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Correction: 10.1007/s00442-021-04856-4, Oecologia (2021) Most small rodent populations in the world have fascinating population dynamics. In the northern hemisphere, voles and lemmings tend to show population cycles with regular fluctuations in numbers. In the southern hemisphere, small rodents tend to have large amplitude outbreaks with less regular intervals. In the light of vast research and debate over almost a century, we here discuss the driving forces of these different rodent population dynamics. We highlight ten questions directly related to the various characteristics of relevant populations and ecosystems that still need to be answered. This overview is not intended as a complete list of questions but rather focuses on the most important issues that are essential for understanding the generality of small rodent population dynamics.

Intergenerational fitness effects on offspring due to the early life of the parent are well studied from the standpoint of the maternal environment, but intergenerational effects owing to the paternal early life environment are often overlooked. Nonetheless, recent laboratory studies in mammals and ecologically relevant studies in invertebrates predict that paternal effects can have a major impact on the offspring's phenotype. These non‐genetic, environment‐dependent paternal effects provide a mechanism for fathers to transmit environmental information to their offspring, and could allow rapid adaptation. We used the bank vole Myodes glareolus, a wild rodent species with no paternal care, to test the hypothesis that a high population density environment in the early life of fathers can affect traits associated with offspring fitness. We show that the protein content in the diet and/or social environment experienced during the father's early life (prenatal and weaning) influence the phenotype and survival of his offspring and may indicate adaptation to density‐dependent costs. Furthermore, we show that experiencing multiple environmental factors during the paternal early life can lead to a different outcome on the offspring phenotype than stimulated by experience of a single environmental factor, highlighting the need to study developmental experiences in tandem rather than independent of each other. peerReviewed

The early life environment can have profound, long‐lasting effects on an individual's fitness. For example, early life quality might (a) positively associate with fitness (a silver spoon effect), (b) stimulate a predictive adaptive response (by adjusting the phenotype to the quality of the environment to maximize fitness) or (c) be obscured by subsequent plasticity. Potentially, the effects of the early life environment can persist beyond one generation, though the intergenerational plasticity on fitness traits of a subsequent generation is unclear. To study both intra‐ and intergenerational effects of the early life environment, we exposed a first generation of bank voles to two early life stimuli (variation in food and social environment) in a controlled environment. To assess possible intra‐generational effects, the reproductive success of female individuals was investigated by placing them in large outdoor enclosures in two different, ecologically relevant environments (population densities). Resulting offspring were raised in the same population densities where they were conceived and their growth was recorded. When adult, half of the offspring were transferred to opposite population densities to evaluate their winter survival, a crucial fitness trait for bank voles. Our setup allowed us to assess: (a) do early life population density cues elicit an intra‐generational adaptive response, that is a higher reproductive success when the density matches the early life cues and (b) can early life stimuli of one generation elicit an intergenerational adaptive response in their offspring, that is a higher growth and winter survival when the density matches the early life cues of their mother. Our results show that the early life environment directly affects the phenotype and reproductive success of the focal generation, but adaptive responses are only evident in the offspring. Growth of the offspring is maintained only when the environment matches their mother's early life environment. Furthermore, winter survival of offspring also tended to be higher in high population densities if their mothers experienced an competitive early life. These results show that the early life environment can contribute to maintain high fitness in challenging environments, but not necessarily in the generation experiencing the early life cues. peerReviewed

International audience; 1. Variation in dominance status determines male mating and reproductive success, butnatural selection for male dominance can be detrimental or antagonistic for female per-formance, and ultimately their fitness. Attaining and maintaining a high dominance statusin a population of competing individuals is physiologically costly for males. But how maledominance status is mediated by maintenance energetics is currently not well under-stood, nor are the corresponding effects of male energetics on his sisters recognized.2. We conducted laboratory and field experiments on rodent populations to testwhether selective breeding for male dominance status (dominant vs. subordinatebreeding lines) antagonistically affected basal metabolic rate (BMR) and fitness offemales under wild conditions.3. Our results showed elevated BMR in females, but not in males, from the dominantbreeding line. However, phenotypically dominant males from the subordinatebreeding line had the highest BMR.4. Males from the dominant line with low BMR sired the most litters and offspring inthe field. Similarly, females from the dominant selection line tended to have moreoffspring if they had lower BMR, while the opposite trend was found in femalesfrom the subordinate selection line. Females with high and low BMR reproducedmost often, as indicated by a significant quadratic selection gradient.5. The increased female BMR resulting from selection for male dominance suggestsgenetic incompatibility between sexes in metabolism inheritance. Elevated BMRin behaviourally dominant males, but not in males from the dominant breedingline, suggests physiological costs in males not genetically suited for dominance.6. Fitness costs of elevated maintenance costs (measured as BMR) shown here sup-port the energetic compensation hypothesis where high BMR is selected againstas it would trade off energy required for other important life-history attributes.

