Effects of birth date and natal dispersal on faecal glucocorticoid concentrations in juvenile Common hamsters

In seasonally breeding animals, timing of reproduction in females can influence offspring development and survival. Temporal and energetic constraints are often more pronounced in juveniles born late in the season, and could activate the stress axis. Common hamsters (Cricetus cricetus) are hibernating rodents, and adult females produce up to three litters during the active season. Birth dates range from May to September, and shortly after natal emergence pups are weaned and start to disperse. In this study, we used faecal cortisol metabolites (FCM) as a non-invasive measure of recent glucocorticoid exposure. We compared FCM levels between early- (June/July) and late- (August/September) born juvenile Common hamsters during their first weeks after natal emergence. We further compared FCM levels in juveniles born in the study area and individuals that immigrated either early or late in the season. Additionally, we investigated potential effects of human disturbances on cortisol secretion patterns. FCM levels in late-born juveniles increased during the first weeks post-emergence and were significantly higher than those in early-born individuals during the post-weaning period. Late-immigrating juveniles had significantly higher FCM levels than early immigrants and residents during the same time period. Individuals that inhabited areas frequently used by humans had higher FCM levels than those in low-impact areas. These results indicate that the seasonal timing of birth and dispersal affect cortisol secretion patterns in juvenile Common hamsters. As all juveniles immerged into their hibernacula during early October, we assume that late-born and late-immigrated individuals have less time to prepare for hibernation. This could elevate energetic demands and increase exposure to predators and/or humans during food caching. These factors might lead to increased adrenal activity and thus, elevated FCM concentrations in these individuals. Potential consequences of these time constraints and associated physiological effects could be reflected in the lower overwinter survival rates of juveniles born late in the season.