This blog post is provided by Kristoffer Wild and tells the #StoryBehindThePaper for the article “Thermal performance curves, activity and survival in a free-ranging ectotherm“, which was recently published in Journal of Animal Ecology. In this study on Australian central bearded dragons, Wild et al. challenge assumptions about locomotor performance and survival.
Most of us instinctively believe that faster is better. Think of Olympic sprinters or animals escaping predators. Surely speed offers a clear survival advantage? But our recent study on Australian central bearded dragons (Pogona vitticeps) flips this assumption on its head. In fact, being too speedy might actually be costly for these lizards in the wild.
A Fitbit for Lizards
To get at this question, we employed cutting-edge gear on these lizards, essentially equipping our dragons with tiny ‘Fitbits’. These small, high-tech devices, combine accelerometers and temperature-sensitive radio transmitters to allow us to measure body temperature and acceleration (a proxy for movement and performance) simultaneously, up to the very second! This intensive monitoring provided detailed insights into the daily lives of these lizards over the course of a year, tracking everything from their fine-scale activity to their maximum performance under natural conditions.
Thermoregulation: A Delicate Balancing Act
Ectotherms – or cold-blooded animals such as lizards – rely on external heat sources to regulate their body temperature. Maintaining optimal temperatures is crucial because it influences their ability to digest food, grow, avoid predators, and reproduce effectively. To achieve this, lizards will move strategically between sunny spots and shady retreats – a delicate balancing act known as behavioural thermoregulation.
Our research confirmed that these dragons were masters at adjusting their behaviour according to seasonal shifts. In spring and summer, when environmental conditions made it easier to maintain optimal temperatures, lizards achieved greater thermoregulatory precision, keeping their bodies within a narrow, ideal temperature range. In winter, when it was more costly (both energetically and in terms of predation risk) to achieve optimal body temperatures, their thermoregulation became less precise.
Sprinting Towards Danger?
The surprising twist came when we linked these thermoregulatory behaviours and thermal performance curves to their survival. Thermal performance curves are a way to visualise how an ectotherm’s ability to perform tasks, such as sprint speed or digestion, changes with changes in body temperature. A lizard will usually regulate its body temperature within a preferred range to optimise these tasks.
Contrary to expectations, higher locomotor performance in the field was associated with greater mortality risk. In other words, being the fastest lizard on the block wasn’t always beneficial. This effect was particularly pronounced in females, suggesting sex-specific trade-offs between performance and survival (Figure 1).
Why would being faster increase their chances of death? It’s possible that speedy individuals might engage in riskier behaviours. Moving around more openly and frequently and thereby becoming more visible and vulnerable to predators like raptors and mammals. Interestingly, this increased risk was especially pronounced during spring, the reproductive season, when predator activity was high, and movement behaviours were more conspicuous.
The Lab Versus The Wild
These findings highlight an important ecological reality: the fitness metrics we measure in controlled laboratory settings don’t always reflect real-world outcomes. Traditionally, researchers measure thermal performance in labs by recording maximum sprint speeds under ideal, predator-free conditions. In the wild, however, performance isn’t just about physiology—it’s deeply intertwined with behaviour, predation risk, and environmental variability. Running from point A to point B is one of the many decisions a lizard must make in the wild.
Our field-based approach, therefore, provides a more accurate picture of how physiological capabilities translate into actual fitness outcomes, demonstrating clearly that the fastest isn’t always the fittest in the case of the Australian central bearded dragon.
The Bigger Picture
Understanding these complex trade-offs is crucial, particularly in the context of climate change. As temperatures rise and environments become more variable, how will ectotherms like our bearded dragons adapt? Will they face even greater challenges balancing the costs and benefits of thermoregulation, potentially impacting their survival?
Moreover, these lizards face additional pressures from invasive predators, such as cats and foxes, which further complicate their ability to survive and reproduce (Figure 2). The presence of these introduced predators, which are not part of the lizards’ natural ecosystem, poses a significant threat, increasing the importance of understanding their ecology.
By combining innovative technology with rigorous ecological methods, our study sheds new light on the nuanced interplay between physiology, behaviour, and theory. It reminds us that the ecological contexts animals live in are just as important as their physiological capacities.
So, next time you cheer for the fastest sprinter, spare a thought for the bearded dragon – sometimes, slow and steady might actually win the race.
Read the article here