Sex determination in rats

[Linds]

Does anyone know what determines the sex of baby rats? Are there outside influences such as temperature involved? I have just been wondering this, because my rats seem to throw predominantly male babies. I would estimate that maybe 10% (maybe even a little less) of the offspring are female. All my rats have good sized litters (10-18), and I rarely get more than one or two females out of them. I have 4 colonies of 1.2 and 1.3 right now, I have replaced my breeding stock with completely unrelated blood at one point, and still this low percentage of females holds true. I like having a high percentage of males because they get much larger, however it is a bit of a pain to have such a low percentage of females because they take up alot of housing space, and in terms of holding any back for breeding purposes... :rolleyes:

[Pixie]

That's a question I was wondering myself but in terms of mice breeding. My 6 last litters have almost all been exclusively males which has me just as puzzled as you!

My best guess is temperature might have something to do with it as it does with many other species and class of animals. Now the question is, how does one determine if that is the exact and only factor and what temps produce what.

Pixie

[Burmies]

I really do not think that you can tell the difference until they get a certian size. Mice are a little harder to tell.

Burmies

[marisa]

I have SO thought about this too guys!

My mice produce almost ALL males. I get a similar ratio as Linds. I seem to remember Linds that you said yours are kept fairly cool? (50-70?)

Mine are unfortunatly kept a little too warm. So I am not sure if temp has something to do with it or not. Maybe the fact that colonies are normally kept with fewer females??? But I doubt that would cause more males. I dunno. A mystery for me!

I don't know about rats but with mice its fairly easy to tell with the size of the space between the anus and the genitals. We don't really even look at the "parts" themselves when they are babies but the space between them is normally a good give away for the young ones. But mistakes are sometimes made.

Marisa

[rattekonigin]

Sex determination in rats is strictly chromosomal, but there could be factors in the environment that affect the viability of female embryos or X chromosome bearing sperm...I don't know if temperature might have something to do with it, but it's entirely possible...Like, as an example of what I mean, in humans the acidity of the female reproductive tract has an affect on the survival of "female" vs "male" sperm, if I recall correctly, with a lower pH the viability of Y chromosome carrying sperm is lowered significantly. Also, I remember reading that human "female" sperm are more likely to fertilize an egg than the "male" sperm, but male embryos, once established, have a higher rate of survival than female embryos...so everything strikes a balance in the end.

[vanderkm]

Interesting observations on the high number of male offspring over multiple generations. Rattekonigin is correct, sex determination in mammals is based on whether male or female sperm fertilize the eggs. The dam determines the maximum number of eggs released, so governs litter size. The sperm are either male or female, so the sire is largely responsible for the sex of the offspring, but conditions in the female reproductive tract (acidity, time of breeding, etc) can influence sperm survival, favoring either male or female sperm.

I would wonder about diet, water source and timing of breeding (immediate rebreeding vs waiting between litters) rather than temperature as the factor that determines sex ratio in the litters. Certain foods can influence the acidity of the female reproductive tract (especially those that are acidic) and water source (very soft or hard water, or acidic water) might have an influence. There are also normal fluxuations in the mucous conditions through the cycle of the female that influence which sperm survive.

Also worth considering that males are surviving the first week of life better, even if equal numbers are born. In most mammals, more males die in the first weeks of life, but in rodents, they seem to be larger from birth and may be more likely to survive. With rats, it may be less likely - they seem to raise a higher percentage of offspring born than mice, but might be a factor with mice.

It might be interesting to monitor over time. My experience has largely been with fairly equal numbers of male and female in mice and gerbils that I breed now -I no longer have rats so cannot offer direct input on them.

mary v.

[Pixie]

The reason I thought that temperature might affect the sexes of the babies is from a documentary I saw a year or so ago talking about human and animal reproduction. In humans and other species, the "female" sperm cells seem to be more active and strong in higher temps than "male" sperm cells. I don't know if this is the same in rodents but it sounds like it could apply to them too as well as other points brought by rattekonigin and vanderkm about acidity in the female's reproductive tract.

Now, if we could only find some documentation on this it would be great!!!

Pixie

[dm101081]

My rats definetly produce more males than females. The only thing I can think of that makes even a little sense besides what has already been posted, is that we all keep colonies of only one may to several females, so nature is trying to balance out the population by producing more males. I don't know if that made sense to anyone, but it does to me.

[Lisa]

I've found we get far more females then males... our house temp is around 80ish....