Fiddle-de-dee: The male fiddler crab is a fine example of how evolutionary pressure can select an exaggerated…

Fiddle-de-dee: The male fiddler crab is a fine example of how evolutionary pressure can select an exaggerated physical trait: while one claw is small and used for feeding, the other is grossly enlarged, reaching up to 2/3 of his body weight! The female has symmetrical, small claws. So, does size matter? 

Ornament vs. Armament: The large claw of the male fiddler crab is a sexual ornament, like the feathers of a peacock. With it, he waves flirtatiously at the susceptible female, enticing her to his sand burrow. The larger and more conspicuous his claw, the greater his chance at mating success. It is also an effective weapon, used to threaten and wage battles with competing males. But these are competing demands: large and light claws may be waved at lower energy costs, whereas heavy claws with powerful muscles are better in fights. Studies show that claws evolved to optimize fitness in both mating and fighting (REF:

Keeping Up Appearances: In a study performed upon a beach in Zanzibar, Tanzania, scientists tethered a female crab by super-glueing a thread to her carapace and anchoring it to a spike in the sand. If she was viewed by a solo male, a friendly waving at a leisurely pace of 11.5 waves/min ensued. But in the presence of male competition, his waving became more urgent, at 16.5 waves/min! (REF: Watch this little guy seemingly inspired by the Village People 🙂 YMCA crab dance with music

Honesty is Not the Best Policy: When a male fiddler loses his major claw, he regenerates a new one of similar size but much weaker fighting ability. Studies have shown that the male can bluff his way through fights with the weaker claw, in a form of dishonest signaling. This unfair advantage presumably makes up for the costs of claw regeneration. (REF:

A Cool Tool: If you think you now know all there is about the fiddler crab claw, consider this. Scientists measured body temperature of fiddler crabs subjected to a heat lamp and showed that the large claw actually acts as a heat sink, allowing the male to cool off more quickly with it. On a hot beach, this advantage may help offset the high energy costs of his exaggerated “male ornament”. (REF: )

✤ GIF:

#ScienceSunday      #HappyBirthdayHalfPintBuddy 

Birthday shout-out to Buddhini Samarasinghe !

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80 Responses to Fiddle-de-dee: The male fiddler crab is a fine example of how evolutionary pressure can select an exaggerated…

  1. Haha! Thank you for the birthday wishes, I am going to take the topic of this post as a compliment because he’s very cute! 😛

  2. Rajini Rao says:

    Birthday pinches, dear Buddhini! We susceptible females have to band together 🙂

  3. Fascinating share Dr. Rao – l love topics about Mother Earth’s creatures.

  4. Rajini Rao says:

    Same here, Vincent Peralta 🙂

  5. Rajini Rao says:

    Watch out for his pincer, Marceline Abadeer ! 🙂

  6. Gary Ray R says:

    Rajini Rao 

    Thanks for the excellent post on one of my favorite little sea creatures.  I always learn something from your work.

  7. Rajini Rao says:

    Thanks, Gary Ray R . I had such fun looking this up. It was impressive how much evolutionary biologists and scientists can dig up about the curious life forms around us. 

  8. Great topic to choose Rajini Rao for #ScienceSunday

  9. Rajini Rao says:

    David Dhannoo , #ScienceSunday  is a great excuse for me to get a post out. Things have been hectic at work, but it’s worth a special effort getting something out on Sunday. Thanks! 

  10. Azlin Bloor says:

    Another fabulous read, Rajini! I do enjoy your posts, I’m always learning something! Hope you’re chilling with a cup of something after your hectic week!

  11. Your post on the Gila Monster was one of my fave posts, still find that fascinating 🙂

  12. Rajini Rao says:

    Azlin Bloor , relaxing with my morning coffee, thanks! I think pumpkin pie is on the agenda today 🙂

  13. Rajini Rao says:

    Avivah Kravitz , thank you. I know my posts can be quite wordy 🙂

  14. Sunil Bajpai says:

    Thank you Rajini Rao! Enjoyed this post immensely. 🙂

  15. Rajini Rao says:

    Delighted that you did, Sunil Bajpai.

  16. Rajini Rao says:

    Marceline Abadeer , I don’t think I’ve been asked that before! I am fascinated by life forms and want to understand them at the most fundamental of levels. At first I wanted to be a doctor, but I tend to be squeamish about sickness 😛 Then I discovered the word “biochemistry” and arbitrarily decided that I would study it..may be when I was `8 years old. Turns out that I did indeed get my doctorate in biochemistry. I find the discovery process is thrilling, and once I learn something I have this urge to pass it along. Thanks for asking! 

