Intergenerational fitness effects of the early life environment in a wild rodent

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.

Nutraceuticals in the Prevention of Neonatal Hypoxia-Ischemia: A Comprehensive Review of their Neuroprotective Properties, Mechanisms of Action and Future Directions

Neonatal hypoxia–ischemia (HI) is a brain injury caused by oxygen deprivation to the brain due to birth asphyxia or reduced cerebral blood perfusion, and it often leads to lifelong limiting sequelae such as cerebral palsy, seizures, or mental retardation. HI remains one of the leading causes of neonatal mortality and morbidity worldwide, and current therapies are limited. Hypothermia has been successful in reducing mortality and some disabilities, but it is only applied to a subset of newborns that meet strict inclusion criteria. Given the unpredictable nature of the obstetric complications that contribute to neonatal HI, prophylactic treatments that prevent, rather than rescue, HI brain injury are emerging as a therapeutic alternative. Nutraceuticals are natural compounds present in the diet or used as dietary supplements that have antioxidant, anti-inflammatory, or antiapoptotic properties. This review summarizes the preclinical in vivo studies, mostly conducted on rodent models, that have investigated the neuroprotective properties of nutraceuticals in preventing and reducing HI-induced brain damage and cognitive impairments. The natural products reviewed include polyphenols, omega-3 fatty acids, vitamins, plant-derived compounds (tanshinones, sulforaphane, and capsaicin), and endogenous compounds (melatonin, carnitine, creatine, and lactate). These nutraceuticals were administered before the damage occurred, either to the mothers as a dietary supplement during pregnancy and/or lactation or to the pups prior to HI induction. To date, very few of these nutritional interventions have been investigated in humans, but we refer to those that have been successful in reducing ischemic stroke in adults. Overall, there is a robust body of preclinical evidence that supports the neuroprotective properties of nutraceuticals, and these may represent a safe and inexpensive nutritional strategy for the prevention of neonatal HI encephalopathy.

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.

A mechanism for population self-regulation: Social density suppresses GnRH expression and reduces reproductivity in voles

Nearly 100 years ago, Charles Elton described lemming and vole population cycles as ecological models for understanding population regulation in nature. Yet, the mechanisms driving these cycles are still not fully understood. These rodent populations can continue to cycle in the absence of predation and with food supplementation, and represent a major unsolved problem in population ecology. It has been hypothesized that the social environment at high population density can drive selection for a low-reproduction phenotype, resulting in population self-regulation as an intrinsic mechanism driving the cycles. However, a physiological mechanism for this self-regulation has not been demonstrated. We manipulated population density in wild meadow voles Microtus pennsylvanicus using large-scale field enclosures over 3 years and examined reproductive performance and physiology. Within the field enclosures, we assessed the proportion of breeding animals, mass at sexual maturation, and faecal androgen and oestrogen metabolites. We then collected brain tissue from juvenile voles born at high or low density, quantified mRNA expression of gonadotropin-releasing hormone (GnRH) and oestrogen receptor alpha (ERα) and measured DNA methylation at six CpG sites in a region that was highly conserved with the mouse GnRH promoter. At high density, there was a lower proportion of reproductive animals. Juvenile voles born at high densities had reduced expression of GnRH in the hypothalamus, accompanied by marginally lower faecal sex hormone metabolites. Female juvenile voles born at high density also had higher methylation levels at two CpG sites while males did not, aligning with prior observations that females (but not males) from high-density environments retain reduced reproduction long term. Our results support a physiological basis for population self-regulation in vole cycles, as altering population density alone induced reproductive downregulation at the hypothalamic level. Our results demonstrate that altering the early-life social environment can fundamentally impact reproductive function in the brain. This, in turn, can drive population demography changes in wild animals.

