# how many spiderlings to buy to guarantee I get a female



## CorySpaceCowboy (Sep 1, 2011)

I would prefer the answer come from a statistics professor. But if none are available, could you just tell me how many spider-lings I should buy to pretty much guarantee that at least one will be female. Is it simply 50/50 per spider? or do some breeders purposely ship out suspected males as a priority? Forgive my ignorance, its just that I've gotten 3 tarantulas in the past sold as spider-lings, but what I got were 2 inch spiders and they all ended up being male. I can not afford to buy a guaranteed female, and would much rather see it grow up anyways.


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## zorora (Sep 1, 2011)

I'm sure you know there is no way to guarantee a female but it would definitely increase your odds of getting one buying 3-5 true slings.  I think once they get a little bigger as your "2 inch" Ts were they would be easy for a seller to figure out the females and sell the rest as "unsexed"
To bad some people are dishonest but they are in every hobby, workplace, family, ect.
Good Luck! 
 Sorry I'm not the Stats Professor.


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## Amoeba (Sep 1, 2011)

I just read this in the Tarantula Keepers Guide. If you buy one spiderling you have a 50/50 of getting what you want if you buy two it's a 75% chance of getting at least one of what you want. I don't remember the next two but if you get five slings there is around a 91% chance of you getting at least one spider of the gender you want.

Reactions: Like 1


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## captmarga (Sep 1, 2011)

If they are tiny slings... three to five (3-5).  I bought three P formosas in a lot (not caring gender) and it looks like all three are male. 

Think about it this way - you can flip a coin ten times in a row, and statistically, it is possible all ten flips could come up heads. 

If you have the room to keep five slings, and they are less than a sexed female, I'd recommend that. This also depends on how popular the T is, and if you will be able to sell off males later, or just want them because you like that species. 

Marga


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## zorora (Sep 1, 2011)

FLAmoeba said:


> I just read this in the Tarantula Keepers Guide. If you buy 1 spiderling you have a 50/50 of getting what you want if you buy two it's a 75% chance of getting at least one of what you want. I don't remember the next two but if you get 5 slings there is around a 91% chance of you getting at least one spider of the gender you want.


Now that's a professor answer!  Sounds pretty good, I wonder though...   and I'm sure it would differ with species, not to mention each sac, but if you have a sac of 100 slings will 50 be male and 50 female?  That's where the numbers get a little crazy, but I think your Stats do the best for getting at least one like you said.  
@OP I think your guarantee stats are as good as Vegas Stats, I hope you get what you want.


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## Amoeba (Sep 1, 2011)

When my roommate(s) aren't around ill pull out my copy of the TKG and give you the exact numbers. I'm sure there is a certain amount of fluctuation with the male to female ratio (as with the human population)  I'm actually in my first stats class this semester. but honestly where is OP gonna be in a year or so to ream us all out for being wrong when he ends up with five sexed males. :bruised: Just kidding and my calculator is dead so I can't do the probability function for you either


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## mickey66 (Sep 1, 2011)

I don't remember where I read this but it said 1 in 4 slings are female.....for what its worth.

Reactions: Like 1


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## zorora (Sep 1, 2011)

could just buy the whole sac?:biggrin:


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## CorySpaceCowboy (Sep 1, 2011)

FLAmoeba said:


> I just read this in the Tarantula Keepers Guide. If you buy one spiderling you have a 50/50 of getting what you want if you buy two it's a 75% chance of getting at least one of what you want. I don't remember the next two but if you get five slings there is around a 91% chance of you getting at least one spider of the gender you want.


I like your answer, I hope your right.



mickey66 said:


> I don't remember where I read this but it said 1 in 4 slings are female.....for what its worth.


I don't like your answer, I hope your wrong.

But if you are, ones odds of getting a female by buying 4 would be 50%?? (is that correct?[if so I REALLY hope your wrong.])


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## Ben Oliver (Sep 1, 2011)

i would say if you were to buy 4 slings you should get at least one female. i don't gamble or play the lottery


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## Zoltan (Sep 1, 2011)

The male-female ratio is not the same for all the species, so...



FLAmoeba said:


> I just read this in the Tarantula Keepers Guide. If you buy one spiderling you have a 50/50 of getting what you want.


