Effects of Population Density on Stress and Maternal Care in a Wild Rodent (Peromyscus maniculatus)
Laboratory studies have demonstrated that variation in maternal care is linked to differences of offspring stress axis dynamics. Offspring who receive a low level nursing face increased exposure to elevated CORT compared to high nursing offspring which, according to the CORT-fitness hypothesis, leads to a reduced fitness. There is a growing body of empirical data which suggests that this relationship between CORT and fitness may depend on the environmental conditions. In the wild, if a female has access to environmental cues, it may be advantageous for a female to program her offspring’s stress axis to match the environment they will experience upon independence. Here, I tested the effects of population density on maternal behaviour by bringing wild-caught Peromyscus maniculatus into captivity to measure their frequency of nursing while experimentally manipulating perceived density cues through the presence of soiled bedding. I predicted the mechanism for this behavioural response to environmental cues to be related to a change in maternal glucocorticoids. As population density increased over the breeding season the frequency of nursing decreased by approximately 27% however, olfactory cues alone do not experimentally alter nursing. We report a U-shaped relationship between population density and maternal stress levels that can be explained by a significant interaction between population density and time of breeding. This suggests that CORT is not the mechanistic link between maternal behaviour and population density but rather may be related to the level of maternal care the female received as a pup. Wild mice face this mismatch of environments where the density in which they were born can be very different from the density in which they breed. These complex relationships between natal and breeding environments might require complex systems for appropriately programming offspring for the current environmental conditions. Matching behavioural phenotypes to a changing environment could be the driver responsible for maintaining variation in maternal behaviour in a wild environment.