  17. Sunil Bajpai says:

    It’s been suggested that peacocks’ long tails are a handicap that only the strongest males can afford because it makes them vulnerable to predators. It is thus an impossible-to-fake signal. Could there be such an element in this crab’s signalling too?

  18. Rajini Rao says:

    Sunil Bajpai , that’s a great analogy. According to the papers I read, there is a huge energetic cost to having large claws.  In the case of the peacock, it is purely an ornament, whereas in the fiddler crabs it is also used as a weapon. So there are competing fitness issues at work. Here is a quote from one of the papers referenced above: “However, it is important to note that the claws of all fiddler crab species that have been studied must effectively perform both of these functions. For instance, an individual with purely signaling morphology would be at an energetic advantage in both development and display, but would not be able to defend territory and should have low fitness. Alternatively, a crab with a shorter or excessively heavy claw, although successful in gaining and defending territory, would incur an energetic cost and may not be able to attract females, therefore having lower fitness.” 

  19. Jae Heme says:

    This is a cool adaptation. But just an observation on your initial premise being slightly misleading. Natural selection does not prove evolution. It’s still a crustacean. In fact, it is still a crab. Also, you commented that it is the evolutionary biologists who find amazing creatures. I would posit that evolution has nothing to do with discovery, or understanding the purpose of such design and adaptation. not does ones world view. Having said all of that, I still love your posts. You always share such colorful and interesting things from nature. I just don’t completely agree with your world view. So, keep up the good work.

  20. you know what they say about big claws…

  21. Rajini Rao says:

    Jae Heme , there is a saying: nothing in biology makes sense except in light of evolution. Natural selection applies to all forms of life, whether human or crustacean or plant, in form. Drug resistance in bacteria is an example of natural selection. In the topic of this post, natural selection explains the major claw of the fiddler crab quite simply: crabs with larger claws have more success in mating because of sexual selection by females. Crabs with stronger claws also defend their territory with more success. The trait (larger claw) is then passed down to the offspring of the successful mating. There is really no controversy about evolution in field of research science as there is in some sections of the public. 

  22. Brad Esau says:

    Most fascinating. And I must say that is a very comely wave (in the GIF). If I were a female fiddler crab, I’d be greatly impressed.

  23. Rajini Rao says:

    Brad Esau , even my non-crab female self fell for his cuteness, Brad Esau 🙂

  24. Just came by to visit.. 

    I love this post, really interesting about the crab. 🙂 Rajini Rao 🙂

  25. Jae Heme says:

    Just because there is a saying doesn’t mean the saying is true, or the subject of said saying is true. Regardless, I certainly don’t deny the role of adaptation and natural selection in nature. But your statement that everything we know about nature can only be explained through the lens of evolution is also false. If, for the moment, you suspend your belief in evolution, it changes nothing about how we understand the mutations of bacteria, the adaptations of crab claws and peacock tails. It would have zero effect on our ability to design medications and test disease, or have our pets cared for by a veterinarian. But we’re all told throughout grade school and college that we need the evolutionary construct in order to understand all these things, so most of us just accept it. But we don’t need evolution. In fact, the only thing evolution tries to bring to the table is the answer to where do we come from. We don’t need that answer in order to observe, study, and understand the world in which we live today. That’s all I’m saying. I’m not trying to change your mind about evolution, because you have your world view, and I doubt anything I say would change your mind. Only the Holy Spirit can bring that change because it is a heart change. My main point is that natural selection and adaptation don’t presume, require, demand, or otherwise need evolution to be true or informative. I do really enjoy your posts, and sometimes I repost them in my daily announcements for my students. But I tend to remove the evolution talk, as it’s not necessary. 

  26. Jae Heme if you don’t teach evolution you are simply being silly.  

  27. It is like a man flouting his biceps?! I wonder how many more of the species do this apart from this crab and man!

  28. Jim Donegan says:

    Sarah Wooller It’s far worse than silly. However – public forums, diverse opining, etc!

  29. Rajini Rao says:

    Actually, it is a major handicap if students in a science class are not taught the principles of evolution. That said, I know of several scientists who overcame this disability and went on to distinguished science careers, so all is not lost 🙂 I should add that in all my years as a research scientist, I have not encountered a single evolution denier among my professional colleagues/students/teachers, not one. 

