The strength of the expression of "egr1" has much variability, dependent on the nature of the song. Songs previously unheard result in strong responses whereas familiar songs result in little or no response. Pure tones or white noise do not evoke any response. It is thought that the purpose of this genetic response to social stimuli is to update the brain's catalog of the changing natural environment. For example, it would be advantageous for a songbird to express more "egr1" in a situation where there is a new song heard (a potential intruder) compared to the familiar song of a known individual. In contrast, for the species of highly social cichlid fish "A. burtoni", the "egr1" gene plays an indirect role in reproduction. In this species, there is an established social dominance hierarchy and an individual's position determines their access to resources for reproduction. If the alpha male is removed from the group, a previous subordinate starts exhibiting dominant behavior and "egr1" is expressed in hypothalamus neurons that are responsible for producing a neuropeptide linked to sexual reproduction. Overall, the role of transcription factor "egr1" in the context of social behavior clearly shows the link between genes and behavior. As a corollary to that described above, given an environmental cue "egr1" will induce or suppress the transcription of other genes. "Egr1" shows how social experience may trigger changes in the brain's gene networks. With higher capacity screening techniques, the expression of many genes simultaneously in response to social stimuli will provide a more complete picture. The neurobiology of drosophila mating behavior is an area of research that has studied in detail and has elucidated the genetic basis for behavior. The male courts a female based on pheromones from the female and previous experience courting other potential mates. The female accepts or rejects the male's courtship also based on pheromones, the acoustics of his courtship song, and her readiness to mate. The mating behavior of drosophila had been described almost a century ago, and the genetics of these behaviors have been studied for several decades. The current interest in neurobiology is trying to understand the neural circuits that provide the basis of action selection—how the brain maps sensory input, internal states, and individual experience to behavioral decisions. The pathways that govern the mapping of pheromones to the brain of drosophila are beginning to be understood in detail. Pheromones are detected by olfactory sensory neurons (OSNs).