repeated in call groups. Calls increase in the number of pulses and note duration from beginning to end of a call group. One egg is laid at a time, generally attached to a piece of vegetation. The 14 millimeters (0.55 in) tadpoles hatch in only a few days, and undergo metamorphosis in early fall. Maturity is usually reached in less than a year. Production of male calls Almost all male frogs have a unique call with the purpose of attracting mates. These calls are defined by unique acoustic characteristics in order to attract female frogs of the same species. In frogs, auditory sounds are produced as a result of the interaction between the structure of the larynx (otherwise known as the voice box), vocal tract, and cartilages that control the flow of air out of lungs. Sound is produced when air hits the vocal cords, causing them to vibrate. The frequency of sound is dictated by the pressure of airflow through the larynx, as well as characteristics of vocal cords such as size and mass. Movements of various muscles in the throat and abdomen can create a pulsing sound. Using snout vent length as an indicator of body size, researchers found that a longer snout vent length corresponded to a lower frequency call and lower pulse rate as well as fewer pulses in general. The production of lower frequency sounds can be attributed to the slower movements of larger physiological structures. High frequency calls were observed to have a shorter duration and faster pulse rate. Other findings of this study on Acris crepitans male calling, which perhaps offers insight into frog calling in general, include relationships between physical anatomy and auditory characteristics. A larger middle ear volume corresponded to a longer call duration, and the number of pulses in the calls showed a significant negative relationship with arytenoid cartilage, vocal cord and constrictor muscle volumes. The pulse rate is also correlated with vocal cord, basal cartilage and constrictor muscle volumes, but not with arytenoid cartilage or dilator muscle volumes. Interestingly, northern cricket frog calls resemble calls of the Bufo genus even though two types of frogs are not closely phylogenetically related. Thus, the vocal mechanisms of the two frogs are assumed to operate the same way. Variation in male calling The calls at the beginning of a call group have been found to vary independently of calls from the middle