Sex ratio bias in tarantulas

jayefbe

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Ok, anyone have any information on whether there is a sex bias in tarantula slings? I've read 50:50, and 5:1 with a male bias. Now, if a male bias were to ever evolve in a group of animals, tarantulas would be the ones. Females have a longer lifespan, while males are generally only sexually mature for a single season. If in fact there was a 50:50 sex ratio in slings, mature males would occur in much smaller numbers than mature females. I know a male can breed with multiple females, but there are also many species in which a good insertion results in an eaten male. Plus, the much higher numbers of mature females due to their much longer lifespans would still outweigh the fact that a male can breed with multiple females. Now, for the most part, sex determination is completely random and a product of meiosis, but I do know of many instances in which this is not the case (many birds, species of deer). It is possible for mechanisms to evolve that skew the sex ratio.

Anyone have any links to papers/discussions/threads?
 

Steve Calceatum

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I am very curious about this topic as well. It seems as if it would be that way, but T's have a way of surprising you.
 

brothaT

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I admit I haven't had sufficient sampling of Ts to satisfy any clear conclusion, but I also have had a nearly 50/50 split and perhaps even a little bias towards females.
 

micheldied

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im curious about this myself.
maybe its a 50% chance of female or 50% chance of male,but thats chances and therefore the actual number would be more random?
im no expert.
 

Talkenlate04

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im curious about this myself.
maybe its a 50% chance of female or 50% chance of male,but thats chances and therefore the actual number would be more random?
im no expert.

To me 50 50 means over all you will get 50 50. You might have a sac that has 55 45, or 40 60, but over all you average 50 50.
 

jayefbe

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im curious about this myself.
maybe its a 50% chance of female or 50% chance of male,but thats chances and therefore the actual number would be more random?
im no expert.
With the large clutch sizes of tarantulas, a single clutch with an accurate sex verification for each sling would go a long way towards showing any bias in sex ratio.

I believe I've read somewhere about a strong bias for males in an Asian arboreal tarantula slings (Cyriopagopus or Lampropelma, I believe) but can no longer find this information.
 

vvx

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It feels like I have more males than females, but I suspect that's just my pessimistic self. When I actually think about it, it's probably closer to 50/50. You get lucky/unlucky sometimes. I bought 6 P. irminia and got 5 females. I bought 6 A. purpurea and got 5 males (not sure on the 6th, it may be male as well.) If you want a sure thing, buy a sexed female.
 

jayefbe

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I didn't start this thread because I personally feel as though I have more males than females. Rather I have read posts stating that some species do have a strongly biased sex ratio (especially for Lampropelma violaceopes) and was wondering if there was any documentation or scientific research verifying this information. I've searched around on google scholar for any information with no results, but that doesn't rule out hobbyists documenting this occurrance.

Of course, it also doesn't rule out the possibility that whoever was doing the exporting held back the T's that looked female.

http://www.meetourspiders.com/SPECIES/blue.htm
http://www.livefoodshop.co.uk/forum/index.php?showtopic=50895
 

micheldied

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To me 50 50 means over all you will get 50 50. You might have a sac that has 55 45, or 40 60, but over all you average 50 50.
what i meant are that for each egg,theres a 50% chance of male,50% chance of female.
this does not mean the brood will be 50/50 does it?
again,im no expert and im just throwing out a guess.
 

micheldied

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With the large clutch sizes of tarantulas, a single clutch with an accurate sex verification for each sling would go a long way towards showing any bias in sex ratio.

I believe I've read somewhere about a strong bias for males in an Asian arboreal tarantula slings (Cyriopagopus or Lampropelma, I believe) but can no longer find this information.
ive read about that too somewhere...maybe cause its harder for males to find mates in arboreals whereas terrestial males would find their mates easier?
therefore arboreals require a higher number of males to increase chances?
 

jayefbe

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ive read about that too somewhere...maybe cause its harder for males to find mates in arboreals whereas terrestial males would find their mates easier?
therefore arboreals require a higher number of males to increase chances?
I don't actually think the main basis for it would have anything to do with the behavior or habitat of the species. If a mutation were to occur that would reliably produce 5x (or however many) as many males as females in any tarantula species, those genes would be favored. Assuming a 50:50 sex ratio, there are many more sexually mature females than males due to a female's significantly longer life span. This would cause a shortage in males, which means that any gene that produces more males than females in an eggsac would provide greater reproductive success for individuals that have it. Without knowing the average number of females each mature male breeds with in its lifetime, I can only assume that any mutation leading to more male than female offspring would be favored in all tarantula species. However, the mechanism would have to evolve in the first place and the probability of that is exceedingly rare.

If Asian arboreals do have a skewed sex ratio (which I've seen no actual evidence of) and no other tarantulas exhibit this characteristic, then it is likely that a common ancestor of Asian arboreals evolved this trait after diverging from other tarantula lineages.
 

