Biological significance, genetics and evolutionary origin of variability in behaviour within and between inbred strains of mice Mus musculus

The biological significance, the origin and the genetical causation of differences in behaviour between inbred strains of mice have been studied. Three inbred strains with different origins were used, these were CPBs (Balb), CBA and C57Black. Exploratory behaviour, nestbuilding behaviour and agonistic behaviour have been observed in environments, which varied with respect to familiarity, presence or absence of nesting material and the possibility of social contact. A comparison of the behaviour of these strains showed the presence of differences in all aspects of the behaviour studied. None of these differences, however, was a qualitative one. The behaviour of each strain was strain specific as appeared from a comparison with earlier reports on related substrains. CPBs mice were territorial with a sensitive aggression-flight balance. They showed a relatively elaborate courtship. They were able to build spherical nests at the surface without support of walls because they treated the nesting material in a special way (fraying). Their exploratory behaviour was mainly directed to open space. C57Black mice were not territorial and did not show a sensitive aggression-flight balance. They hardly showed any courtship. They were not able to build a spherical nest at the surface without support of walls, because they hardly fraved the nesting material. They showed much digging, especially when confronted with nesting material. This digging was well directed and in a suitable substrate holes were made. Their exploratory behaviour was mainly directed to the surface. CBA mice were generally less aggressive than the CPBs mice, but some individuals showed clear territorial behaviour. Their courtship was intermediate to that of the other two strains. CBA mice as a rule did not build proper nests at the surface, either because they did not try to make a nest at all or because they treated the nesting material in such a way that they chopped it up to pieces which were too small to shape more than a simple platform at the surface. They showed much digging but this digging was randomly directed resulting in turning over the whole substrate; no holes resulted from such digging. The exploratory behaviour of the CBA mice was mainly directed to the surface. The nestbuilding behaviour element fraying was chosen for a genetical analysis to find out whether differences in the kind of behaviour studied here were genetically determined or not. Genetical determination appeared to play an important role. Heritabilities for F2 animals and their progeny varied between 17 and 47 per cent. A cross-fostering test between CPBs and C57 Black did not show any cultural influence on the fraying performance in these mice. CPBs mice, which showed a relatively high variability for fraying, appeared to be heterozygous for this character. The heritability found showed a similar size as was found for hybrids (43 per cent). Selection experiments on fraying in this strain were not successful because the lines died out. One selection line for high fraying survived for six generations. It showed no significant reaction to the selection exerted. Heterozygosity in CPBs mice for fraying appeared to be maintained by a kind of balanced system which worked partly by sterility of the high and low fraying types and partly by a negative influence of such types on the physical condition and thus on the fertility of their progeny. Intermediate fraying mice got selective advantage over high and low fraying types in this way, even under inbreeding conditions. Probably only one effective factor is involved in the determination of fraying. Hypotheses concerning the maintenance of genetic variability under inbreeding conditions have been discussed. An analysis of the function and the origin of the differences found between strains suggested that the differences between strains showed strain specific adaptations to habitats in which their ancestors lived. This holds for CPBs mice and for C57Black mice, the ancestors of the first being surface living mice, the ancestors of the second being hole living mice. The behaviour of the CBA mice appeared to be disrupted from a functional point of view, which is in agreement with the fact that CBA mice originate from a cross between two other inbred lines. A specially designed cage to test a differential habitat preference to be present in CPBs and C57Black mice confirmed the hypothesis about the habitat adaptations in these mice and clearly showed that preferences for these habitats are still present in the inbred mice. The results of CBA mice in this test confirmed the unpredictable character of these mice. A comparison for adaptations of this kind between laboratory mice and wild mice revealed that the differences have probably arisen in mice before they were taken into the laboratory. The scarce data available for wild mice suggested that similar adaptations as found here for inbred mice, can also be found in wild mice of different origin. It is discussed that surface living mice have had a better chance to adapt to living in buildings and become commensals, furthermore that the two behavioural types, showing differential adaptations, cannot have gone together as morphs in one polymorphic population. Breeding structure and homozygosity in wild mice and inbred mice have been compared and discussed. It is argued that some wild mice may show more mechanisms to protect them against loss of genetic variability than other organisms because their breeding structure in nature causes a high amount of inbreeding. The way in which differences in mouse behaviour may have evolved was discussed. A primary cause appears to be the differential habitat preference. The possibility of an ecological factor being a primary cause for speciation was suggested many times in studies comparing sympatric species. Differences in non-display behaviour seem to evolve in the same way as display behaviour, i.e. by small changes in threshold of behavioural mechanisms. This could be concluded from the fact that on a subspecies level, like here between different strains of mice, no qualitative differences in behaviour could be found, whereas this is always the case for some of the differences on the species level. It is argued that specific adaptations as found here for mice may have bearings on the outcome of experiments done with mice, furthermore that inbred mice, in addition to studies on wild mice, may be very useful, especially in studying the role of behaviour in regulating population density.