# Why are some Scorpions sexually dimorphic?



## snappleWhiteTea (Jul 8, 2010)

Why would a male scorpion need to look different from a female? they don't see each other, the male doesn't have ornaments that the female is attracted to. The only thing i can think is that with some centruroides species the male is thinner, so maybe it could have survival advantages, or mating advantages. maybe the thinner ones move around better?


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## gromgrom (Jul 8, 2010)

they obviously do it for our convenience 

no clue, it has something obviously to do with social/evolution/breeding patterns, as everything in nature.


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## skips (Jul 8, 2010)

Good question.  I'd love to see if someone has done a study on these scorpions.    In this case there probably is a reason for sexual dimorphism.  Maybe the females need to protect their eggs and babies.  Maybe they need increased body mass to sustain egg production, where the male doesn't have that need so has no reason to sustain that large of a mass.  The study I'm pasting the abstract of below (closest thing I could find) suggests a correlation between the ontogeny of the scorpion and pedipalp size.  Maybe there's some sort of sexual selection for males who wont kill the female why courting...or my favorite explanation, it's a spandrel.  

Speaking just as a student interested in evolution, it's very common even among evolutionary biologists to assume that there has to be some reason for every facet of a species ecology or morphology.  Some things are just not selected strongly against and have made it through the years because they are genetically correlated with another trait.  Things don't always need a concrete discernible benefit directly relating to their existence.  Sometimes traits just come along for the ride (an arguable claim).

"Sexual dimorphism, in the form of sexual difference in pedipalp size, is highly variable among pseudoscorpions in the family Chernetidae. Male pedipalpal chela size ranges from 60 to 150% of that of the female. This study investigated potential life history costs associated with sexual dimorphism, and assessed genetic variation for male chela size in the chernetid Dinocheirus arizonensis. Results demonstrated that nymphal development is prolonged in males. A causal relationship between pedipalpal size and development rate was suggested by (1) a positive correlation between sexual difference in developmental period and sexual dimorphism in chela size, and (2) the absence of sexual differences in adult body mass after removal of the pedipalps. Parent-offspring regression provided some evidence for additive genetic variation underlying male chela size, but also suggested a high genetic correlation between the sexes for the trait. Life history compromises and ecological parameters (population density, patterns of female emergence/receptivity) appear to be important factors influencing sexual dimorphism. However, the degree to which sexual dimorphism actually evolves may depend critically on both the within-sex genetic variance and the between-sex genetic covariance existing in particular chernetid populations."

Life History Consequences of Sexual Dimorphism in a Chernetid Pseudoscorpion
Author(s): David W. Zeh
Source: Ecology, Vol. 68, No. 5 (Oct., 1987), pp. 1495-1501
Published by: Ecological Society of America


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## snappleWhiteTea (Jul 8, 2010)

skips said:


> Maybe there's some sort of sexual selection for males who wont kill the female why courting.


^
this is what i would guess, the males that are strong enough to make it to point of mating, but not to strong to where it kills the females during mating get to reproduce.



gromgrom said:


> they obviously do it for our convenience


and yeah grom, god did it for the hobbyist, especially the arachnoboards members. :}


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## Galapoheros (Jul 8, 2010)

I was wondering about this a few months ago and thought I might have come up with a possibility.  The theory doesn't fit all scorp sexual dimorphism but might in general.  In scorpions and spiders, the tendency for males is to have longer legs, kind of spindly looking and sometimes the trend in size is to be smaller.  In many species, the male is the one doing the traveling to look for a female.  I was thinking that a smaller size and longer legs is more efficient in walking long distances.  But that doesn't explain some other differences:?


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## snappleWhiteTea (Jul 9, 2010)

Galapoheros said:


> I was wondering about this a few months ago and thought I might have come up with a possibility.  The theory doesn't fit all scorp sexual dimorphism but might in general.  In scorpions and spiders, the tendency for males is to have longer legs, kind of spindly looking and sometimes the trend in size is to be smaller.  In many species, the male is the one doing the traveling to look for a female.  I was thinking that a smaller size and longer legs is more efficient in walking long distances.  But that doesn't explain some other differences:?


that makes great sense. the ones that couldn't find mates because of short legs, obviously didn't pass on short legs.


