Altitudinal variation in bumble bee (Bombus) critical thermal limits

Area of Science:

• Ecology
• Physiology
• Climate Change Biology

Background:

• Organismal thermal limits influence geographic distribution and predict range shifts due to climate change.
• Latitudinal gradients in thermal tolerance are known, but altitudinal variations remain understudied.
• Understanding thermal limits is crucial for predicting species' responses to environmental temperature changes.

Purpose of the Study:

• To investigate altitudinal variation in critical thermal limits (CTmin, CTmax, Trec) in bumble bees.
• To determine if thermal tolerance in bumble bees correlates with environmental temperatures across altitudes.
• To provide insights into bumble bee adaptation to thermal extremes in different elevations.

Main Methods:

• Measured critical thermal minimum (CTmin), critical thermal maximum (CTmax), and recovery temperature (Trec) in three bumble bee species.
• Used thermal ramps and tracked righting response to assess thermal tolerance.
• Collected bees from distinct altitudes (2180m and 3290m) in Wyoming, USA.

Main Results:

• Larger body size correlated with tolerance to more extreme temperatures due to thermal inertia.
• High-altitude bumble bees exhibited lower CTmin (~1°C colder) and Trec (~3-4°C colder) compared to low-altitude bees.
• Low-altitude bumble bees tolerated hotter temperatures (~5°C hotter) than high-altitude bees.

Conclusions:

• Altitudinal differences in bumble bee thermal tolerance parallel local environmental temperature gradients.
• This study provides novel data on organismal thermal tolerance across altitudes.
• Demonstrates geographical variation in tolerance to temperature extremes in heterothermic bumble bees.