To attract females, male Uca lactea fiddler crabs wave their larger claw in the air. As soon as an interested female approaches the entrance to his burrow, the male will emit vibrations and repetitive pulses. According to new findings published in The Science of Nature, these signals are very informative and provide females with details ranging from size to stamina. It’s like Morse code.
During the mating season, males construct a raised mound, or "semidome," close to the burrow where they perform the waving display for females searching for mates. As a female approaches, the male enters his burrow and produces vibrations: leg vibrations, rapping, body thumping, and stridulation. He continues to do so after she follows him in. If the courtship is successful, the pair will copulate. While these vibrations are considered acoustic signals that induce mating, it was not clear whether the vibrations influence females’ mating decisions.
To see how vibrations help females pick a mate, Fumio Takeshita of Nagasaki University and Minoru Murai from the University of the Ryukyu used a female dummy (a dried female specimen) to elicit courtship vibrations from 48 Uca lactea males on a tidal flat on Nagaura Island in Kumamoto, Japan.
After analyzing their recordings, the duo found that these acoustic signals consist of repetitive pulses. The dominant frequency of the pulses decreases with male carapace width; the length of the pulses decreases slightly when more vibrations were repeatedly produced, and the interval between pulses increases with increasing repetitions. That means vibrations are a way for males to signal their endurance and stamina to potential mates. It goes hand-in-hand with the initial waving display: Males who are able to wave their claw higher and for a longer period of time typically have the most success.
The team also observed males courting real females in the daytime during outgoing tides in the summer of 2014 and 2015. These females were much more likely to enter the burrow of males who could repeatedly produce a higher rate of pulses in succession. "This indicates that the females use the male vibrational signals to decide whether to enter the burrow or not," Murai explained in a statement. But once the female is in the burrow, his continuous production of vibrations no longer plays a role in her decision to mate.