...I don't think this is true unless you're buying spiderlings from a species with 50-50 M/F ratio. If you buy 4-5 spiderlings, likely at least one of them is a female, but it's not guaranteed. A few years ago I bought four _C. marshalli_ spiderlings, one is female, three are males. On the other hand, in the early 2000s I got _B. emilia_, _B. smithi_, _B. verdezi_, _H. lividum_, _L. parahybana_, _N. chromatus_ (one of each, all unsexed early instars), and all of them were/are females.

Reactions: Like 1


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## Amoeba (Sep 1, 2011)

Zoltan said:


> The male-female ratio is not the same for all the species, so...





FLAmoeba said:


> I'm sure there is a certain amount of fluctuation with the male to female ratio (as with the human population)


 beat you to that punch.


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## DaveM (Sep 1, 2011)

Quick numbers:
spiderlings    % chance of getting at least one female

     1                  50
     2                  75
     3                  87.5
     4                  93.75
     5                  96.875
     6                  98.4375
     7                  99.2188
     8                  99.6094
     9                  99.8074
    10                 99.9023

This, assuming each spiderling has a 50% chance of being female.

---------- Post added 09-01-2011 at 03:32 AM ----------

Ugg...
Here is better formatting.

Quick numbers:
spiderlings --   % chance of getting at least one female

     1     --             50
     2      --            75
     3     --             87.5
     4      --           93.75
     5     --             96.875
     6     --             98.4375
     7     --             99.2188
     8      --            99.6094
     9       --           99.8074
    10      --           99.9023

---------- Post added 09-01-2011 at 03:52 AM ----------

The correct formula for calculating these statistics is:

( 1 - ( 1 / ( 2 ^ number of spiderlings ) ) ) x 100%

I'm using the symbol ^ like computer programmers do to mean 'raised to the power of' --
So 2 ^ 3 means 2 x 2 x 2, which equals 8. 

Here's an example:

If you buy 3 spiderlings. then:

( 1 - ( 1 / ( 2 ^ 3 ) ) ) x 100% simplifies to
( 1 - ( 1 / 8 ) ) x 100% simplifies to
0.875 x 100% = 87.5 %

Here's a plain language explanation of the formula:

Since there are 2 genders, the number of possible gender combinations equals ( 2 ^ number of spiderlings ).
So if you buy 3 spiderlings, then 2 ^ 3 = 8. There are 8 combinations:
FFF, MFF, FMF, FFM, MMF, FMM, MFM, MMM
Only one combination would mean you get no females: MMM.
So that 1 combination out of the 8 (in this example) or 1 / 8 is the fraction of the time that you get all males.
Then the fraction of the time that you don't get all males is 1 - ( 1 / 8 ) = 7 / 8 or 0.875.
Then multiple that by 100 to express that fraction as a percent.  0.875 x 100% = 87.5% 

The formula works as such because no matter how many spiders you buy, only one combination of genders would give you no females.
MM if you buy 2
MMM if you buy 3, and so on...
It also works if you buy no spiderlings because any number raised to the power of zero equals 1.
So buy 0 spiderlings and (1 - ( 1 / ( 2 ^ 0 ) ) ) x 100% = ( 1 - ( 1 / 1 ) ) x 100% = 0 x 100% = 0%
So you have no chance of getting females if you get no spiders, but I guess that's obvious. *blush*

By the way, tarantula gender is determined by chromosome allotment, not by temperature or external factors like with some reptiles and amphibians and fish. So for sure, in every T species half of the sperm cells should be male and half female. Whether male or female sperm cells might have any advantage in fertilizing eggs under certain conditions -- I think that's unknown. In humans, male sperm swim slightly faster, while female sperm are slightly more resilient (survive longer, or survive better under sub-optimal conditions). In Ts, the sperm is transfered directly onto the eggs, not far to swim.

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## Chris_Skeleton (Sep 1, 2011)

mickey66 said:


> I don't remember where I read this but it said 1 in 4 slings are female.....for what its worth.





CorySpaceCowboy said:


> I don't like your answer, I hope your wrong.
> 
> But if you are, ones odds of getting a female by buying 4 would be 50%?? (is that correct?[if so I REALLY hope your wrong.])


If it's a 1 in 4 chance, then buying 4 would mean 25%.


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## Amoeba (Sep 1, 2011)

DaveM said:


> The formula works as such because no matter how many spiders you buy, only one combination of genders would give you no females.
> MM if you buy 2
> MMM if you buy 3, and so on...
> It also works if you buy no spiderlings because any number raised to the power of zero equals 1.
> ...


