Litter deficits of socially stressed and low status hamster dams

Maternal bereavement during pregnancy and the risk of stillbirth: a nationwide cohort study in Sweden

Increasing evidence suggests that maternal stress during pregnancy may influence pregnancy outcomes. In a nationwide Swedish study including almost 3 million births taking place during 1973-2006, we investigated whether maternal bereavement during pregnancy is associated with stillbirth risk. Through individual record linkage between several population-based registers, we obtained information on demographic, health-related, and pregnancy-related factors and deaths of mothers' first-degree relatives. There were 11,071 stillbirths (3.8 per 1,000 births) in the cohort. After adjustment for potential confounders, infants of mothers who had lost any first-degree relative the year before or during pregnancy had an 18% higher risk of stillbirth than unexposed offspring (95% confidence interval (CI): 1.06, 1.31). Corresponding hazard ratios were 1.67 (95% CI: 1.18, 2.36) for maternal loss of an older child, 2.06 (95% CI: 1.44, 2.94) for loss of a sibling, and 1.07 (95% CI: 0.95, 1.21) for loss of a parent. The relationship between maternal bereavement and stillbirth did not vary by time of death or by whether the relative's death was expected or unexpected. Death of a close relative is one of the most severe sources of stress, and future studies need to investigate whether less severe but more common stressors also increase stillbirth risk.

Potential mechanisms for sex ratio adjustment in mammals and birds

Sex ratio skews in relation to a variety of environmental or parental conditions have frequently been reported among mammals and, though less commonly, among birds. However, the adaptive significance of such sex ratio variation remains unclear. This has, in part, been attributed to the absence of a low-cost physiological mechanism for sex ratio manipulation by the parent. It is shown here that several recent findings in reproductive biology are suggestive of many potential pathways by which gonadotropins and steroid hormones could interfere with the sex ratio at birth. And these hormone levels are well-known to be influenced by many parameters which have been invoked in correlating with offspring sex ratios. Hence, it is argued that the significant, but inconsistent sex ratio biases reported in mammalian and avian populations are coherent with current knowledge on reproductive physiology in those species. However, whether such variations can be viewed at as a consequence of physiological constraint or as adaptive sex ratio adjustment, has still to be determined.

Sex-specific embryonic mortality during concurrent pregnancy and lactation in house mice

To evaluate the importance of lactational stress for sex ratio manipulation in postpartum inseminated house mice, 163 sexually inexperienced male and female BALB/c ABom inbred mice were mated and stud males were removed the day following the initial parturition. Randomly chosen dams were allowed to suckle their young for either one or 14 or 21 days. The results showed that two weeks of lactation enhance the incidence of total abortion of the litter conceived postpartum. If lactation continued another week, dams which gave birth to a second litter lost relatively more embryos during uterine development and the percentage of lost embryos was increased by the number of pups suckled. Among these dams, the number of embryos lost in utero correlated positively with viable litter sex ratios (percentage male pups) at second parturition, indicating that lactational stress enhances embryonic mortality and that this additional mortality predominantly affects female embryos.

Social dominance and reproductive success in pregnant and lactating golden hamsters (Mesocricetus auratus) under seminatural conditions

Pairs of female hamsters were acclimated to seminatural enclosures and permitted to interact for brief periods on days 12-14 of gestation or on days 2-4 of lactation. In one experimental paradigm, food was provided in excess of the daily requirements while in a second, food rations were restricted. Control females were similarly housed and fed but were not exposed to another animal. Paired females readily established dominant-subordinate relationships. Dominant females successfully removed food from the subordinate females and, as a result, accumulated larger food hoards. Subordinate females gave birth to fewer pups than either dominant or subordinate animals when social interactions occurred late in pregnancy and maintained smaller litters when interactions occurred during lactation. Reductions in the fecundity of subordinate females were more pronounced when the food ration was restricted. Under ad lib feeding conditions dominant females also maintained fewer young than control animals, but when food was restricted they were more successful than the control group. Subordinate females successfully defended their burrows against intrusions by dominant animals; maternal cannibalism accounted for most reductions in litter size. These results suggest that a defensible burrow and the acquisition of a food hoard are essential for successful reproduction in this species.

Maternal social stress disrupts reproduction of hamsters drinking high-calorie fluids

Adding maternal social stress, induced by housing in pairs, to maternal consumption of high-calorie fluids adversely affected hamster reproduction. Pregnancy outcomes of 67 dams housed either alone or in pairs, and given either water (WAT), or isocaloric solutions of either 30% ethanol (ETH), sucrose (SUC) or propylene glycol (PG), were compared. Pups of unpaired dams drinking any high-calorie fluid weighed less than those drinking water, but no other deficits were found. Among paired dams, those designated submissive on the basis of agonistic behavior and drinking either SUC, ETH, or PG, had fewer viable pups and more fetal resorptions and stillborn pups than WAT dams. Also, dominant PG dams delivered few pups and both dominant and submissive PG dams delivered malformed pups or embryos. Whether paired or unpaired, litters of SUC and ETH dams were similar. Drinking any high-calorie fluid reduced maternal food and fluid intake and weight gains below that of water, but pairing had no additional effect on dams.