DDaake

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What about c. crawshayi? I've heard the males are really hard to find. I don't think I've ever seen one available myself. I heard awhile back, not sure where, that the crawshayi ratio is 1:10 in favor of the females. Anybody know more info on this subject? D
 

rustym3talh3ad

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I myself have pondered this thought recently and tried discussing it with my boss. he said he wasnt sure and we need more research into it. MY personal thought was based on the fact that in the last year and a half of selling T's a VERY large majority of my customers come back with males.....we sell them as slings so obviously unsexable at the time of purchase, but we keep good contact with people and when they are big enuff we will sex them.

now......a thought of mine that may or may not have any basis....heat and moisture? in reptiles (geckos being what im familiar with the most) in high heat u get alot of males...in cooler or "perfect" temps you get almost all female. Could this be a possibility in inverts as well? if we are saying that Asian arboreal's is getting mostly males perhaps local and incubation temps have a huge part. pokies could be 50/50 due to local as well....and then desert dewellers could be a totally different number all together.

i agree in the thought that more research should be done..if we could control the outcome of the sac we would further our selves in the hobby by being able to make sure that u get a male for your C. crawshayi or a female for all the dang males i seem to be encountering lol.
 

MIC

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The 'equation' has a lot of parameters - factors that could play a significant role for the result.

I can list just some of them:

Temperature, Humidity (as rustym3talh3ad said)
Number of eggs in a sac
Expectation of life for male - female
Time to reach maturity
Females behavior in regard to males
Natural enemies (males are more vulnerable to them)
Habitat and more...

The difficulty comes also from the interdependency between the most of the aforementioned parameters - factors.

Another focal point is that such parameters may have played a role for very long periods (thousands or even millions of years) and today we are just observing what changes those periods have worked to tarantulas sex bias.
 
Last edited:

xhexdx

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what i meant are that for each egg,theres a 50% chance of male,50% chance of female.
this does not mean the brood will be 50/50 does it?
again,im no expert and im just throwing out a guess.
What it means is:

To me 50 50 means over all you will get 50 50. You might have a sac that has 55 45, or 40 60, but over all you average 50 50.
No, it does not mean the sac will have exactly 50% male and 50% female. What it means is there are the same odds of an egg being male as there are female. Statistically, that means the sac average is the same; 50%male, 50% female.

It feels like I have more males than females, but I suspect that's just my pessimistic self. When I actually think about it, it's probably closer to 50/50. You get lucky/unlucky sometimes. I bought 6 P. irminia and got 5 females. I bought 6 A. purpurea and got 5 males (not sure on the 6th, it may be male as well.) If you want a sure thing, buy a sexed female.
Even then, you don't always get a female. We've all either read threads about it or been unfortunate enough to be the recipient of a 'sexed' female only to find it out was male.

Until I find data that proves otherwise, I'm of the opinion that sacs are 50/50, on average.

--Joe
 

micheldied

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I don't actually think the main basis for it would have anything to do with the behavior or habitat of the species. If a mutation were to occur that would reliably produce 5x (or however many) as many males as females in any tarantula species, those genes would be favored. Assuming a 50:50 sex ratio, there are many more sexually mature females than males due to a female's significantly longer life span. This would cause a shortage in males, which means that any gene that produces more males than females in an eggsac would provide greater reproductive success for individuals that have it. Without knowing the average number of females each mature male breeds with in its lifetime, I can only assume that any mutation leading to more male than female offspring would be favored in all tarantula species. However, the mechanism would have to evolve in the first place and the probability of that is exceedingly rare.

If Asian arboreals do have a skewed sex ratio (which I've seen no actual evidence of) and no other tarantulas exhibit this characteristic, then it is likely that a common ancestor of Asian arboreals evolved this trait after diverging from other tarantula lineages.
hmmm...very true.
if only tarantulas knew how to speak english.:rolleyes:
 

micheldied

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now......a thought of mine that may or may not have any basis....heat and moisture? in reptiles (geckos being what im familiar with the most) in high heat u get alot of males...in cooler or "perfect" temps you get almost all female. Could this be a possibility in inverts as well? if we are saying that Asian arboreal's is getting mostly males perhaps local and incubation temps have a huge part. pokies could be 50/50 due to local as well....and then desert dewellers could be a totally different number all together.

i agree in the thought that more research should be done..if we could control the outcome of the sac we would further our selves in the hobby by being able to make sure that u get a male for your C. crawshayi or a female for all the dang males i seem to be encountering lol.
however,if that were true wouldnt the all tropical Ts have the same sex ratio?
 

micheldied

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What it means is:



No, it does not mean the sac will have exactly 50% male and 50% female. What it means is there are the same odds of an egg being male as there are female. Statistically, that means the sac average is the same; 50%male, 50% female.



Even then, you don't always get a female. We've all either read threads about it or been unfortunate enough to be the recipient of a 'sexed' female only to find it out was male.

Until I find data that proves otherwise, I'm of the opinion that sacs are 50/50, on average.

--Joe
yeah,thats what i meant.
thanks joe.
 
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