Seasonal effects on digging activity and burrow architecture in the Cape dune mole-rat, Bathyergus suillus (Rodentia: Bathyergidae)

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Function of burrows

The majority of fossorial mammals use burrows simply as a resting place when they are not foraging, searching for mates or defending their territory (Lacey, 2000). Even so, a large proportion of their time is spent underground, up to as much as three quarters of the day e.g. the Great basin kangaroo rat, (Dipodomys microps) (Kenagy, 1976). The advantage of using a burrow is that at a depth of approximately 30cm a significant amount of the daily temperature fluctuations disappear although long term seasonal changes in temperature do occur (Reichman et al., 1985).
Burrows provide effective protection against many predators (Lacey, 2000). Avian predators and most terrestrial carnivores cannot chase prey down small burrows (Reichman & Smith, 1990). The exceptions to this are weasels (Mustela nivalis: Mustelidae), ferrets (Mustela putorius furo: Mustelidae) (Smith, 1967; Halpin, 1983) and badgers (Taxidea taxus: Mustelidae), the latter of which can dig their prey out of burrows (Knopf & Balph, 1969). Plugging the burrow may inhibit some snakes from entering (Hickman, 1973), but predatory mammals will dig through the soil plug to get to the animal (Hickman, 1973) and may actually be drawn to fresh soil as a sign of recent excavations by potential prey (Brett, 1990).
Burrows that provide basic protection can be quite simple, comprising of little more than an entrance, a short tunnel and an expanded chamber as a nest (Reichman & Smith, 1990). The vast majority of mammals that construct burrows use such simple designs (Reichman & Smith, 1990). An example of one of the simplest types of excavation is shown by the Namib golden mole (Eremitalpa granti namibensis: Chrysochloridae) and the marsupial mole (Notorytctes typhlops: Notoryctidae) both of which species tunnel through loose sandy soil that is too fluid in form to support a permanent structure (Mason & Narins, 2001).
Many large mammals construct simple linear burrows, for example, platypuses (Ornithorhynchus anatinus : Ornithorhynchidae) construct tunnels into the banks of rivers or ponds (Grant, 1983). Armadillos (Dasypus novemcinctatus: Dasypodidae) excavate burrows that are slightly angled into the ground and are up to 1.25m in length and terminate with a simple nest chamber (Taber, 1945; Clark, 1951). Pangolins (Manis culionensis: Manidae) excavate very similar structures to armadillos
except they may extend up to 3m in length (Walker, 1983). Carnivores such as honeybadgers (Mellivora capensis: Mustelidae), weasels (Mustela nivalis: Mustelidae), and genets (Genetta genetta: Viverridae) utilise similar tunnel systems, although suricates (Suricata suricatta: Herpestidae) and mongooses (Cynictis penicillata: Herpestidae) that live in communal groups may excavate longer and more complex structures (Smithers, 1983) (See illustrations in Bronner, 1992). Most burrows contain nests which are usually expanded chambers, which may be lined with vegetation or fur, which keeps the inhabitant dry and warm (Reichman & Smith, 1990). The beneficial feature of a nest for an individual is realised when females give birth to young that are relatively helpless with poor thermoregulatory capabilities, and remain in the nest for days or even weeks (Bronson, 1989). Among mammals, rodents excavate some of the most complex burrow systems that are (assumed to be) used for protection (Reichman & Smith, 1990). These burrows frequently have numerous entrances that interconnect below ground and have more than one nest site (Butynski & Mattingly, 1979).

