Faculty of Science
School of Biology
9410199j
TEL: 011 673 8139 / I studied the sociality and activity patterns of free-living ice rats Otomys sloggetti robertsi.
The ice rat is a medium-sized, diurnal, herbivorous rodent, endemic to the alpine habitats of
the southern Drakensberg and Maluti Mountains in southern Africa. These environments are
characterized by sub-zero temperatures in winter and high levels of solar radiation in summer,
and snow can be expected at any time of the year. Previous research by other scientists
indicated that the physiology of ice rats is more similar to congeners living at lower altitudes,
and, instead the taxon has morphological and behavioural adaptations for coping with the
temperature extremes in its environment. Predator pressure on ice rats was negligible in my
study site, making the ice rat an ideal model for testing how environmental factors influence
sociality and activity patterns.
Sociality was investigated by using direct observations and through experimental
manipulations. Colonies comprised 4-17 individuals with several reproductively active males
and females. Colony members had a high degree of home range overlap, whereas interactions
between colony members were rare and usually resulted in agonistic behaviour. Ice rats
responded aggressively to experimentally-caged colony members positioned in different parts
of their own colony and these were treated with the same level of aggression as were strangers.
Moreover, colony members competed aggressively for better-quality introduced food,
particularly in winter.
From direct observations of male-female interactions, it appeared that ice rats mate
promiscuously, which is most likely a consequence of the multi-male and multi-female
colonies. Females spent long periods of time belowground with unweaned young which
emerged aboveground independently at four weeks of age. The first litter born in a season
dispersed at sexual maturity (males±14 weeks; females±9 weeks), but, since I did not observe
the behaviour of litters born later in the breeding season, I was not able to tell if these
dispersed as well.
I excavated the burrow systems of ice rats and found an intricate interlinking
underground tunnel system with sometimes more than 25 entrance holes and 1-2 nesting
chambers, which would provide a thermoneutral refuge for the ice rats at night as well as
during adverse weather conditions. Because the ratio of the number of animals in the colony to
III
the number of nest chambers exceeded one, I predicted that huddling occurs belowground,
which was confirmed by video recordings of nest chambers.
Diurnal aboveground activity patterns of ice rats were influenced by prevailing
environmental conditions, which resulted in synchronous aboveground appearance of
members in a colony. The summer activity pattern was bimodal, dominated by foraging and
sun basking behaviour aboveground, with animals retreating belowground to escape high
temperatures and radiation levels prevalent around midday. Ice rats utilized the warmer
temperatures throughout the day in winter for aboveground foraging and basking.
Otomys s. robertsi displays a spatial shift in its social system: colony members huddle
belowground but display temporal territoriality aboveground. Such a social system is contrary
to predictions previously made for this taxon (i.e. they live as family groups). Ice rat relatives
living at lower altitudes mainly occur as non-social aggregations and one relative, the vlei rat
O. irroratus, also displays temporal territoriality. Although sociality is common in rodents
inhabiting temperate environments in the northern hemisphere, these rodents do not show
territoriality at other times. I conclude that the social system of ice rats, in the absence of
significant predation pressure, is determined by a combination of 1) environmental factors
driving communal thermoregulation and 2) phylogenetic constraints imposed by competition
for limited food resources.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/1803 |
| Date | 16 November 2006 |
| Creators | Hinze, Andrea |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 516525 bytes, application/pdf, application/pdf |
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