emergence of juveniles from the natal burrow ranged from 1 to 14; the most common litter sizes, collectively accounting for 41.0% of 999 litters, were 6 and 7. The number of offspring surviving to adulthood (attained on emergence from hibernation as yearlings) increased with increasing litter size, a result that was not predicted by Lack's "optimal litter size" hypothesis, Mountford's "cliff-edge" effect, or the "bad-years" effect. Contrary to the negative effects predicted by the "cost of reproduction" hypothesis, litter size had no significant influence on survival of mothers to the subsequent year or on the size of the subsequent litter. Rather, our results best fit the predictions of the "individual optimization" hypothesis, which suggests that litter size is determined by the body condition and environmental circumstances of each mother. Supporting this hypothesis, survival of individual offspring was not significantly associated with litter size. Additionally, year-to-year changes in maternal body mass at mating were positively associated with concurrent changes in litter size (r = 0.56), suggesting that litter size depends on the body condition of the mother. Because the mean number of recruits to adulthood increased as litter size increased (r2 = 0.96) and litter size increased with maternal condition, offspring productivity was greater for mothers in better body condition.

Eimeria-parasites are associated with a lowered mother's and offspring's body condition in island and mainland populations of the bank vole

This study, based on correlative data, tests the hypothesis that infections withEimeriaspp. parasites exert a significant loss of fitness of bank voles (Clethrionomys glareolus) reflected in lower reproductive success and survival, declining host population densities and are associated positively with population size. The study was conducted in 20 mainland and 27 island populations in central Finland during May–September in 1999. Faecal samples showed that 28% of 767 individuals were infected withEimeriaspp. The presence ofEimeriaparasites was higher in dense mainland populations than in sparsely populated islands. Eimerian infections increased during the course of the breeding season, probably as a result of the high infection rate of young individuals. Accordingly, the body masses of bank voles were negatively related to the presence ofEimeriaspp. Reproductive output, as measured by the breeding probability of females and litter size, was not associated with the presence of eimerian infection. Interestingly, the body condition of the infected mothers appeared to be low. Moreover, mother's body condition was the single most important variable studied that showed a positive correlation to pup's body condition at birth. On small islands ([les ]3·2 ha) that were comprehensively trapped, the mean number ofEimeriaspp. in the bank vole population was negatively related to density changes of the bank vole population during the study. Our data are consistent with the idea that infection with coccidian parasites may be one of the factors responsible for declining host populations in small, isolated populations.

Sex-biased maternal investment in voles: importance of environmental conditions

Adaptive bias in sex allocation is traditionally proposed to be related to the condition of mothers as well as to the unequal fitness values of produced sexes. A positive relationship between mother condition and investment into male offspring is often predicted. This relationship was also recently found to depend on environmental conditions. We studied these causalities experimentally using a design where winter food supply was manipulated in eight outdoor-enclosed populations of field voles Microtus agrestis. At the beginning of the breeding season in spring, food-supplemented mothers seemed to be in a similar condition, measured as body mass, head width, body condition index and parasite load (blood parasite Trypanosoma), to non-supplemented mothers. Food supplements affected neither the litter size, the reproductive effort of mothers, nor the litter sex ratios at birth. However, food supplementation significantly increased the birth size of male offspring and improved their condition, as indicated by reduced parasite loads (intestinal Eimeria). Interestingly, mothers in good body condition produced larger male offspring only when environmental conditions were improved by food supplements. Although the adaptiveness of variation in mammalian sex ratios is still questionable, our study indicates that mothers in good condition bias their investment towards male offspring, but only when environmental conditions are favourable.

Effects of extra food on nestling growth and survival in red-backed voles (Clethrionomys gapperi)

Nestling mortality is typically high among small mammals, with food often considered an important limiting factor because of the increased energetic costs females incur during lactation. We provided female red-backed voles, Clethrionomys gapperi (Vigors, 1830), with sunflower seeds during lactation to test the hypothesis that food supply influences nestling growth and survival. Food supplementation did not increase nestling survival, but newly emerged young from fed mothers were significantly heavier than those from unfed mothers. High mortality/dispersal of postemergent young, particularly females, was associated with high densities of breeding females. We suggest that any survival advantage incurred from higher nestling growth rates may be mitigated by density-dependent mortality/ dispersal postemergence.