Burrows ventilated by eddy penetration

Abstract

Just because we aren't burrow dwellers doesn't mean that we can't appreciate the challenges of a subterranean lifestyle. Anyone who has travelled on an underground system cannot have failed to notice the stale atmosphere in the tunnels and it has long been assumed that burrowing species must contend with high concentrations of carbon dioxide in their subterranean lairs. However, a team from Ben-Gurion University of the Negev, Israel, and ESF-SUNY, USA, suspected that burrow dwellers' homes might be better ventilated than we assume. Inbal Brickner-Braun, Daniel Zucker-Milwerger, Avi Braun, Berry Pinshow, Scott Turner and Pedro Berliner explain that although CO2 levels in the burrows of some rodent species were found to be high, carbon dioxide measurements in the burrows of other rodents were essentially the same as those on the surface. So, the team decided to find out how the burrows of one small species of rodent, Sundevall's jird, are ventilated (p. 4141).The team say that various processes may contribute to the ventilation of burrows. They explain that air might be forced through the tunnels by the inhabitants moving like pistons, moved by convection or diffusion, or driven by air movements at the surface. Intrigued, the team built artificial jird burrow systems to find out whether eddies from the surface could replenish air deep inside a burrow.Improvising with kitchen sieves to build the nest chamber and light-wire mesh for the burrow walls, Brickner-Braun and Zucker-Milwerger constructed two, 2 m long U-shaped tunnels, each of which descended 60 cm down from the surface and was connected to the nest chamber at the deepest point by a short tunnel. Wrapping one burrow in plastic (to seal it and allow air movement along the tunnel by convection alone) and the other in medical gauze (to permit diffusion of gases across the unsealed walls), the team buried the simulated burrows in light soil, and aligned them with the direction of the prevailing wind. Then they waited for windy days to measure the air temperature in the burrow – in the hope of seeing puffs of warm air driven by surface air currents penetrate the tunnel – to find out whether atmospheric turbulence can drive air circulation through subterranean burrows.Analysing the temperature profiles of the burrows, the team saw that eddies from the surface were able to travel deep into the burrows, but they never reached the nest in the most remote regions. However, when the team simulated the presence of a mother with four pups inside the burrows by pumping CO2 into the nest chambers, they could see that air currents at the surface dramatically affected the chamber's CO2 levels. The CO2 concentration in the sealed burrow fell spectacularly from 25,660 ppm (65 times atmospheric CO2 levels) at the lowest wind speeds to ~4000 ppm at wind speeds of 4 m s−1. Meanwhile, in the unsealed burrow with both entrances plugged – so that gases could only leave or enter by diffusion across the burrow walls – the CO2 levels were unaffected by wind speed, remaining between 8600 and 10,400 ppm. However, when both mechanisms of gas exchange were possible, the nest CO2 measurements fell as low as 2800 ppm at the top wind speed of 3.0 m s−1.The team say, ‘The nest chamber seems not to be directly ventilated by eddy penetration’; however, they suspect that fresh air carried into the burrow probably increases the CO2 gradient between the nest and main channel to boost its diffusion away from the chamber and keep the air fresh.