Lucas Invernizzi (he/him) provides the story behind his paper, “The Expensive Son hypothesis“, which was shortlisted for this year’s Elton Prize. We also hear a little about his journey into animal ecology

About the paper:

What is your shortlisted paper about, and what are you seeking to answer with your research? 

In many species, males and females differ in size and developmental needs. Our paper formalises the “Expensive Son hypothesis” (ESH) as a unifying conceptual framework. We argue that in sexually dimorphic species, producing the larger sex (generally males) entails higher fitness costs for the mother, and we pointed out factors neglected so far. I wanted to highlight a major gap in the literature: while we know why mothers bias offspring sex ratios (evolutionary causes), we rarely quantify the evolutionary consequences of rearing those expensive sons.

Were you surprised by anything when working on it? Did you have any challenges to overcome?

I was genuinely surprised by the sheer number of different terms used across disciplines to describe the exact same phenomenon. Harmonising this fragmented, impractical vocabulary into a cohesive hypothesis was tough but necessary to help future research.

What is the next step in this field going to be? 

The next frontier is uncovering the behavioural and physiological mechanisms driving these fitness costs. Specifically, we need to understand the bridge between the higher energy cost of producing the larger sex and the maternal fitness consequences.

What are the broader impacts or implications of your research for policy or practice? 

I believe this framework also has crucial implications for conservation biology; as climate change and resource scarcity intensify life-history trade-offs, the cost of having a son will become increasingly burdensome for wild mothers, making it vital to consider offspring sex to refine demographic model predictions.

About the author:

How did you get involved in ecology?

My fascination with biology began with high school, but it was during my undergraduate studies that I realised ecology and evolution were the disciplines that captivated me the most. During my master’s degree, I was still unsure whether I wanted to pursue a PhD. Ultimately, it was my research internship that convinced me to take the leap to continue, and I couldn’t be happier with that decision today.

What is your current position?

I have recently completed my PhD in November 2025 at the Laboratoire de Biologie et Biométrie Evolutive (LBBE) in Lyon (France) and am currently transitioning into my postdoctoral research phase, actively securing funding to expand my research into other animal models.

Have you continued the research your paper is about?

Absolutely! After formalising the Expensive Son hypothesis framework, I empirically tested its predictions in a pre-industrial human population during my thesis. Now, I am preparing to test his framework in other model species, the purpose being to highlight the universality of the Expensive Son hypothesis.

What one piece of advice would you give to someone in your field? 

If I could give one piece of advice to fellow early-career researchers, it would be to not be afraid to embrace interdisciplinary approaches! When you tackle a problem through different spectrums of vision, such as combining evolutionary demography, physiology, and behavioural ecology you uncover blind spots you didn’t even know you had. Different disciplines bring different perspectives, opinions, and tools to the table, which is essential to truly understand complex biological mechanisms.