Biologist Discovers What May Be World's 'Pickiest' Mates

A study conductedby a biologist at the University of California, San Diego thatappears in the August issue of the journal Animal Behaviourfound that females of the species Uca crenulata maycheck out 100 or more male fiddler crabs and their burrows beforefinally deciding on a mate.

"As far as Iknow, no other species has been observed sampling nearly asmany candidates as the California fiddler crab," saidCatherine deRivera, who conducted the study while a doctoralstudent and a lecturer at UCSD. She is now a research biologistat the Aquatic Bioinvasions Research and Policy Institute, ajoint entity of the Smithsonian Environmental Research Centerand Portland State University.

deRivera and a groupof UCSD students who assisted her conducted their observationsin the Sweetwater River estuary in Chula Vista, south of SanDiego, near the Mexico-U.S. border. She said previous studiesof mate selection in other animals, such as birds and the natterjacktoad, found that females of most species typically sampled onlya handful of potential mates before making a final selection.

"Most animalssample just a few mates, presumably because search costs overridethe benefits of lengthy searches," she said in her paper.But female California fiddler crabs are much pickier, she discoveredin her study, checking out male suitors and their bachelor padsan average of 23 times before making a final selection. Oneparticularly choosy crab visited 106 male burrows, fully entering15 of them, during her one hour and six minute search.

Why are female fiddlercrabs so picky? The survival of their offspring, deRivera foundin her experiments, appears to be strongly linked to the sizeof their mate and, more importantly, his corresponding abode.

"The size ofthe male's burrow affects the development time of hislarvae," she said. "A burrow of just the right sizeallows larvae to hatch at the safest time, the peak outwardnighttime flow of the biweekly tidal cycle."

"Wide burrowsspeed incubation, so they cause the larvae to hatch too earlyand miss the peak tides. This research provides one of the firstexamples of how choosy resource selection can help offspringsurvivorship."

Male fiddler crabsattract suitors by standing in front of their burrows and wavingtheir enlarged claws at prospective female passers by, muchas humans motions "come here" with their arms andhands

"The Californiafiddler crabs use a lateral wave that looks much like a humanbeckoning 'come here'," deRivera said. "Italso seems to serve as a 'come hither' signal, as a male waves,standing at his burrow entrance, and interested females comeover."

Interested femalesinitially eye the males, who select their burrows based upontheir body size, and if they're interested, partiallyor fully enter a burrow to size it up.

"The burrow openings,which are circular, are just big enough for the owners to getin," deRivera said. "Crabs enter burrows sidewaysso have to fit in front to back and top to bottom."

When a female hasfound a mate and burrow to her liking, typically one that isabout the same size as she, either she or the male will plugup the opening of the burrow and the couple will mate and incubatetheir eggs, which later hatch and release tiny crab larvae thatare quickly flushed from the estuary by high night tides.

deRivera found thatlarger female crabs couldn't be as picky about choosingmates as their smaller counterparts. They took less time, shenoted, because they entered fewer burrows, primarily becausemany of the burrows they passed were too small to accommodatethem and successfully incubate their eggs and release theirlarvae.

"Larvae weresuccessfully released during high-amplitude nocturnal tidesonly when females incubated in burrows that allowed the larvaeto exit the estuary swiftly and thus reduce predation risk,but not when females incubated in burrows that were too wideor narrow," deRivera writes in her paper. "The effectof burrow aperture on incubation duration may explain why femalessampled many male burrows as they searched for a mate and whyfemales of different size classes selected and sampled differently."