# What affects gender of mice?



## lifelongcannibal (Dec 23, 2012)

So I have mice that are 5 weeks old, 32 total from 3 different litters. I just now got to separating the sexes, And found that only 8 of them are females!!! that means 75% of these mice, all born within one day of eachother, are males! Is this common? I would think, especially with 3 litters, that the numbers would be closer to half and half. So because they were all born around the same time, I am led to believe that something like temperature affected the genders. Here in florida it was probably lows of high 50's and highs of high 70s when they were conceived, and a few degrees chillier when they were born. What gives?


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## GibblyGiblets (Oct 2, 2011)

I'm no science wizz but I'm pretty sure gender is a random factor decided by X and Y chromosomes

I always get more males than females, for me it's usually around 60/40

for example out of my two newest litters there are 13 babies and it looks like only 3 or 4 of them are female.

I had heard somewhere that the temperature can effect the number of male vs female offspring, the warmer the area the more males are born or something to that effect but I don't know if there is any truth in the matter :shock:


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## littlelovesmousery (May 19, 2012)

Most of my litters tend to be pretty male heavy. Except for my last one, it was a litter of 5 and had 4 females & 1 male. I can live with those numbers!


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## tinyhartmouseries (Dec 30, 2010)

I have heard something about daylight hours and types of light affecting gender of babies. Man, we are being so specific. :lol:


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## onionpencil (Jan 6, 2013)

tiny, do you have any links to that info? that would be very handy, if we could somehow encourage more girls (or boys if desired).


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## lifelongcannibal (Dec 23, 2012)

huh, weird. I wonder why males are more likely 0_o


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## tinyhartmouseries (Dec 30, 2010)

Ah, no, I'm extremely specific and helpful today, it seems.  I'd love any further info myself.


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## onionpencil (Jan 6, 2013)

quick search turned up this, and i may look for a few more-

http://www.biolreprod.org/content/71/4/1063.full

http://www.pnas.org/content/100/8/4628.full


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## onionpencil (Jan 6, 2013)

also found one about crowding, but as that seems less humane, i'm not including it


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## MojoMouse (Apr 20, 2012)

I get slightly more females. I can only conclude it's related to whether you live on the top or on the bottom of the world. Maybe more girls born down under in Australia.


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## HannahBanana (Dec 23, 2012)

I wrote an essay on this last year and the evolutionary mechanisms behind it. I think because female mice are kept in groups and only exposed to makes for breeding there is an underlying mechanism that makes them skew litters towards males, as if they think there are no males around. As though subconsciously trying to keep the species going. Didnt see any evidence in the literature but if I had to make up a reason.....


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## candycorn (Oct 31, 2011)

In the wild...males fight among each other for territory and in most species they are "expendable" because they are able to spread sperm quickly and if they don't live long, they did their job. They tend to wander more and that causes more risk of death by predator etc. And hey...it's a good thing because snakes and hawks and owls need to eat too! So in many species more males are born since they don't tend to live as long. I am pretty sure this is biological. Nothing can really affect mammal gender in the environment. Just like nothing works for humans naturally. *sure you can spin sperm out and try to increase the odds...but that would be very impractical for mice. 
But the nice thing is mice have huge litters that do need to be culled down to create a healthy size litter...so just make the odds better yourself!


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## maddeh (Jul 14, 2011)

In terms of 'fitness' (i.e. passing on your genes) it is more beneficial produce sons than daughters, as it is more likely in a polygamous species that your son will father many more young than your daughter would. Just a thought


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## HannahBanana (Dec 23, 2012)

Yes, that's true! Good thinking.


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## dunctonhams (Jun 15, 2011)

maddeh said:


> In terms of 'fitness' (i.e. passing on your genes) it is more beneficial produce sons than daughters, as it is more likely in a polygamous species that your son will father many more young than your daughter would. Just a thought


If you have a son who reproduces, this is true. However, in polygamous species most males never breed. Since, in theory at least, roughly equal numbers of male and females are born, every extra female that a polygamous male mates with means that another male misses out the chance to mate. For example, in the extremely polygamous elephant seals, with a harem system, less than 5% of the males father 90% of the young.

In polygamous species, females are a "safe bet" for grandchildren, (passing on genes); they almost certainly will have SOME offspring but not huge numbers. Males are the "gamble"; they'll most likely have NO offspring but if they do breed they will give you shed loads of grandchildren.


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## Sizzlea89 (Jan 19, 2013)

I always have either 50/50 in a litter or more females, it could be to do with the fact that even with the heating on in my house it is still really cold, that and the fact that I live in scotland


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