Selective breeding experiments and determination of heritabilities are common tools in behavioral genetics as well as population biology. Many of these projects originally were started in connection with agricultural animal breeding, in order to gain strains or races that would fit better into their artificial environments. Others were originally related to questions of aggressiveness, or learning performance in laboratory animals. The outcomes of these different approaches were remarkably similar in many cases. In poultry, farm fox (see below), and lab as well as wild mice, often already after 10-15 generations of selective breeding, a significant change in behavioral performance could be found. Nervertheless, heritabilities often lie around 30-35%. One famous breeding experiment was started almost 40 years ago with a rare color type of the red fox called silver fox. This animal was planned to be produced in the then Soviet Union, as a fur farm animal. The animals were subjected to a very simple behavioral test - they had to take food from a person and allow themselves to be stroked. Only those animals passing this test were kept in the breeding program. Remarkably soon, starting after less than ten generations, foxes became significantly more people-oriented; later on, they also showed other traits, their eyes and ears opened earlier in pup life, corticosteroid levels were much lower, morphological changes toward larger specimen with different skeletal proportions, etc., followed, and the animals quite literally became pet foxes. This is another example for obviously pleiotropic effects. In another project, wild-caught house mice were put into an arena in same-sex pairs, and the mouse attacking earlier was used to breed in one line, the attacked (hesitant) mouse in another. Again, after about 15 generations, SAL (short attack latency) and LAL (long attack latency) mice differed highly significantly. Again, this was not restricted to the trait that the mice had originally been selected for. SAL mice also entered unfamiliar rooms sooner, were more active in exploring novel objects, novel food, or unknown conspe-cies, and under conditions of a stable environment learned quicker. However, under unpredictably changing conditions, LAL mice could adapt better! These and other results often refer to a certain personality classification, which is called bold versus shy. SAL mice, also endocrinologically, are typically bold animals, whereas LAL mice are shy. In a study of great tits, it could be shown not only that again heritabilities of around 35% exist, but also that both types are present, obviously in a rather stable equilibrium, in wild populations, and that the same advantages or disadvantages relating to predictable conditions (food supply, predators) exist as in mice!
Selection experiments with laboratory mice have also been performed with regard to open-field tests, nest building, and other traits. The outcome often is not only separations between two lines (e.g., fast moving vs. freezing in the open field) that are stable over generations. It is also often observed that these differences, even after many generations, tend to increase. This is normally taken as a cue for polygeneous heredity.
In a different line of studies, heritabilities for behavioral traits have been studied in different breeds of domestic working dogs. There is, of course, no doubt that breeds of working dogs differ from each other for at least partly genetical reasons. However, heritability, as we have seen, refers to differences within one population, here within one breed. And in most cases, be it shepherd dogs, hunting dogs, retrievers, or others, heritabilities were very low, only rarely over 10-12%, often under 10%. Only some traits that again could be summarized under the shy/bold classification, such as 'nervousness', 'temperament', or 'sharpness', sometimes achieved heritabil-ities between 40% and 50% (for comparison, morphometric traits such as shoulder height, chest girth, or body weight in breed dogs have heritabilities of mostly between 65% and 75%). All of these data thus suggest that the contribution of individual genes to the performance ofspecific behaviors is rather limited.
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