Foraging biology and habitat use of the southern African ice rat, Otomys sloggetti robertsi
Schwaibold, Ute Heidrun
Animals living in cold environments show physiological, morphological and behavioural adaptations to low temperatures. The African ice rat, Otomys sloggetti robertsi, which is endemic to the southern African Drakensberg and Maluti mountains above 2000m, is an interesting exception since, unlike most alpine small mammals, it does not hibernate or display torpor and is physiologically poorly adapted to low temperatures. It is a strict herbivore, feeding on a low quality diet. Ice rats do show some morphological (e.g. short tails) and behavioural (e.g. communal huddling; constructing underground burrows) adaptations, but little else is known about their biology, particularly how they maximise energy gain to meet thermoregulatory requirements, especially during cold periods. Since feeding represents the primary method of energy gain in endotherms, I studied aspects of the foraging biology of ice rats, including gut structure, foraging patterns and habitat choice. The gut structure of O. s. robertsi is well adapted for a high fibre, herbivorous diet and shows broad similarities with those of its mesic- and arid-occurring relatives. However, O. s. robertsi showed increased dimensions of several foregut organs which may be adaptations for increased energy uptake and/or poor diet quality in alpine environments. Furthermore, females had a larger stomach as well as a longer caecum, small and large intestine in summer than in winter but the gut of males was unaffected; such sexual asymmetry may be related to increased energy requirements of females during pregnancy and lactation. Environmental influences on the aboveground behaviour of O. s. robertsi were investigated by recording the duration of behaviours as well as sequential transitions among behaviours. Ice rats spent most of their day foraging and basking, and much time was spent in their underground burrows. Seasonal comparisons revealed that ice rats spent significantly more time acquiring energy through foraging in winter, whereas they remained below ground for longer periods of time during the middle of the day in summer to escape extreme heat and solar radiation. To understand how low temperatures and predation influenced foraging patterns, the behaviour of ice rats was studied in summer and winter in a population where predators were minimal and in another population which experienced higher levels of predation. Ice rats are central place foragers that travel short distances to forage and display significant seasonal variation in their foraging patterns. In the absence of predation risk, ice rats generally returned to a central place with forage, even though returning to a burrow after foraging in winter was energetically costly. However, these costs must be weighed against the benefits of avoiding exposure to low temperatures by feeding under cover as well as the loss of collected food and possible injury associated with aggressive interactions with conspecifics. Under moderate predation pressure in both seasons, ice rats followed a central place foraging strategy to minimise predation risk, always returning to a burrow entrance with forage collected elsewhere. However, when no perceivable threat was observed, ice rats displayed ‘optimal’ foraging patterns in summer similar to those recorded in the absence of predation pressure and only returned to a burrow with forage as distance from that burrow increased, suggesting that ice rats display facultative foraging decision making in response to multiple environmental cues. The distribution of occupied ice rat burrows was correlated against several environmental factors to determine microhabitat requirements. Ice rat burrows were situated in close proximity to herbaceous and wetland plants, but away from woody vegetation, suggesting that habitat choice is related to the presence of food plants and reduction of shade, facilitating short travel distances during foraging as well as promoting basking. Despite the physiological shortcomings of ice rats, the gut structure, foraging behaviour, and habitat choice of the taxon are adapted for life in cold alpine habitats, most likely by maximising energy intake. Similarities in foraging behaviour and habitat use between O. s. robertsi with its closely-related arid-occurring relative Parotomys spp. suggest phylogenetic influences, but it is possibly more a reflection of similar phenotypic responses to the extreme habitats inhabited by these otomyines.
Student Number : 9613963J - PhD thesis - School of Animal, Plant and Environmental Sciences - Faculty of Science
Otomys sloggetti robertsi, foraging, habitat use, gut morphology, thermoregulation, seasonality