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## Michiel (Jul 11, 2010)

snappleWhiteTea said:


> Why would a male scorpion need to look different from a female? they don't see each other, the male doesn't have ornaments that the female is attracted to. The only thing i can think is that with some centruroides species the male is thinner, so maybe it could have survival advantages, or mating advantages. maybe the thinner ones move around better?



Why do you have a penis and your mom doesn't?? 
Come on man, nature invented this so we can separate them.....

Okay, that was the funny bit. This is a interesting question indeed, and I don't have a beautyfull concise answer to it. Mabye males have longer legs, because they are far more active (looking for females every night) than females, who spend more time in the burrows/ retreats. 

Question: where do Centruroides species hide?
Answer: under bark, in crevices
Question: why do some males of Centruroides have elongated metasomal (= sexual dimorphism) segments and not a "Androctonus like tail"? 
Possible answer: Because they might also mate in the crevices, under bark. Then the male could easily reach the female to make the sexual sting, if necessary. That's why I think they have flattened bodies and elongated metasomal segment. 

So we already have two possibilities: a) higher activity level in males, radiating more far from their retreat. b) elongated metasomal segments (=sexual dimorphism) in males for purpose of sexual sting (female does not apply a sexual sting and lacks the elongated metasomal segments)..


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## Galapoheros (Jul 11, 2010)

Hey that's a good theory about the longer tail on some male species.  I used to think the sex sting was kind of a rare thing but have seen it more often since then, common in some species and not others.  When I put my H. arizonensis pallidus together, the male grabbed her.  There was no struggle from her.  He reached over and poked her in the back, left the aculeus in for several seconds.  Since I had not seen H. arizonensis mate before, I wasn't 100% sure if it was aggression or mating behavior at first.  So I said, ..hey, that's enough of that and took the stinger out with my finger.  I wonder how much venom and/or if it's a different mixture ratio than usual when mating.  Here's the male and female right after the sting, made me think it's routine with the species.


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## skips (Jul 11, 2010)

Do scorpions identify mates by sight or more by pheromone or body language?


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## Galapoheros (Jul 12, 2010)

imo, ime, pheromones and "body language", vibrations.


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## snappleWhiteTea (Jul 12, 2010)

> Because they might also mate in the crevices, under bark. Then the male could easily reach the female to make the sexual sting, if necessary. That's why I think they have flattened bodies and elongated metasomal segment.


you know, i was thinking the same thing after i posted the thread.

also,
if any one has any more examples of species that are sexually dimorphic please share if you have pics.


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## snappleWhiteTea (Jul 12, 2010)

skips said:


> Do scorpions identify mates by sight or more by pheromone or body language?


i think the second things you stated are more likely, according to what i read about scorpions eye sight. (which isn't much).


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## RyoKenzaki (Jul 12, 2010)

In my humble opinion, it could be to make the mating ritual easier
EG: Longer metasoma in bark scorp allowing male to sting the female easier to calm her down, more bulbous or gap between chela allowing male to grap the female firmly, female body mass bigger to enable her to have more energy to give birth etc 
Nature is the best biological engineer ain't it?


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## Michiel (Jul 12, 2010)

The median eyes can form a crude image of the surroundings of the scorpion. The lateral eyes are even less developed, and they are more or less "light-dark/day-night" detectors.....

The differences in metasomal segments and in the form of the chela (males having bulbous chela), one or both characters, can be seen in Centruroides, Babycurus, Lychas, Tityus, Uroplectes and some other Buthid genera. And species of other families.

Now a different question. Why do males of some species have different coloration patterns than females of the same species? (like in Tityus zulianus and Rhopalurus junceus i.e.). Is it a coincidence or does it have a certain use.
In birds, males are more prominently and brightly colored, but they have well developed eyesight, unlike scorpions.....


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## Galapoheros (Jul 12, 2010)

It makes me wonder that, if the mature males tend to be out and about more, maybe the color diff helps them be undetected by their local predators(?).


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## snappleWhiteTea (Jul 12, 2010)

RyoKenzaki said:


> In my humble opinion, it could be to make the mating ritual easier
> EG: Longer metasoma in bark scorp allowing male to sting the female easier to calm her down, more bulbous or gap between chela allowing male to grap the female firmly, female body mass bigger to enable her to have more energy to give birth etc
> Nature is the best biological engineer ain't it?


yeah,
 that pretty much whats being said.


