Vocal cords are present only in terrestrial vertebrates who breathe air using lungs. Vocal cords were formed during the evolution of upper respiratory pathways, specifically larynx. As air inhalation or exhalation are required to produce sounds, they could not be formed in vertebrates who live only in water (fishes). Although birds have a larynx, they use syrinx to produce sound. This organ is located in the trachea. As a result, birds do not have vocal cords too. As a result, amphibians, reptiles, and mammals have larynx covered with vocal cords.