Adaptiveness of tunnel system features in subterranean mammal burrows.

Most moles and mole rats spend their lifetimes within the confines of tunnel walls, constructing, maintaining, and modifying burrow system structure in response to changing external circumstances and changing internal needs. Thus, anatomy, physiology, behavior, and distribution are all greatly affected by the burrow environment, with temperature, humidity, gas concentrations, living space, availability of food and mating partners, and protection from flooding and predation all being influenced by burrow architecture. Regrettably, the burrow structure of almost all subterranean mammals is unknown, or known only from several incomplete excavations. Moreover, existing data is often vague without any mention of particular structures or surrounding habitat, which makes the placing of burrow structure in an adaptive perspective difficult. Reasons for these short-comings are understandable: excavations require laborious excavation which can extend for hundreds of meters, reaching depths in excess of a meter; it is often difficult to remove the occupant prior to excavation which avoids modification of the system by the animal during excavation; particular types of subterranean mammals are limited in distribution to particular continents so that the literature is often localized and a global perspective difficult to obtain; and there is no standardization of terms relating to burrow structure, making intergroup comparison confusing. This paper addresses the above difficulties and encourages further investigation in this central area of study by making obvious the paucity of information for most subterranean mammals, presenting a synopsis of known burrow structures of major groups which will make particular gaps in our knowledge evident, providing a bibliography of major papers on burrow structure with coverage of most moles and mole rats, establishing a checklist of burrow features to better analyze future excavations and facilitate intergroup comparison, and lastly, to indicate possible adaptive significance of various burrow features to promote further observation, speculation and experimentation.