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## Michiel (Jul 13, 2010)

Galapoheros said:


> It makes me wonder that, if the mature males tend to be out and about more, maybe the color diff helps them be undetected by their local predators(?).



I don't think so, because scorpions are nocturnal. Most of their predators also and I doubt that these can differentiate between, let's say: dark reddish and dark brownish at night. Even human eyes are not able to do this, and scorpion eyes are much less developed. 

On the other hand, scorpions that live on lighter substrates, are lightly colored and vice versa. Frankly, I have no idea why there is difference in coloration between sexes of some scorpions. I can't think of a single sound theory, maybe the experts in here have an answer...


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## Nomadinexile (Jul 13, 2010)

One way to start looking into it, would be to try and compare sexually dimorphic and other species.    It would be a bit complicated trying to compare say 20 total species.   But it would be interesting.   It should serve some purpose though right?   Otherwise, they wouldn't evolve that way, I think.  :?


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## Galapoheros (Jul 13, 2010)

Michiel said:


> I don't think so, because scorpions are nocturnal. Most of their predators also and I doubt that these can differentiate between, let's say: dark reddish and dark brownish at night. Even human eyes are not able to do this, and scorpion eyes are much less developed.
> 
> On the other hand, scorpions that live on lighter substrates, are lightly colored and vice versa. Frankly, I have no idea why there is difference in coloration between sexes of some scorpions. I can't think of a single sound theory, maybe the experts in here have an answer...


Yeah I was reaching there, I was considering the nocturnal thing and the night vision of some nocturnal mammals is really good, who knows exactly how well some see with so little light at night, some much better than humans do(?)  I wonder if trained scientists have tried to figure it out yet, sure seems like they would have but maybe not.


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## Aztek (Jul 13, 2010)

And also the males roam more.
I guess they need tighter bodies to find hiding spots as well.


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## skips (Jul 15, 2010)

Nomadinexile said:


> One way to start looking into it, would be to try and compare sexually dimorphic and other species.    It would be a bit complicated trying to compare say 20 total species.   But it would be interesting.   It should serve some purpose though right?   Otherwise, they wouldn't evolve that way, I think.  :?


you could compare in the way that you can get ideas, but the reasons probably differ widely between species due to differing life history and ontology.  Birds are sexually dimorphic, color-wise, because of competition (i.e. peacocks and birds of paradise).  But again, though these traits may have evolutionary reason to happen, some traits are just correlated to other traits genetically and persist through time because they're not heavily selected against.  Not many people get published though for "there is no underlying reason for my research."



Galapoheros said:


> Yeah I was reaching there, I was considering the nocturnal thing and the night vision of some nocturnal mammals is really good, who knows exactly how well some see with so little light at night, some much better than humans do(?)  I wonder if trained scientists have tried to figure it out yet, sure seems like they would have but maybe not.


I tried to look around but came up with bubkiss on google scholar.  I feel like they probably see poorly, but may have far more rods than cones allowing them to see at night, not in great color, but at least see movement well.  Having a lot more cones wouldn't be so beneficial because rods allow you to see movement better, if I remember correctly that is...

it may not have to do with hiding so much as standing out, like poison dart frogs?  i'm not up at all on the ecology of scorpions.  what are these scorpions main predators?


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## Michiel (Jul 16, 2010)

Predators can be large frogs, lizards, snakes, birds, rodents and other arthropods.


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## snappleWhiteTea (Jul 28, 2010)

skips said:


> you could compare in the way that you can get ideas, but the reasons probably differ widely between species due to differing life history and ontology.  Birds are sexually dimorphic, color-wise, because of competition (i.e. peacocks and birds of paradise).  But again, though these traits may have evolutionary reason to happen, some traits are just correlated to other traits genetically and persist through time because they're not heavily selected against.  Not many people get published though for "there is no underlying reason for my research."
> 
> 
> 
> ...


I would assume ground squirrels to be there main predator, only because they are really tough and I've seen them take down some nasty prey. not to mention there are tons of them here!






and the comparing to other species is what made me think to ask here, peacocks have ornaments that attract mates, they prove fitness to the females this way, but scorpions in a way are blind, at least they would be to ornaments. but people here have made a lot of sense with theories, i think it simply that males have a longer way to roam.


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