A widely-used pollutant causes reversal of conspecific mate preference in a freshwater fish

Chemical communication is an important mechanism of mate choice across the animal tree of life. However, anthropogenic perturbation of the signaling environment can disrupt chemical communication and result in a breakdown of behavioral reproductive isolation. Here we find that calcium hydroxide (Ca(OH)2), a common and deliberately introduced anthropogenic pollutant, profoundly disrupts chemical communication in the swordtail fish Xiphophorus birchmanni. Moreover, it acts in a way that should promote hybridization, causing female X. birchmanni to prefer the chemical cues of the parapatric sister species X. malinche. We find that this flip in the direction of preference is attributable both to a reduced preference for conspecific signals and to a coupled strengthening of preference for sister-species signals.