Foraging habits

As herbivores, subterranean rodents feed extensively on vegetation and appropriate plant species must be available to support the animals. In addition to providing critical food resources, the type of vegetation present may determine the distribution of subterranean rodents through effects on patterns of ventilation (Busch et al., 1989). In particular, the structure and density of vegetation may influence patterns of heat flux within subterranean burrows which may, in turn, determine whether a given habitat is suitable for underground existence; for example Comparatore et al. (1991) found that in warmer months, favourable zones for Ctenomys talarum were those with greater density and height of the vegetation. Subterranean plant tissues may represent a more variable resource, in terms of nutritional value, than above-ground plant tissues (Andersen 1987), This difference in nutritional quality may influence food selectivity. In addition to the availability of suitable roots and tubers, the cost of foraging for these items may prevent some species from specializing on subterranean plant parts. Heth et al. (1989) argued that subterranean herbivores cannot afford to be selective feeders because the costs of searching for food items would exceed the benefits of this
selectivity; as a result, subterranean rodents should utilise all food that they encounter. Generally, most species of mole-rats feed on corms, bulbs and geophytes of plants (Bennett & Jarvis, 1995). Exceptions to this are B. suillus and G. capensis which also consume the aerial portions of vegetation (Beviss Challinor, Broll and Jarvis unpublished data).

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Excavation of burrow systems

Upon removal of the occupant, burrow systems were excavated manually with hoes to expose the tunnels along their entire length. A total of 40 burrows were excavated (20 during winter and 20 during summer).The lengths of the tunnels and their dimensions and shape were recorded sensu Thomas et al., (2009) for B. suillus. The depth from the ground surface to the top of the burrow; height and width of the burrow were measured using a tape measure (± 0.1cm). Tunnels were defined as either being deep, semi-permanent (> 20cm) or shallow, foraging (< 20cm) tunnels. The distinction between tunnel usages were determined by the depth of the bulbs and roots of the plants reached in the localities. Tunnels were defined as arched if the ratio of the tunnel height divided by the tunnel width exceeded 1.4 or circular if not. A map of each burrow system was recorded relative to magnetic north and later digitised. Due to the shorter length of G. capensis burrow systems compared to B. suillus (Thomas et al., 2009); tunnel depths were recorded approximately every 1m instead of every 2m.
The location and dimensions of any nests, food stores, bolt holes and latrines were recorded. Nests were defined as chambers with only a single entrance and filled with nesting material (Thomas et al., 2009). Food stores were blind-ended tunnels filled with bulbs or roots. Bolt holes were steep-angled tunnels (almost vertical) that were greater than 30cm in length used as anti-predatory function, thermoregulation or as drainage sumps (Hickman, 1990; Nevo, 1999). Latrines were blind-ended tunnels packed with soil and faeces. The position and ages of the mounds were recorded as in Thomas et al., (2009).

Chapter 1 – General introduction
 Introduction
 Shelters
 Burrows
 Function of burrows
 Mole-rats
 Mole-rat burrow systems
 Foraging habits
 Exploring the environment
 Seasonality
 Morphology
 Excavation methods
 Chisel-tooth digging
 Bite force
 Forelimb digging
 Forelimb structure
 Sex differences in forelimb structure
 Summary
Chapter 2 – Seasonal effects on digging activity and burrow architecture in the Cape dune mole-rat, Bathyergus suillus (Rodentia: Bathyergidae)
 Abstract
 Keywords
 Introduction
 Materials and methods
 Results
 Discussion
Chapter 3 – Season but not sex influences burrow length and complexity in the non sexually dimorphic solitary Cape mole-rat (Rodentia: Bathyergidae)
 Abstract
 Keywords
 Introduction
 Materials and methods
 Results
 Discussion
Chapter 4 – Seasonal changes in burrow geometry of the common mole-rat, Cryptomys hottentotus hottentotus (Rodentia: Bathyergidae)
 Abstract
 Keywords
 Introduction
 Materials and methods
 Results
 Discussion
Chapter 5 – Bite force and sociality in African mole-rats (Rodentia: Bathyergidae)
 Abstract
 Keywords
 Introduction
 Materials and methods
 Results
 Discussion
Chapter 6 – Skeletal structure and function in the forelimb bones of three species of
southern African mole-rat (Bathyergidae)
 Abstract
 Keywords
 Introduction
 Materials and methods
 Results
 Discussion
Chapter 7 – Synthesis

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