:biggrin: You sir will be getting a thank from me. There is much learning in this post. Per Tarantula Keepers Guide Third edition (page 119) The table says almost exactly what DaveM posted just in more roundy readable numbers (i.e. not as exact) and didn't provide the formula: 
1 = 50% (1 out of 2) 
2 = 75% (3 out of 4) 
3 = 87.5 (7 out of 8) 
4 = 93.75% (15 out of 16) 
5 = 95% (19 out of 20) 

Didn't need to post this but I wanted to correct my numbers


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## xhexdx (Sep 1, 2011)

Zoltan said:


> The male-female ratio is not the same for all the species, so...
> 
> 
> ...I don't think this is true unless you're buying spiderlings from a species with 50-50 M/F ratio. If you buy 4-5 spiderlings, likely at least one of them is a female, but it's not guaranteed. A few years ago I bought four _C. marshalli_ spiderlings, one is female, three are males. On the other hand, in the early 2000s I got _B. emilia_, _B. smithi_, _B. verdezi_, _H. lividum_, _L. parahybana_, _N. chromatus_ (one of each, all unsexed early instars), and all of them were/are females.


Zoltan,

Have you got links to any studies that have been done regarding this topic?  I'd be curious to see what has been documented.

Thanks.


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## catfishrod69 (Sep 1, 2011)

when i buy slings of a new species to me, i usually buy 4 if they are higher priced...its really just a game of luck..

Reactions: Agree 1


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## Quazgar (Sep 1, 2011)

how many should you buy?  all of them :biggrin: then you really are guaranteed a female

Reactions: Like 1


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## captmarga (Sep 2, 2011)

Dealing as I do with equine genetics, when we are using a reference sire (horse stallion or donkey jack) for progeny reports, we work with a sire that has 100 or more offspring first.  Anything smaller than that sample is not statistically useful.  First choice is always 100, one with 50 would be useful, but a sire with only 7 offspring isn't considered as a statistical choice.  Therefore it would be great to be able to have true numbers from a sac of 100 spiderlings, from hatch to sexing.  Wishful thinking, I'm sure, but I can hope...

Marga


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## Zoltan (Sep 2, 2011)

xhexdx said:


> Have you got links to any studies that have been done regarding this topic?  I'd be curious to see what has been documented.


I haven't read any article like a comparison of sex ratios between theraphosid species, so I actually can't say if there's significant difference between different species, but I remember reading a post of an experienced keeper (Martin Huber if I remember correctly) about a species stating there are very few males. Biased sex ratio has been documented in araneomorphs, and Steve Nunn cites an article in this topic:

Main B. Y. 1990. Dwarf males in mygalomorph spiders: adaptation to environmental hazards. _Acta Zoologica Fennica_ *190*: 273–278.

I could only find the abstract of this work:

Male dwarfism of araneomorph spiders has attracted much attention, but the phenomenon in mygalomorphs has received previously little comment apart from its systematic value. It is suggested that in some Australian mygalomorph species the selective advantage for small males is an adaptation to aridity and/or flooding of microhabitats. Examples are drawn from the ctenizid genera _Arbanitis_ and _Eucyrtops_ and the nemesiid genera _Teyl_ and _Yilgarnia_. The postulated accelerated development of males (resulting in dwarfism) may also lead to a more even adult sex ratio than is generally found in mygalomorphs. This hypothesis is presented as a speculative basis for directing future field investigations.

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## DaveM (Sep 4, 2011)

FLAmoeba said:


> :biggrin: You sir will be getting a thank from me. There is much learning in this post. Per Tarantula Keepers Guide Third edition (page 119) The table says almost exactly what DaveM posted just in more roundy readable numbers (i.e. not as exact) and didn't provide the formula:
> 1 = 50% (1 out of 2)
> 2 = 75% (3 out of 4)
> 3 = 87.5 (7 out of 8)
> ...



Thanks, FLAmoeba!
I tend to think that the average sac would have close to 50/50 males/females, and would be curious to find if any T species is consistently different.
Most sexually reproducing species in nature fit with Fisher's principle: http://en.wikipedia.org/wiki/Fisher's_principle
...and for the species that don't, the reasons are usually interesting...:sarcasm:


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