The act of living partially or entirely underground (e.g. caves, burrows, dens) or using hidden environments (e.g. tree hollows, with leaf litter, dense vegetation) makes research into the ecology and health of many animal species a challenge. The essential issue arises from difficulty in obvserving species within these hidden environments—meaning that aspects of their ecology are not easily understood. For example, aspects of the social ecology of some mole species, and aspects of the reproductive biology of winter hibernating bears have remained long-standing research challenges. Additionally, some of the most significant wildlife pathogens can be transmitted in hidden environments. For example, the fungal agents that cause bat White Nose Syndrome and amphibian chytridiomycosis can be transmitted in caves and moist substrates under rocks and leaf litter, respectively. Thus, understanding the ‘hidden’ ecology of species and environments is both a difficult and important issue. To overcome such issues technological solutions are often the best answer (e.g. hidden or remote cameras, attachment of retrievable accelerometers to individuals, and vehicles).
Wombats are medium sized, herbivorous marsupials which are fossorial. They create burrows for shelter and thermoregulation from diurnal temperature extremes [32]. Three species of wombats occur in Australia, the bare-nosed wombat (Vombatus ursinus, a.k.a. common wombat) across southeastern temperate regions (far southeast Queensland, eastern New South Wales [NSW], Victoria, Tasmania, and southeastern South Australia), the southern hairy-nosed wombat (Lasiorhinus latifrons) in arid south areas (South Australia and into Western Australia) and the critically endangered northern hairy-nosed wombat (Lasiorhinus krefftii) is restricted to two arid areas of inland Queensland [32]. All three species are primarily nocturnal and prefer cooler temperatures, although they will also forage in diurnal periods during cooler conditions. Burrows are dug into substrate that varies in composition of clay, sand, and obstructions (e.g., rocks, tree roots). Bare-nosed wombats typically dig single burrows, often occupied by a single individual, whereas the hairy-nosed species tend to create warrens each of which can be occupied by several individuals [32]. Evidence indicates the environmental conditions (temperature and humidity) within wombat burrows are relatively stable over circadian cycles [28]. However, the stability of environmental conditions across seasons is less well understood. Indeed, for bare-nosed wombats (the focal species of this study), it is well known that they can vary their use of burrows owing to seasonal changes in environmental conditions (e.g. flooding from changing water tables).
The subterranean environment which wombats utilize, has been a long-standing challenge to study. The earliest investigations of wombat borrows came from the sketches of the Australian Peter Nicholson during his childhood in the 1960’s. He crawled into the burrows of bare-nosed wombats in NSW, partially excavating aspects to facilitate maneuvering [32]. Of course, certain safety and ethical risks are presented by this—entrapment in confined spaces, cave-ins, aggressive wombat encounters, and the necessity of small-statured individuals (children) for exploration—hence other research methods are necessary. Other studies to document the structure and environmental conditions of wombat burrows have included excavation (destruction), drilling of a sequential series of portholes directed by mirrors and cameras and, more recently, ground-penetrating radar (GPR) [19, 31, 32]. These studies document heterogeneity in the length of burrows (generally \({<}10\hbox { meters}\), longest 89 meters), number of entrances (generally 1-2, up to 28), amount of branching, and terrain (including substrate [compact soil, dry friable sand, submerged sections, muddy conditions], vertical inclines [up to 45\(^{\circ }\)] with angles [up to 180\(^{\circ }\) within 50cm]) [19, 28, 31, 32]. A single study has investigated the behaviour of wombats within burrows using accelerometers glued into the hair of the wombat dorsum [17]. During early experiments we trialed the use of wireless remote vehicles to traverse burrows but found that this was infeasible due to rapid signal attenuation as the burrow was traversed.
Understanding the environmental conditions within the burrows of wombats has important implications for their health. The most important disease of wombats (particularly bare-nosed wombats) is sarcoptic mange, which is caused by the parasitic mite (Sarcoptes scabiei Linnaeus, 1758) [15]. This parasite has been documented to infect greater than 100 species of mammals around the world [4, 23]. It is also a significant human parasitic disease (scabies), considered a Neglected Tropical Disease by the World Health Organisation [20]. Evidence indicates that S. scabiei is transmitted environmentally among bare-nosed wombats [16, 29]. Wombat individuals are largely solitary, rarely coming into direct contact, but will switch the burrow in which they reside/sleep every 4-10 days. This burrow can then become occupied by another wombat [18]. Because asynchronous burrow sharing occurs among bare-nosed wombats it is thought that bedding chambers within burrows are the likely site where mite fomites transfer to and from wombats. Laboratory data show that off-host survival of S. scabiei is strongly influenced by temperature and humidity conditions, with a peak survival of 19 days at 10\(^{\circ }\)C and 97% relative humidity [1]. Thus, technological advances in the capacity to enter, navigate and record conditions within wombat burrows are valuable for understanding the ecology of this iconic marsupial and also the transmission of S. scabiei among individuals.
Given the safety, size, and environmental constraints, we have turned to robotics which have a long history in operating similarly constrained environments. Existing constrained robotic operating environments cover a wide range including infrastructure sectors (e.g. sewer pipe [7, 26], bridge [30] or HVAC inspection [3]), disaster management [13, 21] and powerline inspection [24]).
Aside from some aerial radio-telemetry and surveying applications [2, 8, 9] robots haven’t been heavily used in wildlife and conservation ecology besides getting close-up footage for documentaries [12]. What we see far more commonly in robotics research is biomimicry where engineers derive inspiration for their designs from clever design elements that we see in the natural world. These may vary from methods of locomotion (e.g. flapping Micro Air Vehicles) through sensing, decision making, and construction [6, 10, 22, 27].
For robotic locomotion, there are many possible solutions that may be applied (e.g. tracks, wheels, inch-worm, walking, rotors, adhesion) with specific system requirements (e.g. speed, environment, traction, stability) acting as constraints. Likewise, there are numerous possibilities in the selection of pickup tools (e.g. finger grippers, electro-magnets, jamming, soft) with a selection to be made based on suitability and system requirements (e.g. material, size, compliance) [11].
The purpose of this study is to evaluate the efficacy of a mobile robot (WomBot as shown in Fig. 1) designed specifically for subterranean exploration of wombat burrows. This exploration provides a low impact, rapid means of burrow exploration and understanding of environmental conditions which may have significant implications on wombat health.
This paper is structured as follows: Section 2 presents the system requirements which govern the design and performance aspects of the robot design. Following this, Sect. 3 describes the design and implementation of the robot and covers all the major sub-systems. Section 4 presents and discusses the experimental results both in the laboratory and in field-testing. Finally concluding remarks are provided in Sect. 5.