2025 HALDANE PRIZE SHORTLIST: Nicole Walasek discusses her paper “The evolution of sensitive periods beyond early ontogeny: Bridging theory and data“, which has been shortlisted for Functional Ecology’s 2025 Haldane Prize for Early Career Researchers.
About the paper
From bacteria to humans, all organisms tailor their traits to environmental demands. Such phenotypic plasticity tends to be highest early in life, when experiences can have a large and long-lasting impact on behaviour, physiology and morphology. For example, exposure to early-life adversity can durably shape behavioural development in children, while early visual experience shapes how the brain processes visual information across humans and other animals. Yet, heightened plasticity is not limited to early development. Many species also have sensitive periods later in life, such as during adolescence. During these windows, the impact of experience is large and long-lasting relative to other life stages. For example, guinea pigs housed in isolation throughout adolescence show higher levels of aggression as adults than those raised in isolation as juveniles. Similar patterns occur across diverse taxa, from invertebrates to primates. Despite growing interest across developmental biology, animal behavior and neuroscience—we still know surprisingly little about the evolutionary selection pressures that influence the timing and duration of later sensitive periods.
In our paper, we synthesize mathematical models and empirical research to gain insight into the evolutionary origins of later sensitive periods. Across models, organisms’ uncertainty about their environment emerges as a common mechanism. Sensitive periods arise when uncertainty increases, either because the environment fluctuates or because the reliability of environmental cues—observations that provide information about current or future states of the environment—changes over time. Comparing mathematical theory with empirical data, we find that sensitive periods in adolescence are common for social traits and occur predominantly in mammals. Based on this finding, we propose that mammals may have evolved to use information gained from peers during adolescence to reduce uncertainty about their social environment, such as their social status or mating opportunities. Our synthesis generates novel hypotheses about the conditions that favour sensitive periods later in life, highlighting key directions for future research. Testing these predictions across species and traits are important next steps. Identifying when organisms are most sensitive to their environment may also inform developmental interventions and conservation strategies. For example, understanding whether positive later-life experiences can offset early-life adversity remains a central question across biology, psychology, and public health, while conservation efforts depend on how the timing of plasticity interacts with life history and population dynamics. By providing a theoretical framework for later sensitive periods, our work helps integrate research across these disciplines.
Plasticity tracks uncertainty across ontogeny. The height of the triangles indicates the level of plasticity; its shading the level of uncertainty; with darker shades indicating more uncertainty. Panel (A) shows that sensitive periods can occur mid-ontogeny if organisms become more uncertain about their phenotype-environment fit across early ontogeny. For example, rodents may have evolved to expect a safe environment (sun) and become more uncertain when they sample contradicting cues (thunder). Panel (B) shows that sensitive periods can also occur mid-ontogeny when cues (upward pointing arrows) become more reliable towards that time period, enabling organisms to reduce their uncertainty; with larger arrows indicating higher cue reliability. As an example, rodents might sample reliable cues from peers about their mate value. Panel (C) illustrates that sensitive periods towards the end of ontogeny can occur if changing environmental conditions (black, jiggly line) induce uncertainty about the adult environment. In such conditions, organisms might rely on the most recent information at the end of ontogeny to reduce their uncertainty and adjust their phenotypes. For example, rodents may exhibit heightened plasticity for social behaviors near adulthood because the population’s sex ratio changes within their lifetimes (Credit: Walasek et al. 2024)
Plasticity tracks uncertainty across ontogeny (adjusted from Walasek et al., 2025). The height of the triangles indicates the level of plasticity; its shading the level of uncertainty; with darker shades indicating more uncertainty. Panel (A) shows that sensitive periods can occur mid-ontogeny if organisms become more uncertain about their phenotype-environment fit across early development or cues (upward pointing arrows) become more informative, enabling organisms to reduce uncertainty. As an example, rodents might sample reliable cues from peers about their mate value. Panel (B) illustrates that sensitive periods towards the end of ontogeny can occur if changing environmental conditions (black, jiggly line) induce uncertainty about the adult environment. In such conditions, organisms might rely on the most recent information at the end of ontogeny to reduce their uncertainty and adjust their phenotypes.
About the author
I have taken an interdisciplinary route towards ecology. Between my bachelor’s studies and my early postdoctoral experiences, I moved between different disciplines, including cognitive science, computer science, artificial intelligence, psychology, and biology. As I experienced these diverse disciplines, I became increasingly interested in how environmental changes shape the evolution of developmental processes.
Much of my work combines mathematical and statistical modelling with empirical data from humans and other animals to study how development responds to environmental uncertainty. Recently, I was awarded a Veni grant, which supports three years of independent research and allows me the unique opportunity to empirically test predictions about the evolution of sensitive periods. I will conduct this research at the Institute for Biodiversity and Ecosystem Dynamics where I currently work as a postdoctoral researcher. Building on the ideas in this paper, I will use experimental evolution in invertebrates to test whether changing environments favour the evolution of later sensitive periods.
If I could offer one piece of advice to early-career researchers, it would be to embrace interdisciplinary curiosity. Working across fields can feel uncertain at times, but it also creates opportunities to ask new questions and integrate perspectives into something greater than the sum of their parts.
Read the full list of articles shortlisted for the 2024 Haldane Prize here.
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