Thesis defence Lucas Invernizzi

Jury composition:

Dominique Pontier - PU, UCBL - Examiner

Michel Raymond - DR, CNRS - Reviewer 

Virpi Lummaa - PU, University of Turku - Reviewer

Sarah Benhaiem - CR, Leibniz-IZW - Examiner

Mathieu Douhard - CR, CNRS - Thesis director

Jean-François Lemaître - DR, CNRS - Thesis co-director

Fanie Pelletier - PU, University of Sherbrooke, Guest

Patrick Bergeron - PU, Bishop's University, Guest

Thesis summary:

According to the principle of allocation, the energy acquired by individuals must be distributed among different vital functions: reproduction, growth, and maintenance. The energy invested in one function cannot be invested in another, generating negative relationships between the traits involved, which are evolutionary trade-offs. Thus, when an individual reproduces, it devotes most of its energy to reproduction at the expense of somatic maintenance. This leads to evolutionary trade-offs called reproductive costs. This topic has attracted considerable interest in evolutionary biology, but one important factor still remains largely neglected: the offspring sex. In sexually dimorphic species, the production and rearing of the larger sex (generally males) require more energy from the mother. It is therefore expected that mothers of sons suffer from higher reproductive costs. The primary objective of this thesis was to formalise this hypothesis, known as “the Expensive Son hypothesis”, and then to test its predictions using longitudinal data from the human pre-industrial population of Nouvelle-France (Québec). I have examined short-term reproductive costs, i.e., those occurring at a single reproductive event, by investigating whether having a son delays the next birth and increases the maternal mortality risk (up to 42 days after childbirth). Then, I have focused on long-term reproductive costs by investigating whether having majority sons leads to a shorter lifespan or an earlier and stronger reproductive ageing later in life in mothers. I conclude this thesis by discussing my research in a broader evolutionary context and proposing new research perspectives within the framework of the Expensive Son hypothesis.