Attracting the ladies is all about putting on the best performance, so male túngara frogs make sure that they stage a spectacular extravaganza. ‘They have these huge vocal sacs that are almost twice the size of their normal body’, describes Wouter Halfwerk from the Smithsonian Tropical Research Institute, Republic of Panama, adding that the males inflate the sacs as they serenade the females. But Halfwerk was intrigued; if the male frogs were so effective at advertising to females, who else was picking up on the ostentatious display and which sensory systems were these eavesdroppers using to check out the action? Halfwerk explains that fringe-lipped bats are particularly partial to a túngara frog snack, so he wondered which part of the amorous display a hungry bat might tune in to and which senses could the stealthy hunters use to home in on their boastful prey (p. 3038)?
Intrigued, Halfwerk set off into the rainforest to catch some bats. ‘That's the fun part’, he chuckles, describing how he used recordings of frog choruses to lure bats into a net. However, instead of using live frogs to test the bats' reactions, Halfwerk had access to incredibly life-like túngara frog models – made by artist Barrett Klein in collaboration with Halfwerk's colleagues Ryan Taylor, Rachel Page and Michael Ryan – that could put on a mating display realistic enough to seduce female frogs.
Having settled the bats into their new home, Halfwerk and colleague Patricia Jones, helped by May Dixon and Kristina Ottens, tested the animals' responses to the robot frogs' performances. Offering the bats a choice between a static robofrog and a singing robofrog that were 0.5 m apart, the duo was impressed to see that the bats only attacked the vocalising frog models. Robofrog's mating performance was definitely attracting the predators, but which component of the frog display were the bats locking on to?
The team tested whether the bats were attracted to the movement of the vocal sac or the shear size of the inflated organ. The bats enthusiastically attacked robofrog models that had been inflating their vocal sacs and they even responded to frogs that had been moving but froze as soon as the bat left its perch. The team also tested the range over which the bats could pick out the frogs and were surprised that the bats could detect their prey over an impressive distance of 4 m.
But which sense were the bats using to detect their frog victims? According to Halfwerk, the female frogs use vision and hearing to assess their males, but could the bats be using echolocation alone? First, the team placed the bats in darkness to force them to rely on echolocation and waited to see how well the bats targeted the robofrogs. The bats had no difficulties identifying calling robofrogs in the dark. However, when the team interfered with the bats' echolocation – by shielding the frogs' moving vocal sac with a transparent cup – the animals were completely flummoxed, selecting the static and mobile frog models at random.
Instead of relying on vision to identify male túngara frog victims, predatory fringe-lipped bats use their echolocation channel, and Halfwerk admits that he is impressed that it only takes two echolocation squeaks for the bats to locate their victims. However, he adds that predation by the bats is pretty bad for the sexual signal and he suggests that it could be one of the factors that helps to keep the size of the male's vocal sac in check, otherwise what would stop them expanding to three or even four times their own body size?