Spatial dichotomy of sociality in the African ice rat

Abstract

Sociality is environmentally and phylogenetically determined and can vary intraspecifically and interspecifically. We investigated the reasons for group living in the African ice rat Otomys sloggetti robertsi, a diurnal, herbivorous, nonhibernating murid rodent, endemic to the sub-alpine and alpine regions of the southern African Drakensberg and Maluti mountains. We expected ice rats to be group living, nesting communally in underground burrows. We documented the spatial organization and social behaviour of free-living ice rats through direct observations and experimental manipulations. Colonies comprised 4-17 adults of both sexes. Members of a colony had a high degree of spatial home-range overlap but no temporal overlap because interactions between members were rare aboveground. Individuals experimentally displaced within their own colony were attacked by members of their own colony and were treated in the same way as strangers from other colonies. Members of a colony competed aggressively for prized food, particularly in winter. Ice rats displayed a vertical spatial separation in social behaviour, from huddling and tolerance belowground to solitary foraging and mutual avoidance aboveground. Such a dichotomy in sociality reflects the compromise between the benefits of social thermoregulation and burrow sharing on the one hand and the constraints of competing for resources, mainly food, on the other. Such a compromise may ultimately be related to the poor physiological adaptation of ice rats to the cold environments they inhabit.

Behavioural correlates of group size and group persistence in the African ice rat Otomys sloggetti robertsi

“…Plural breeding without communal nursing can lead to competition among females for 152 resources (Silk 2007), which might partially explain the above-ground competition for 153 resources (Hinze et al 2013). Colony members are also highly intolerant of strangers, which 154 reduces the probability of emigration into existing colonies, resulting in spatial segregation of 155 colonies (Hinze et al 2013).…”

“…Colony members are also highly intolerant of strangers, which 154 reduces the probability of emigration into existing colonies, resulting in spatial segregation of 155 colonies (Hinze et al 2013). New colonies form in vacant areas by dispersing sexually 156 mature adults from existing colonies (Willan 1990).…”

“…Ranges6 (Kenward et al 2002) as described by Hinze et al (2013). Briefly, we demarcated 319 each colony into 4x4m grid squares using coloured pegs (30 cm high) based on pilot 320 observations of the behaviour of adults; the number of squares varied based on the size of, 321…”

“…and area occupied by, the colony (Hinze et al 2013). We recorded the position of all 322 individuals in a colony every 1 h during the 60 h of behavioural sampling, noting their 323 position within a quarter square on the grid.…”

“…We recorded the position of all 322 individuals in a colony every 1 h during the 60 h of behavioural sampling, noting their 323 position within a quarter square on the grid. Locations taken greater than 35 min apart are not 324 auto-correlated (Hinze et al 2013). The data from all individuals in a colony were plotted on 325 a schematic diagram, which was used to generate the convex polygons.…”