Helical burrows as a palaeoclimate response: Daimonelix by Palaeocastor
Abstract The extra effort needed to dig a helical burrow versus a straight burrow with the same passage radius has been quantified in an equation that depends only on the angles of incline. For the helical burrows (Daimonelix) of the Miocene beaver Palaeocastor fossor, extra and/or unusual effort was needed for the helical passage versus a straight passage. That effort for the helical passage was probably not justified to avoid the small likelihood of burrow interference by straight passages among closely packed burrows since there was already a greater chance (but so far unobserved) of interference with their living-chambers which had a larger diameter and a smaller average angle of incline. That effort was probably not justified either as a deterrent to predators since smaller predators (such as Zodiolestes daimonelixensis) have already been found in these helical burrows, the type of beaver remains found in the burrows suggest predation, and the great and consistent depth would not have been needed. Helical burrows are more likely a response to palaeoclimates since the extra and/or unusual effort for helical passages did result in corresponding increases in both burrow subsurface air volume and burrow wall surface area and/or in limiting air circulation, which would have provided the environmental advantages of increased heat loss or gain resulting in more consistent subsurface temperature and humidity when there were seasonal or extreme surface conditions.