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dogs, etc.). Similarly, mice and rats can be bred for various behaviors, such as learning performances or aggression. Usually, animals are selected for the opposite of the desired behavior (directional selection).

For aggressive behavior, various pairs of selection lines exist. One of the more conspicuous ones originated from animals from a feral population caught in The Netherlands that were subsequently selectively bred for behavior in a test cage developed by Geert van Oortmerssen. Different chambers of the cage were connected through slide doors such that home territories and a border area were created. Since in the wild most agonistic encounters occur at the border of the territories, this design takes the natural settings of the test animals into account. Moreover, the aggression test takes 3 days, with one encounter every day. This procedure not only reduces the effects of chance but also creates the opportunity to investigate the development of aggression over time. Using the previously mentioned paradigm, van Oortmerssen selected two lines for attack latency (Fig. 1): one highly aggressive line, characterized by short attack latencies (SALs), and one low to nonaggressive line, characterized by long attack latencies (LALs).

Although it is beyond the scope of this article to discuss the theory and practice of selected lines in detail, the following strict conditions must be met in order to reliably reveal the genetic underpinnings of behavior. First, one has to bear in mind the obvious. Behavior without genetic variation underlying it cannot be selected for or against. Therefore, if no differences for the desired trait are observed in a panel of inbred strains, it is theoretically impossible to selectively breed for that characteristic. Second, the simpler the phenotype, the better the experiment generally works. Third, if possible, a genetically defined, replicable founding population should be used. In this case, one can always use the parental population if needed. These conditions can be met relatively easily and without great financial resources. However, a last theoretical requirement results in almost prohibitive costs. Since selection depends on genetic variation, inbreeding is its natural enemy. Inbreeding often leads to mortality or loss of fertility, but it also conflicts with the desired result of selection: to increase the frequency of only those alleles that favor the trait under selection. Inbreeding is not desired for the vast majority of genes that need to be kept segregated. To this end, one would have to maintain an unrealistically large number of animals. In practice, selective breeding leads to the subsequent loss of alleles (genetic drift). However, it is possible to

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Figure 1 Artificial bidirectional selection for attack latency in male wild house mice. Three lines have been established: one control line (Ctrl) and two selection lines—one highly aggressive line, characterized by short attack latencies (SAL), and one low to nonaggressive line, characterized by long attack latencies (LAL). Differences in the number of generations between SAL and LAL mice originated from difficulties in developing the LAL line per se (f, unsuccessful attempts) and in unequal rates of reproduction, with the SAL females producing both earlier and larger litters.

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Figure 1 Artificial bidirectional selection for attack latency in male wild house mice. Three lines have been established: one control line (Ctrl) and two selection lines—one highly aggressive line, characterized by short attack latencies (SAL), and one low to nonaggressive line, characterized by long attack latencies (LAL). Differences in the number of generations between SAL and LAL mice originated from difficulties in developing the LAL line per se (f, unsuccessful attempts) and in unequal rates of reproduction, with the SAL females producing both earlier and larger litters.

control to a certain extent for genetic drift through the establishment of separate, replicated lines.

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