  30. Jim Donegan Well that depends. Silly really means that anything further that the person goes on to say is not worth taking into consideration, which is not totally minor, so I don’t use it lightly. 

    However, all is not lost – tap the word adaptation into wikipedia and then do a word count of evolution in the resulting article – it comes up 91 times, which will leave few students in much serious doubt. No doubt the article is under constant attack – but equally of no doubt there will be fight back.

  31. Jim Donegan says:

    Sarah Wooller Indeed! I knew what you meant but just wanted to underline your point which was perfectly sound and correct.

  32. Jae Heme says:

    I’m sorry, I really wasn’t trying to start a debate on the veracity of evolution. Just meant to point out that you don’t need evolution to understand how adaptation and natural selection work. I do teach the principals of evolution. But I personally do not believe in evolution. Calling me silly or irrelevant doesn’t prove evolution to be true. Nor does the fact that lots of people think it’s true. But, if Wikipedia has lots of references to evolution when you search for adaptation, oh, well then it must be true. What exactly is the deficit experienced by those not convinced by the mob rules belief in evolution?

  33. Rajini Rao says:

    Got it, Jae Heme .

  34. Jae Heme says:

    Rajin Raj thanks, and seriously, Keep up the good work. I really do enjoy your posts. 🙂

  35. When I was nine I waged a daily and relentless battle against these guys. I feel badly about it now but at the time throwing large rocks into the mud made for great sport.

  36. so adorable ,I think I call him Synko. 🙂

  37. Rajini Rao says:

    That’s a good name for a fiddler crab, Carson Kennedy 🙂

  38. For some reason he makes me think of Resident Evil…

  39. Mary T says:

    Cool post Rajini Rao ~ He’s a very cute crab 🙂

  40. Rajini Rao says:

    Nice try at trolling, Ana Lopez 😉

  41. If the new claw is a little smaller, a larger claw implies a greater likelihood of never having lost a fight

  42. Ana Lopez

    You’re uncool but boring

  43. Linda Cohen says:

    Fascinating the crab  appears to be dancing to the Native American music I was listening to, now it is switched to an African chant and he is still keeping perfect time.

  44. I’ve always wondered, why is it that in animals it is the male that put’s on this display of ornaments and colour to attract the female of the species whereas in humans it is the female of the species who have evolved and also dress up to attract the male??

  45. Rajini Rao says:

    ajith kunnathully , an excellent question that would require us to delve into the evolution of social behavior in humans. I suspect that the skewed emphasis on “adornment” of women is quite recent in the overall time scale. 

  46. The crab dancing YMCA is hilarious!! Certainly made my day!

  47. I tend to disagree that adornment of women is a recent phenomenon. In humans it is the female who is designed to attract the male, a case in point is that the human female breasts are definitely not the ideal design for ease of giving milk and ease of suckling to the offspring, unlike in other mammals, it serves more as an item of display of fertility and allure to the male of the species.

  48. Rajini Rao says:

    ajith kunnathully but studies show that men are not hard wired to find breasts attractive. See:

    Breast feeding is a biological function, as in all mammals. But seeing breasts as a sexual object is cultural. There’s even a Wiki article on breast fetishism, which is relatively recent. 

  49. Interesting, I went through it, but does it mean that in the stone age a man could not differentiate visually between a beautiful and an ugly woman just like animals? Tribal men may not be turned on by breasts due to over exposure but even early humans would have found some women more attractive than others, this would be unique to humans I presume.

    The breasts were just one example, men are turned on by the texture of the skin, the attractiveness of the eyes, the curves in the body, the smile, the hair….hell I feel women have been designed to turn men on…some women more so than others, and thats unique to the human species.

  50. And I further feel that the way modern women dress and make up to make themselves more attractive to men is an extension to age old pre programmed dna coding, any amount of modernity cannot completely change an ancient core programme.

  51. Rajini Rao says:

    ajith kunnathully , you say, “hell I feel women have been designed to turn men on…some women more so than others, and thats unique to the human species.” From a biologist’s view point, nothing could be more wrong or willfully anthropocentric. All animals have developed means of attracting the other sex, via appearance, pheromones or behavioral cues. We are not that special, really. Also, I know of no genetic or other scientific evidence that the way one dresses is “age old pre programmed DNA coding”.

