In the early 20th century, scientist often used simple organisms such as bacteria and bacteriophages to explain how cells work at the molecular level. With the rise of genetics, researchers focused on more complex systems by using higher organisms such as the fruit fly Drosophila melanogaster which has a short generation time (7-9 days from egg to adult;[1]). Furthermore, it has four pairs of large chromosomes which facilitate the study of eurokaryotic genetics [2]. Use of the nematode Caenorhabditis elegans, yeast, mice, rats and the chicken embryo as animal models increased in various fields of science [3, 4]. These model organisms, especially mice, remained the focal point for fundamental biological and clinical research and were mainstays of biomedical research [5].

In vitro assays also helped in various disciplines of science. However, due to their lack of a complex physiological environment, cell-culture studies sometimes correlate poorly with results in the whole animal [6]. A good example is provided by behaviour, a phenomenon which cannot easily be studied with the help of in vitro assays. The study of behaviour dates back to the time of Aristotle, when he had many interesting observations concerning animal behaviour [7]. In the modern era, Charles Darwin (1809-82) wrote a whole chapter ‘instincts’ in his book Origin of Species [8]. In the twentieth century, animal behaviour was studied in the context of learning by Ivan Pavlov and Edward Thorndike. Later it became an independent scientific discipline ‘ethology’due to the efforts of Konard Lorenz and the Dutchman Niko Tinbergen [9].