VTMaxBox : A reproducible practical tool for measuring voluntary thermal maximum in terrestrial ectotherms

Thermal traits are central to understanding how ectotherms respond to environmental warming. However, not all thermal traits describe the same biological process. Critical thermal maxima quantify physiological failure, whereas preferred temperatures describe thermal selection under choice conditions. Voluntary thermal maximum (VTMax) instead captures the body temperature at which an individual voluntarily withdraws from a progressively heated environment. Here, we treat VTMax as a behavioural upper thermal avoidance threshold, rather than as a direct proxy for physiological thermal limits or preferred temperature.

We present VTMaxBox, a low‐cost, modular, 3D‐printable practical tool designed to quantify voluntary upper thermal avoidance in terrestrial ectotherms. The system consists of a controlled heating chamber, an escape zone, thermocouples and continuous temperature recording. This design allows researchers to record body temperature at the moment of voluntary withdrawal while controlling heating rate and monitoring internal chamber conditions.

We describe the construction, assembly, calibration and scaling of the VTMaxBox and provide open‐source design files to facilitate replication and adaptation. We also outline a recommended experimental workflow, including animal acclimation, chamber stabilization, heating‐rate control, endpoint definition and data extraction. In doing so, we distinguish the hardware itself from the broader experimental protocol required to generate comparable VTMax estimates.

We further discuss the performance, limitations and appropriate applications of the VTMaxBox. The tool is best suited for terrestrial ectotherms large enough to carry or contact a thermocouple without major restriction of movement, and small enough to experience stable and homogeneous heating within the chamber. Its use requires careful standardization of chamber size, ramping rate, animal handling and ethical safeguards.

VTMaxBox offers an accessible and reproducible platform for incorporating behavioural thermal avoidance into comparative thermal ecology, conservation physiology and teaching. By complementing, rather than replacing, approaches based on critical thermal limits or preferred temperatures, it provides a practical route for studying how animals behaviourally respond to increasing heat exposure under controlled conditions.