Understanding how offspring development is affected by the maternal environmental is crucial in ecology and evolution, because this environment directly impacts offspring survival and reproduction. Embedded in the concept of parental care, maternal effects are defined as the influence of the mother’s phenotype on the offspring phenotype, beyond direct genetic transmission. In a changing environment, plasticity in maternal traits is one way maternal effects can be investigated, to better understand females’ capacity to adjust to rapid changes. The extended period of maternal care in mammals can provide important insights on how mothers differ and affect their offspring growth and survival. In northern and temperate environments, birthdates have to coincide with the peak in resource availability, to ensure proper offspring growth. As part of my PhD project, I explore how individual female bighorn sheep (Ovis canadensis) adjust their timing of lambing dates to changing environmental conditions, in relation to yearly climatic and meteorological variation, and maternal characteristics. In addition, I investigate how maternal care differs between individual mothers, and what are the consequences on offspring growth. I specifically study lactation, and milk samples are analysed for their fat, lactose, protein and mineral content, in conjunction with maternal characteristics. Lastly, I investigate potential differences among mothers in patterns of neonatal mortality. Close monitoring of marked bighorn ewes allows the determination of their lactation status and thus, their reproductive success. Analyses of maternal characteristics and ecological factors such as predation could potentially bring insights into early-life survival of bighorn neonates, which entirely depend on their mother’s nourishment and protection for optimal early development. To answer my questions, I use long-term data from a bighorn sheep population at Ram Mountain, Alberta, where all individuals have been marked and followed from birth to death. Monitoring of the population started in 1971. Pedigree and mother-young associations are all known, through molecular techniques and behavioural observations; all lambs are marked in their first summer.