Social behaviour and activity patterns of the African ice rat Otomys sloggetti robertsi

[摘要] 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 ofthe southern Drakensberg and Maluti Mountains in southern Africa. These environments arecharacterized 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 scientistsindicated 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 thetemperature extremes in its environment. Predator pressure on ice rats was negligible in mystudy site, making the ice rat an ideal model for testing how environmental factors influencesociality and activity patterns.Sociality was investigated by using direct observations and through experimentalmanipulations. Colonies comprised 4-17 individuals with several reproductively active malesand females. Colony members had a high degree of home range overlap, whereas interactionsbetween colony members were rare and usually resulted in agonistic behaviour. Ice ratsresponded aggressively to experimentally-caged colony members positioned in different partsof 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 matepromiscuously, which is most likely a consequence of the multi-male and multi-femalecolonies. Females spent long periods of time belowground with unweaned young whichemerged aboveground independently at four weeks of age. The first litter born in a seasondispersed at sexual maturity (males±14 weeks; females±9 weeks), but, since I did not observethe behaviour of litters born later in the breeding season, I was not able to tell if thesedispersed as well.I excavated the burrow systems of ice rats and found an intricate interlinkingunderground tunnel system with sometimes more than 25 entrance holes and 1-2 nestingchambers, which would provide a thermoneutral refuge for the ice rats at night as well asduring adverse weather conditions. Because the ratio of the number of animals in the colony toIIIthe 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 prevailingenvironmental conditions, which resulted in synchronous aboveground appearance ofmembers in a colony. The summer activity pattern was bimodal, dominated by foraging andsun basking behaviour aboveground, with animals retreating belowground to escape hightemperatures and radiation levels prevalent around midday. Ice rats utilized the warmertemperatures throughout the day in winter for aboveground foraging and basking.Otomys s. robertsi displays a spatial shift in its social system: colony members huddlebelowground but display temporal territoriality aboveground. Such a social system is contraryto predictions previously made for this taxon (i.e. they live as family groups). Ice rat relativesliving at lower altitudes mainly occur as non-social aggregations and one relative, the vlei ratO. irroratus, also displays temporal territoriality. Although sociality is common in rodentsinhabiting temperate environments in the northern hemisphere, these rodents do not showterritoriality at other times. I conclude that the social system of ice rats, in the absence ofsignificant predation pressure, is determined by a combination of 1) environmental factorsdriving communal thermoregulation and 2) phylogenetic constraints imposed by competitionfor limited food resources.

[发布机构] University of the Witwatersrand