DILLON, M.E.*; DANIEL, T.L.; HUEY, R.B.: Bumblebee Thermoregulation at High Altitude: it Ain’t as Bad as it Seems

Thermoregulation at high altitude is a challenge for flying insects. Achieving body temperatures warm enough for flight is made difficult by low air temperatures, and metabolic heat production is potentially constrained by low O₂ partial pressures. Nevertheless, bumblebees are remarkably abundant and adept fliers in high alpine ecosystems. To explore how bumblebees deal with altitudinal challenges to thermoregulation, we developed a deterministic heat-balance model to estimate equilibrium body temperatures (T_e) of bumblebees of different body sizes. We used bumblebee models in a wind tunnel to measure convective heat-loss coefficients, published measurements of metabolic rates and of flight muscle efficiencies to estimate metabolic heat production, and standard heat transfer theory to estimate radiative heat flux and evaporative cooling. All else being equal, the drop in ambient temperature with altitude should hamper thermoregulation by increasing convective heat loss. However, all else isn’t equal. The convection coefficient (h_c) actually decreases with altitude because air density is reduced; plus solar heat gain increases with altitude. In combination, these changes effectively buffer – especially for large bumblebees – the convective impact of declining ambient temperatures. Consequently, thermoregulation in thin air may not be as challenging as it appears.