35-minute time spans in bins of a 5-minute duration. The PPI measures the acoustic startle response, which is an exaggerated response to an unexpected stimulus. The acoustic startle is measured over a range of frequencies and amplitudes to calculate the average response. These tests allow us to detect abnormal behaviours and document them. Behavioural degradation under spontaneous mutation accumulation In each generation, the genetic variation within a population increases due to accumulation of mutations and decreases in response to natural selection and genetic drift. Mutation accumulation occurs when mutations of small effect accumulate at certain loci, yielding a large phenotypic effect in the aggregate. Multiple genes may simultaneously affect behavioural traits. Spontaneous mutations arise from sources including errors in DNA replication, spontaneous lesions, and transposable genetic elements in the absence of mutagens. Spontaneous mutations play a central role in the maintenance of genetic variation and persistence of natural population of many organisms. Evolutionary biologists have used mutation accumulation experiments, in which mutations are allowed to drift to fixation in inbred lines, to study the effect of spontaneous mutations on phenotype character. Phenotypic assays significantly determine whether and how quickly population with accumulated deleterious mutational loads can result in degradation of behavioural responses over time. Based on laboratory experimental evolution with long-term mutation accumulation (MA) lines of the nematode Caenorhabditis elegans, a team of researchers at the University of Oregon investigated that mutation accumulation of behaviour is capable of generating significant levels of individual variation in ecologically relevant behavioural traits within populations. This variation will be dependent largely on the genetic structure and demographic characteristics of individuals. As a result, small or isolated populations are at high risk of experiencing behavioural degradation. For instance, the rate of mutation for behavioural traits has more effects for behavioural mutation within captive populations and some endangered species. The study of two closely related behavioural traits of the free-living soil nematode C. elegans, chemotaxis and locomotion, indicates that behavioural degradation is a direct source of competitive fitness loss under genomic mutation accumulation. Raymond B. Huey and his colleagues used the same MA lines method, suggesting that mutation accumulation in Drosophila melanogaster significantly depresses only some behavioural traits. There are several explanations for this. It is possible that traits are influenced only by few loci so that their mutational target is small. Alternatively, the values of the behavioural traits are not maximized by directional selection, but