Early life of fathers affects offspring fitness in a wild rodent

Stress in the social context: a behavioural and eco-evolutionary perspective

The social environment is one of the primary sources of challenging stimuli that can induce a stress response in animals. It comprises both short-term and stable interactions among conspecifics (including unrelated individuals, mates, potential mates and kin). Social stress is of unique interest in the field of stress research because (1) the social domain is arguably the most complex and fluctuating component of an animal's environment; (2) stress is socially transmissible; and (3) stress can be buffered by social partners. Thus, social interactions can be both the cause and cure of stress. Here, we review the history of social stress research, and discuss social stressors and their effects on organisms across early life and adulthood. We also consider cross-generational effects. We discuss the physiological mechanisms underpinning social stressors and stress responses, as well as the potential adaptive value of responses to social stressors. Finally, we identify outstanding challenges in social stress research, and propose a framework for addressing these in future work.

Is chronic stress a causal mechanism for small mammal population cycles? Reconciling the evidence

Chronic stress has long been hypothesized to play a role in driving population cycles. Christian (1950) hypothesized that high population density results in chronic stress and mass "die-offs" in small mammal populations. Updated variations of this hypothesis propose that chronic stress at high population density may reduce fitness, reproduction, or program aspects of phenotype, driving population declines. We tested the effect of density on the stress axis in meadow voles (Microtus pennsylvanicus) by manipulating population density in field enclosures over three years. Using fecal corticosterone metabolites as a non-invasive measure of glucocorticoid (GC) concentrations, we found that density alone was not associated with GC differences. However, we found that the seasonal relationship of GC levels differed by density treatment, with high-density populations having elevated GC levels early in the breeding season and decreasing towards late summer. We additionally tested hippocampal glucocorticoid receptor and mineralocorticoid receptor gene expression in juvenile voles born at different densities, with the hypothesis that high density may reduce receptor expression, altering negative feedback of the stress axis. We found that females had marginally higher glucocorticoid receptor expression at high density, no effect in males, and no detectable effect of density on mineralocorticoid receptor expression in either sex. Hence, we found no evidence that high density directly impairs negative feedback in the hippocampus, but rather female offspring may be better equipped for negative feedback. We compare our findings with prior studies to attempt to disentangle the complicated relationship between density, seasonality, sex, reproduction and the stress axis.

The effects of Borrelia infection on its wintering rodent host

In seasonal environments, appropriate adaptations are crucial for organisms to maximize their fitness. For instance, in many species, the immune function has been noticed to decrease during winter, which is assumed to be an adaptation to the season's limited food availability. Consequences of an infection on the health and survival of the host organism could thus be more severe in winter than in summer. Here, we experimentally investigated the effect of a zoonotic, endemic pathogen, Borrelia afzelii infection on the survival and body condition in its host, the bank vole (Myodes glareolus), during late autumn-early winter under semi-natural field conditions in 11 large outdoor enclosures. To test the interaction of Borrelia infection and energetic condition, four populations received supplementary nutrition, while remaining seven populations exploited only natural food sources. Supplementary food during winter increased the body mass independent of the infection status, however, Borrelia afzelii infection did not cause severe increase in the host mortality or affect the host body condition in the late autumn-early winter. While our study suggests that no severe effects are caused by B. afzelii infection on bank vole, further studies are warranted to identify any potentially smaller effects the pathogen may cause on the host fitness over the period of whole winter.

Effect of maternal zinc deficiency on offspring health: The epigenetic impact

BACKGROUND

Zinc deficiency is associated with adverse effects on maternal health and pregnancy outcomes. These consequences have been reported over the years from zinc supplementation trials and observational studies whereby outcomes of maternal, foetal and infant health were measured. Owing to the importance of zinc in the functions of epigenetic enzymes, pre-clinical studies have shown that its deficiency could disrupt biological activities that involve epigenetic mechanisms in offspring. Thus, this review assessed the link between epigenetics and the effects of maternal zinc deficiency on the offspring's health in animal studies.

METHODS

Research articles were retrieved without date restriction from PubMed, Web of Science, ScienceDirect, and Google Scholar databases, as well as reference lists of relevant articles. The search terms used were "zinc deficiency", "maternal zinc deficiency", "epigenetics", and "offspring." Six studies met the eligibility criteria and were reviewed.

RESULTS

All the eligible studies reported maternal zinc deficiency and observed changes in epigenetic markers on the progeny during prenatal and postnatal stages of development. The main epigenetic markers reported were global and gene specific methylation and/ or acetylation. The epigenetic changes led to mortality, disruption in development, and risk of later life diseases.

CONCLUSION

Maternal zinc deficiency is associated with epigenetic modifications in offspring, which induce pathologies and increase the risk of later life diseases. More research and insight into the epigenetic mechanisms could spring up new approaches to combat the associated disease conditions.

Population cycles and outbreaks of small rodents: ten essential questions we still need to solve

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.