Inter- and intrasex habitat partitioning in the highly dimorphic southern elephant seal

Partitioning resources is a key mechanism for avoiding intraspecific competition and maximizing individual energy gain. However, in sexually dimorphic species it is difficult to discern if partitioning is due to competition or the different resource needs of morphologically distinct individuals. In the highly dimorphic southern elephant seal, there are intersexual differences in habitat use; at Iles Kerguelen, males predominantly use shelf waters, while females use deeper oceanic waters. There are equally marked intrasexual differences, with some males using the nearby Kerguelen Plateau, and others using the much more distant Antarctic continental shelf (~2,000 km away). We used this combination of inter and intrasexual behavior to test two hypotheses regarding habitat partitioning in highly dimorphic species. (a) that intersexual differences in habitat use will not appear until the seals diverge in body size and (b) that some habitats have higher rates of energy return than others. In particular, that the Antarctic shelf would provide higher energy returns than the Kerguelen Shelf, to offset the greater cost of travel. We quantified the habitat use of 187 southern elephant seals (102 adult females and 85 subadult males). The seals in the two groups were the same size (~2.4 m) removing the confounding effect of body size. We found that the intersexual differences in habitat use existed before the divergence in body size. Also, we found that the amount of energy gained was the same in all of the major habitats. This suggests that the use of shelf habitats by males is innate, and a trade-off between the need to access the large benthic prey available on shelf waters, against the higher risk of predation there. Intrasexual differences in habitat use are another trade-off; although there are fewer predators on the Antarctic shelf, it is subject to considerable interannual fluctuations in sea-ice extent. In contrast, the Kerguelen Plateau presents more consistent foraging opportunities, but contains higher levels of predation. Habitat partitioning in this highly dimorphic species is therefore the result of complex interplay of life history strategies, environmental conditions and predation pressure.

Two hundred and fifty-four metagenome-assembled bacterial genomes from the bank vole gut microbiota

Vertebrate gut microbiota provide many essential services to their host. To better understand the diversity of such services provided by gut microbiota in wild rodents, we assembled metagenome shotgun sequence data from a small mammal, the bank vole Myodes glareolus (Rodentia, Cricetidae). We were able to identify 254 metagenome assembled genomes (MAGs) that were at least 50% (n = 133 MAGs), 80% (n = 77 MAGs) or 95% (n = 44 MAGs) complete. As typical for a rodent gut microbiota, these MAGs are dominated by taxa assigned to the phyla Bacteroidetes (n = 132 MAGs) and Firmicutes (n = 80), with some Spirochaetes (n = 15) and Proteobacteria (n = 11). Based on coverage over contigs, Bacteroidetes were estimated to be most abundant group, followed by Firmicutes, Spirochaetes and Proteobacteria. These draft bacterial genomes can be used freely to determine the likely functions of gut microbiota community composition in wild rodents.

Measurement(s)	genome • DNA • gut microbiome measurement • metagenomic data
Technology Type(s)	DNA sequencing
Sample Characteristic - Organism	Bacteroidetes • Firmicutes • Spirochaetes • Proteobacteria • Myodes glareolus

Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12936797

The relevance of a rodent cohort in the Consortium on Individual Development

One of the features of the Consortium on Individual Development is the existence of a rodent cohort, in parallel with the human cohorts. Here we give an overview of the current status. We first elaborate on the choice of rat and mouse models mimicking early life adverse or beneficial conditions during development. We performed a systematic literature search on early life adversity and adult social behavior to address the status quo. Next, we describe the behavioral tasks we used and designed to examine behavioral control and social competence in rodents. The results so far indicate that manipulation of the environment in the first postnatal week only subtly affects social behavior. Stronger effects were seen in the model that targeted early adolescence; once adult, these rats are characterized by increased attention, a higher degree of impulsiveness and reduced social interest in peers. Many experiments in our rodent models with tightly controlled conditions were inspired by findings in human cohorts, and now allow in-depth mechanistic investigations. Vice versa, some of the findings in rodents are currently followed up by dedicated investigations in the human cohorts. This exemplifies the added value of animal investigations in a consortium encompassing primarily human developmental cohorts.