Male fiddler crabs (Uca annulipes) try to get the ladies by waving their one big claw. Females prefer fast wavers, which is a lot of work (see some examples in the videos below). A new study shows that males are willing to wave harder when there is competition.
In a study published last month in Biology Letters, scientists working on the beaches in Zanzibar, Tanzania, tethered a female fiddler crab 10 cm in front of a randomly designated male that could either see or not see his neighbors. (The female was held in place by superglueing a 1 cm piece of cotton thread to the crab’s carapace and attaching it to a nail pushed into the sand.)
In the low-competition treatment, scientists startled the neighboring males into their burrows and enclosed them by covering the holes with bottle caps. Without other males competing, the solo male wave rate was 11.5 waves per minute. When the male crab could see his fellow competition, his median wave rate was about 40 percent faster — 16.5 waves per minute.
When it seems fiddler crabs throw their claws in the air, and wave them like they just don’t care — they do. The crabs care whether there is immediate competition.