The role of social behaviour and spacing in populations of the bush rat, Rattus fuscipes

Social behaviour and social spacing was studied in natural and enclosed populations of the bush rat Rattus fuscipes. The aim of this study was to elucidate the role of social behaviour in the population biology of this species. The approach taken involved the manipulation of resources and spatial environments of rats in enclosures in order to reveal details on the relationship b tween social behaviour and resource availability (primarily resource distribution)and to reveal the behavioural mechanisms responsible for the partitioning of resources among conspecifics. 71 trapping and tracking study in the field was used to reveal aspects of the social structure of natural populations and changes in spacing associated with natural changes in resource distribution. 71 comparison of information from three widely separated populations (from Kioloa, N,S,W., Lees Creek, A.C.T. and Mt Glorious, Qld.) was used to establish the general demographic and social characteristics of the species (using similarities in the data) and to establish the range of social responses to varying environments (differences in data.) The similarities in demography of rats in the three areas were as follows: (a)there was an annual cycle in abundance of animals and annual turnover of membership in the population, (b) the sequence of life history events in each cohort resulted in similar timing of peaks and depressions in population size in all study areas; <cl mating took place in spring and early summer, young emerged and became trappable in mid-summer, (d) populations peaked in late summer and autumn and reached lo11est levels just before juveniles appeared, (e) males shelled lo11er residency than females and experience high turnover and typically disperse in spring; (f)females sho11ed high residency and, were probably responsible for the overall predictability of population size from cohort to cohort. The three populations differed primarily in density end stability of numbers over the study periods. While the population at Kiola declined drammaticelly the populations at Lees Creek and Mt Glorious sho11ed only the typical annual oscillations described above. Both of these latter t11o populations 11ere consistently more dense than the population at Kioloa, A more detailed study of one population (at Kioloa, N.S.W.) provided information on the relationship bet11een resources and the density of individuals and thus helped to explain the observed decline in the size of the loc.al population. A combination of deterioration patches of appropriate habitat and physical isolation of the population from other sources of a. fuscipes contributed to the decline in numbers and lack of immigration. Spring immigration of males and the availability of sufficient resources for over11intering of resident animals (particularly females)may be <'3sential to stable population size in this species, Five measures of animal dispersion and one measure of social relationship based on spacing were applied to trapping data from all three field populations and revealed the follo11ing general characteristics regarding a. fuscipes: Animal dispersion and animal associations varied seasonally and 11ith the sex and age of the individuals. In particular, adult females maintained discrete spacing in all seasons and showed seasonal association 11ith other types of individuals. In 11inter some subadults (particularly overlapped with adult females and shared complex burrow systems. Consequently, there was a significant difference in clumping of females in winter compared with their dispersion in other seasons. Hales consistently showed a spring increase in movements associated with the onset of breeding. The social associations of males was highly variable through the year. Information on the use of resources and the diet of R. fuscipes showed that it has a diverse food base with seasonal preferences and that in general both the food resources and p6tential shelter of bush rats were patchily distributed. At Kioloa it ate and cached the highly toxic seeds of cycads in autumn but did not a pear to suffer ill effects. The bush rats dug burrows in autumn and winter and use shallow nests in log hollows for shelter in other seasons. In all seasons and particularly in winter, the bush rats make use of runways which were sometimes clearly discernable. Enclosure experiments were designed to explore the relationship between changes in the availability and distribution of resources and changes in social behaviour and spacing of R. fuscipes. The establishment of the social order which functions to partition resources was observed in enclosures and attempts were made to show that findings from enclosure studies were consistent with observations in natural populations. The behavioural mechanisms by which the social structure was. maintained were largely revealed by using large outdoor enclosures for short-term 6 weeks experiments involving variab les such as season breeding and non-breeding, the nuffiber of potentially interacting individuals, and the distribution ofood resources(central and scattered). Dominance hierarchies and the Use of runways emerged as the key behaviour mechanisms. The maintenance of spacing and some of the effects of spacing behaviour were studied in semi-natural outdoor enclosures. Linear dominance hierarchies were quickly established and shown to be related to space use patterns of tne animals of different social rank. The relationship was more distinct in breeding season when dominance was generally attained by a female and not necessarily the largest individual. She was extremely intolerant of the presence of others in her space and attempted to exclude them. Their attempts to escape were prevented by the walls of the enclosure and the consequence of avoidance beh viour led to an increase in the complexity of the runway system to which all movements were restricted. Intolerant behaviour, and the patrolling of space using runways showed peaks during oestrus and pregnancy. Since breeding and social dominance were correlated, it was that being first to enter oestrus contributed to the probability of being socially dominant and being the first and only female to breed. Observations in smaller indoor enclosure; (called "pools") confirmed the findings from outdoor enclosures that the dominant animal WitS usually a female and was the only individual to breed successfully and that soci lly dominant animals affect the use of space and resoUl-ces by subordinates. The use of space was linked with development of a runway system and relocation of thesrunways by changing the positions of specially designed floor tiles revealed that animals rely on olfactory cues to locate the runways. Disturbance of the runways, led to a significant drop in social interactions and animal moveme ts remained infrequent until runways were reestablished. Since runways were marked with urine it was postulated that the runways may bear some information ebout the dominant individual, her ownership of space are reproductive condition, and behavioural state, Bush rat spacing is probably the consequence of avoidance responses by individuals encountering the fresh odours of dominant animals(space owners)on runways, and is a means of avoidifl1 direct encounters, In the field secondary cues may be only occasionally reinforced through overt social behaviour, thus accounting for the lack of stylized agonistic behaviour in this species. Long-term experiments in the pools s o11ed that therare incid ntal effects on rats living at high density, When confined populations to pools for several months populations stopped gro11ilig and'young animals which remained confined 11ith a dominant female experienced suppressed growth. Social behaviour may be responsible for impaired fitness of individuals which cannot disperse