My, what a big claw you have


Porcelain fiddler crab (Uca annulipes)

Yes, ladies, I *am* happy to see you! Image by Ria Tan (via Flickr).

Male fiddler crabs wave their giant claws to get the attention of females. Females prefer males that wave a lot, in line with a common theme in female choice: making the male work for it. Waving that big thing around is more than just an advertisement. The thing is heavy to lift, so the male has to be in good shape to keep up all that waving. It’s also dangerous. The more he waves that big sign saying, “Hey, look at me!” the more predators will take notice.

How do males determine how often to wave if too much waving will get them dead, and too little will keep them from getting mates? As we’ve seen before, many males in the animal kingdom have to walk this fine line between life and sex, and they all cope with it in different ways.

New research from Richard Milner, Michael Jennions, and Patricia Backwell at The Australian National University has found the answer for the fiddler crab, Uca annulipes. The more competition there is, the more a male will wave. The research appears in the Journal Biology Letters.

These are the things a female fiddler crab finds sexy:

  • Males with a faster waving rate (the ones who wave most often);
  • Males who wave slightly earlier than competitors. In this species, males tend to wave in sync with each other, though some may wave a little before the others. And it’s hot;
  • Bigger claws. That’s right: size matters.
  • An awesome bachelor pad (females make their final decision based on the quality of the male’s burrow).

Because the first item on the Lady Fiddler Crab’s Guide to Finding True Love is one most likely to get a guy killed, the authors focused on this one. They wanted to know how the trade-off was managed from the male’s point of view. Specifically, they asked whether males were more likely to put themselves at risk if they were up against competition.

They directly manipulated the level of competition experienced by a given male. Females were tethered in place by a piece of string in front of the test male. Then, the researchers created a situation with either low or high competition.

For the low competition scenario, other males in the vicinity were first cleared out. The researchers “startled all crabs into their burrows by standing up and approaching them.” I’m sure the other male crabs wished they also possessed these awesome powers to drive off their enemies.¬†They then made sure males couldn’t come back out before the trial was over, by placing bottle caps over all the burrow entrances, except one.

When the lucky guy came out, they measured how often he waved at the female, over 30 seconds, once he had time to get started.

For high competition, only some burrows were closed off. They then measured the wave rate of the same male as in the low competition scenario. This whole process was repeated with 50 different males.

They found that males will wave more if they are surrounded by competition. In the high competition scenario, the median wave rate was 16.5 waves per 30 seconds. This was compared to about 11 in the low competition scenario.

100911 - Porcelain fiddler crab, female

All right, boys, let’s see you shake it! (Image via Flickr)

Even without competition, males still want to put on a good show for the female, but they’re not going to go into overkill without more incentive. In fact, the wave frequency went up with increasing numbers of rival males. They save the really intense waving for the toughest situations. When only one rival is near the focal male, he waves about 16.5 times in 30 seconds. That goes up to over 2o when 4 rivals are present.

The results from this study weren’t necessarily the way it had to be. Males might benefit from putting their best claw forward, regardless of the competition. Receptive females are rare. For every 45 males, there is only one female with eggs ready to be fertilized.

Besides that, females don’t always pick the best male from a single group. She might shop around, looking at males in several groups before making up her mind. Males should therefore try hard to get her interested, even if no one else is around.

The finding that males save the good stuff for when they need to stand out from the immediate crowd suggests that it really is costly for males to always perform at their peak. After all, it’s no good being super sexy if you’re just going to die before cashing it in.

Reference:

Milner, R., Jennions, M., & Backwell, P. (2011). Keeping up appearances: male fiddler crabs wave faster in a crowd Biology Letters DOI: 10.1098/rsbl.2011.0926

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