Seasonal patterns of burrow architecture and morphological adaptations to digging in three sympatric species of South African mole-rat, Bathyergus suillus (Shreber, 1782), Georychus capensis (Pallas, 1778) and Cryptomys hottentotus hottentotus (Lesson, 1826)

Abstract

Mammals use burrows for numerous functions. As these functions become more complex so does the burrow system. Although this underground environment is buffered from many climatic factors, it does pose an energetically expensive way of life. Due to the increased energy expenditure that is needed to forage and live underground, most subterranean mammals have evolved morphological adaptations to living in such a specialised environment. To this end, the burrow systems of three different African mole rats were considered, which range in body size, sociality and apparent foraging behaviour. Bathyergus suillus excavate their burrow systems using their enlarged forelimbs. Their burrow systems generally have one or two main, deeper, central tunnels that connect to a varying number of shallow 'foraging' tunnels in both sexes. In this study, winter burrow geometry did not differ from summer burrow geometry. The fractal dimension and thus habitat exploration differed with sex; males exploring the environment more efficiently than females.

The burrows of G. capensis did not differ with the sex of the occupant but rather with season. The burrows in winter were generally longer and covered a greater area, but they did not necessarily explore the surrounding environment any more efficiently than burrows in summer. The lack of difference in burrow geometry coupled with the sexual dimorphism of the skull (reverse sexual size dimorphism) indicates that an alternative mating strategy may be employed by G. capensis compared to other species of mole-rats.

Finally, Cryptomys hottentotus hottentotus create elaborate multi-layered burrow systems with numerous foraging tunnels and a few central deep tunnels. The burrow systems in this case differed between the winter and summer. Burrows in the winter were generally longer, covered a greater area and had more branching of the tunnels than those burrows in the summer, i.e. were more efficient. Burrow size appears to be dependent upon colony size; the larger the colony, the larger and the more branching the burrow system.

At a glance the mole-rats face suggests that their incisors may play a special role in mole-rat behaviour. Mole-rats move objects, excavate burrows, carry young, eat and display in social interactions using external procumbent incisors. Mole-rats also use their incisors as a somatosensory organ and Cryptomys h. hottentotus has been suggested to use its incisors to sense vibrations (Poduschka, 1978). This study revealed no evidence of sexual dimorphism in bite force even within solitary species that have marked sexual size dimorphism. Bite force does not appear to be allometric to body size as previously reported in other studies.

Macro-anatomical osteological descriptions are non-existent for South African mole-rats. Within the forelimb, the skeletal system has presumably adapted to withstand the greater pressures exerted due to the excavation methods employed by the mole-rats. To this end, the forelimb bones are generally more robust and have larger areas than terrestrial rodents with more tuberosities for extra muscle attachment to enable them to dig proficiently.