Does thermal convection occur in mammalian burrows during the night?

Burrowing is a common habit of mammals in arid zones, yet knowledge of environmental conditions within animal burrows, and especially of the way burrows are ventilated, is scarce. The ventilation rate of a burrow controls air composition within the burrow by driving gas exchange between the lower part of the burrow where the animal typically lives, and the atmosphere. Ventilation can be achieved by the following mechanisms: (1) diffusion; (2) external winds; (3) movement of the inhabitant within the burrow (the 'piston-effect'); and (4) natural thermal convection, a process by which a natural thermal gradient between burrow and atmosphere creates a density gradient which induces air flow. Here we investigate the role of thermal convection in burrow ventilation. For this purpose, artificial burrows (65 cm in depth and 7 cm in diameter) were drilled in loess soil in the Negev Desert of Israel and a network of thermocouples was installed to continuously monitor and record temperature distribution within these burrows. The results show that free convection occurs on a daily basis during the night and early morning. During these times, burrow air temperature was warmer than atmospheric air, and temperature readings pointed to the regular occurrence of convection flow in a thermosyphon pattern. Volume fluxes were calculated based on analytical solution and empirical correlations. For the artificial burrows investigated, an average CO2 volume flux of about 15 liter/hour was calculated during the night when convective conditions prevailed. For comparison, CO2 volume flux by steady-state diffusion alone is 3 orders of magnitude lower.