Supplementary material from "Borrelia afzelii alters reproductive success in a rodent host". Detailed material and methods, supplementary tables, figures and results.

Variation in dominance status determines male mating and reproductive success, but natural selection for male dominance can be detrimental or antagonistic for female performance, and ultimately their fitness. Attaining and maintaining a high dominance status in a population of competing individuals is physiologically costly for males. But how male dominance status is mediated by maintenance energetics is currently not well understood, nor are the corresponding effects of male energetics on his sisters recognized. We conducted laboratory and field experiments on rodent populations to test whether selective breeding for male dominance status (dominant vs. subordinate breeding lines) antagonistically affected basal metabolic rate (BMR) and fitness of females under wild conditions. Our results showed elevated BMR in females, but not in males, from the dominant breeding line. However, phenotypically dominant males from the subordinate breeding line had the highest BMR. Males from the dominant line with low BMR sired the most litters and offspring in the field. Similarly, females from the dominant selection line tended to have more offspring if they had lower BMR, while the opposite trend was found in females from the subordinate selection line. Females with high and low BMR reproduced most often, as indicated by a significant quadratic selection gradient. The increased female BMR resulting from selection for male dominance suggests genetic incompatibility between sexes in metabolism inheritance. Elevated BMR in behaviourally dominant males, but not in males from the dominant breeding line, suggests physiological costs in males not genetically suited for dominance. Fitness costs of elevated maintenance costs (measured as BMR) shown here support the energetic compensation hypothesis where high BMR is selected against as it would trade off energy required for other important life‐history attributes. peerReviewed

Average monthly saturation deficit and temperature during the monitoring years. Figure S2. Observed mean abundance of ticks in vegetation per session, from May 2012 to October 2015. Figure S3. Mean number of vole captured per trap-night at each session and in each site, from May 2012 to October 2015. Table S1. Selection table for models explaining the abundance of ticks questing in the vegetation. Figure S4. Predicted number of larvae, nymphs and pooled nymphs and females per 100Â m2 of vegetation explained by bank vole abundance. Table S2. Selection table for models explaining the abundance of ticks questing in the vegetation. Table S3. Total number of ticks (per species and stage) collected on voles. Figure S5. Vole infestation per session with I. ricinus larvae and nymphs from May 2012 to October 2015. Table S4. Selection table for models explaining the abundance of infesting larvae. Table S5. Selection table for models explaining the abundance of infesting nymphs. Table S6. Additional model for the abundance of nymphs questing in the vegetation. Table S7. Selection table for the additional model explaining the abundance of questing nymphs. (DOCX 3706 kb)

Details of model selection procedure (Supplementary Tables 1, 2) and analyses of effect of microsatellite allele length on litter size (Supplementary Methods, Results, Supplementary Table 3).

Killing conspecific infants (infanticide) is among the most puzzling phenomena in nature. Stable polymorphism in such behaviour could be maintained by negative frequency-dependent selection (benefit of rare types). However, it is currently unknown whether there is genetic polymorphism in infanticidal behaviour or whether infanticide may have any fitness advantages when rare. Here we show genetic polymorphism in non-parental infanticide. Our novel invasion experiment confirms negative frequency-dependent selection in wild bank vole populations, where resource benefits allow an infanticidal strategy to invade a population of non-infanticidal individuals. The results show that infanticidal behaviour is highly heritable with genetic correlation across the sexes. Thus, a positive correlative response in male behaviour is expected when selection operates on females only and vice versa. Our results, on one hand, demonstrate potential benefits of infanticide, and on the other, they open a new perspective of correlative evolution of infanticide in females and males. peerReviewed

Conspicuous cyclic changes in population density characterize many populations of small northern rodents. The extreme crashes in individual number are expected to reduce the amount of genetic variation within a population during the crash phases of the population cycle. By long-term monitoring of a bank vole (Myodes glareolus) population we show that despite the substantial and repetitive crashes in the population size, high heterozygosity is maintained throughout the population cycle. The striking population density fluctuation in fact only slightly reduced the allelic richness of the population during the crash phases. Effective population sizes of vole populations remained also relatively high even during the crash phases. The peak phases are characterized by both a change in spatial pattern of individuals and a rapid accession of new alleles. peerReviewed