Females significantly preferred calls accompanied by vocal sac inflation (Fig. 1a, b) over those with no inflation (Fig. 1a; 14 females responded to inflated versus 6 to not inflated; one-tailed exact binomial test: P!0:05) and failed to prefer the not inflated stimulus (Fig. 1a) to the blank control (9 females responded to not inflated versus 13 to blank, NS). The rectangle stimulus (Fig. 1c) failed to increase call attractiveness (10 females responded to rectangle versus 9 to not inflated, NS).