    I’m afraid there are personal opinions and then there are scientifically informed opinions. This isn’t my area of expertise, and I’m pretty sure that you are not a biologist. We should just leave this discussion at that! 🙂  

  52. I like crabs (said like the little boy who says I like turtles)

  53. Rajini Rao says:

    The ones that scuttle sideways are so cute, David Nicholl !

  54. At Wrightsville Beach me and the kids have a great time searching for ghost crabs with a flashlight at night.

  55. Rajini Rao says:

    This is the second time someone has mentioned ghost crabs. I need to look them up! 

  56. We’ll, ur right, iam an engineer in electronics and communication by academics and a senior manager by profession :), but be it electronics, computers or biology, logic always applies and holds true even in evolution and dna programming even though we may not yet understand the underlying logic now, but I’m sure that the future will provide the right answer. 🙂 anyway thanks for engaging my query and let us leave it at that :))

  57. ajith kunnathully

    How do we know that human males DON’T have methods for ornamentation and attracting females? Several studies indicate that females prefer males with beards as beards are thought of as giving men a more “paternal” face. Also when trying to think about why humans tend to have relatively little ornamentation it pays to look at our close relatives. Most great apes and other monkeys aren’t very ornamented either. Instead, male apes tend to be larger and more aggressive, and aggressiveness can be shown in display. And this tends to be true of male humans as well. Larger, more “confident” men (aka just the right amount of aggressiveness) tend to be the most attractive to women. Lastly, the alteration of human mating patterns from the great ape paradigm might explain why females ornament themselves. In gorillas, a single male tends to be in control of a group of females and is responsible for protection of all of the group and in return he gets mating privileges. In humans, we have a semi-monogamous mating system, in that the ideal we point towards is monogamy while in practice it’s more complicated. In a monogamous system, the energetically expensive act of child-raising is placed on the mother, i.e. Pregnancy, lactation, etc. while males would tend to be free towards multiple matings. However, our “aggressive” nature means that any child left with just a mother was more likely to die so therefore the development a two parent system was more efficient. But how does a woman ensure the attentions of a single male? Looking as attractive as possible, aka ornamentation. Now keep in mind we’re talking about humans and removing our own bias is tough but this is the set if hypotheses behind ornamentation and why it has developed in humans the way it has. It could still be disproven but it makes sense and some of it’s aspects have been borne out in studies.

  58. Joshua Baecker you have a point, I accept that in human males looking strong and manly would be attractive to female’s. But it is only in humans that the female of the species also develop physical traits that visually attract the male, the early humans were not semi monogamous and had a similar family structure as the great apes with one alpha male having multiple mates, but I would still assume (maybe wrongly) that the males would still be more attracted to the women with larger breasts and curves, despite some current research suggesting that in some African tribes over exposure to naked breasts had an effect of it not being visually a turn on for men, but I am sure that if a lineup of women of the tribe was done and the males asked to select their mate, most would go for women with the greater assets.

    To assume that ornamentation in women evolved due to social necessities appears to me to be a bit far fetched, compared to any other species the human female appears to have developed physical characteistics similar to ornamentation in males of other species that would make some women stand apart from others.

    Maybe our current evolutionary theories of humans are not quite right??

    Iam not an expert in evolution, but logically speaking something does not quite gel here.

  59. Why is it so difficult for us to believe that if beards and muscles were designed to attract women, then long hair and big breasts and curves were designed to attract men??

  60. Deeksha Tare says:

    An now I wake up 😦

    Amazing post Rajini Rao !! Cheers! 🙂

  61. Kawthar A says:

    Oh..I have learned something new today..that’s a great post!! hands down 🙂 Rajini Rao 

  62. Rajini Rao says:

    Thanks, Kawthar A 🙂

  63. Kawthar A says:

    Hello back at ya 🙂

  64. Rajini Rao says:

    Even if we do have smaller claws, Kawthar A , we women can use them well 😉

  65. Kilee Bough says:

    Lmao!! I had some and saw them do it. It was like they are doing the robot to….. Domo arigato Mr roboto. Haaaa

  66. or perhaps the claw is a tissue modification from the bacteria that the crabs eat and we are retro engineering the reasons for use .Also large claws help in excavation of the burrows. cool post .

  67. 1 0 says:

    Such is a tool after all, no doubt!!!

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