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When to start reproducing is a key question for any species since it can impact survival, total reproduction over a lifetime (number of offspring), longevity, but also population dynamics and generation time. Reproduction is a process that requires a lot of energy and resources. Breeding early has been shown to decrease survival and/or future fecundity. However, postponing reproduction means lost in opportunities to reproduce and increases the chances of dying without producing any offspring. Then, a trade-off in age at first reproduction arises, with large variation observed even within population of certain species. In addition, behavioural interactions can also affect age at first reproduction. For instance, reproduction can be delayed in subordinates. I plan to investigate the causes and consequences of age at first reproduction in yellow-bellied marmots. This hibernator can start to reproduce before reaching adult size, which is around 4 years of age. It also reproduces only once per year, shows wide variation in age at first reproduction (ranging from 2 to 7 years), and exhibits lot of variation in its social organisation. Indeed, females can be solitary or be part of a group of two to several dozens of individuals. I will use behavioural, reproductive, life-history and pedigree data obtained from a long-term study of individually marked marmots for over 60 years. Maximizing the richness of the data, I will also determine the impact of age of first reproduction of the mother on her offspring's survival and reproduction. This work will therefore provide new insights on the biological factors regarding an important life-history parameter.